– Improper installation or repair of Webasto heating and cooling systems can cause fire
or the leakage of deadly carbon monoxide leading to serious injury or death.
– Installation and repair of Webasto heating and cooling systems requires special
Webasto training, technical information, special tools and special equipment.
– NEVER attempt to install or repair a Webasto heating or cooling system unless you
have successfully completed the factory training course and have the technical skills,
technical information, tools and equipment required to properly complete the
necessary procedures.
– ALWAYS carefully follow Webasto installation and repair instructions and heed all
WARNINGS.
– Webasto rejects any liability for problems and damage caused by the system being
The purpose of safety symbols is to attract your attention to possible hazardous conditions. This manual uses a series
of symbols and signal words which are intended to convey the level of importance of the safety messages. The
progression of symbols is described below. Remember that safety messages by themselves do not eliminate danger
and are not a substitute for proper accident prevention measures.
Indicates an imminently hazardous situation which, if not avoided,
WILL result in death or serious injury.
DANGER
Indicates a potentially hazardous situation which, if not avoided,
COULD result in death or serious injury.
WARNING
Indicates a potentially hazardous situation which, if not avoided,
MAY result in minor or moderate injury or property damage.
CAUTION
It may also be used to alert against unsafe practices.
These symbols are used to alert the installer to important or useful information
OR
about proper installation of the equipment.
1.2General Information
Webasto Product North America, Inc. is pleased to provide this installation manual for use with this new heater. When
installed in accordance with the guidelines stated in this manual, you can expect to provide years of trouble-free,
enjoyable operation for your customer.
This manual represents our latest effort to produce the best technical documentation possible.
In our efforts towards continuous, ongoing product improvement, we encourage our customers to write to us with
their comments or criticisms concerning this manual or product.
Please write to us at:
Webasto Product North America, Inc.
Technical Documentation Group
15083 North Road
Fenton MI 48430
You are also invited to fill out our online questionnaire concerning our technical documentation and web site at:
If you have any immediate questions concerning this manual, the installation procedures within or the product itself,
please call us at:
www.webasto.us3Webasto Product N.A., Inc.
www.techwebasto.com
(800) 860-7866 or send a fax to: (810) 593-6001
Page 6
Regulation for Installation in the VehicleThermo 90 ST
2.Regulation for Installation in the
Vehicle
Read this installation manual in its entirety before
installing this equipment.
2.1Scope
2.1.1Subject to the provisions of paragraph 2.1.2,
internal combustion heaters must be installed in
accordance with the requirements contained in
this Annex.
2.1.2In the case of vehicles and trailers with heaters
for liquid fuel, it is presumed that these vehicles
comply with the requirements in this Annex.
2.2Position of the Heater
2.2.1Parts of the vehicle body and other components
in the immediate vicinity of the heater must be
protected against excessive heat and the danger
of contamination by fuel or oil.
2.2.2The internal combustion heater must not pose a
fire hazard even when overheated. This
requirement is deemed to have been met if care
is taken during installation to ensure an
adequate distance from all parts, as well as
adequate ventilation and if fire-resistant
materials or heat shields are used.
2.2.3For passenger carrying vehicles such as shuttle
busses, transit busses, and coaches, the heater
must not be installed in the passenger cabin. A
device in a sealed cover, which also meets the
requirements set out in paragraph 2.2.2, may be
used, however.
2.2.4The model/ Serial plate or a duplicate thereof
(duplicate model/ Serial plate) must be fitted in
such a way that it is still clearly legible when the
heater has been installed in the vehicle.
2.2.5When positioning the heater, all reasonable
precautions must be taken to minimize the risk
of personal injury or damage to items in the
vehicle.
2.2.6A clearly visible indicator within the user's field
of vision must show when the heater is switched
on or off.
2.3Fuel Supply
2.3.1The fuel filler neck must not be located in the
passenger compartment and must have a tightly
fitting cap to prevent any fuel leaks.
2.3.2The type of fuel and the fuel filler neck must be
clearly identified on heaters for liquid fuel, for
which the fuel supply is separate from the fuel
supply for the vehicle.
2.3.3A sign must be affixed to the fuel filler neck
warning that the heater must be switched off
before refuelling. An identical warning must also
be included in the manufacturer's operating
instructions.
2.4Exhaust System
2.4.1The exhaust outlet must be positioned in such a
way that exhaust fumes cannot get into the
interior of the vehicle through ventilation
devices, hot-air inlets or open windows.
2.5Combustion Air Inlet
2.5.1The air for the combustion chamber of the
heater must not be extracted from the
passenger cabin of the vehicle.
2.5.2The air inlet must be positioned in such a way
that it cannot be obstructed by other objects.
2.6Water Inlet
2.6.1The supply of water to be heated must be
protected against freezing with a suitable
antifreeze additive to the required strength as
directed by the manufacturer. The supply of
water to be heated must be uncontaminated by
corrosive compounds and other impurities.
2.6.2The inlet line must be protected from damage.
2.7Hot Water Outlet
2.7.1Hot water lines within the vehicle must be
positioned or protected in such a way as to
exclude all risk of injury or damage caused by
direct contact.
Webasto Product N.A., Inc.4www.techwebasto.com
Page 7
Thermo 90 STRegulation for Installation in the Vehicle
2.7.2The water outlet line must be protected so that
it cannot be obstructed by other objects or the
flow of water through the lines be blocked.
IMPORTANT!
Failure to follow the installation
instructions and the notes contained
therein will lead to all liability being
refused by Webasto. The same applies if
repairs are carried out incorrectly or with
the use of parts other than genuine
Webasto service parts. This will result in
the invalidation of the type approval for
the heater and therefore of its
homologation / type licence.
www.webasto.us5Webasto Product N.A., Inc.
Page 8
Purpose / VersionThermo 90 ST
3.Purpose / Version
3.1Purpose of the Coolant Heater
Thermo 90 ST coolant heaters are used in connection
with the vehicle’s own heating system
– to heat the cab,
– to defrost vehicle windows and
– to preheat water-cooled engines,
– to heat boats and motor-homes (recreational vehicles)
The water heater operates independently of the engine
and is connected to the cooling system, the fuel system
and the electrical system of the vehicle.
3.2Versions
3.2.1Thermo 90 ST version
Thermo 90 ST gasoline
Water heater for "gasoline" fuel
Thermo 90 ST diesel
Water heater for "diesel" fuel
The water heaters are designed for 12 V (Thermo 90 ST
gasoline) and for 12 or 24 V (Thermo 90 ST diesel)
Webasto Product N.A., Inc.6www.techwebasto.com
Page 9
Thermo 90 STInstallation
4.Installation
IMPORTANT!
The water heater must be installed outside
the passenger cabin.
The requirements of the latest version of
“Hazmat” must also be observed for the
installation of the heater into vehicles used
to transport hazardous substances.
4.1Installation site / Installation position
The heater must be installed in as low a position as
possible to allow the heater and circulating pump to be
bled automatically. This is particularly important as the
circulating pump is not self-priming.
The installation must be performed in accordance with
the installation instructions provided in this manual.
NOTE:
If the vehicle manufacturer has issued specific installation
instructions, they must be followed.
As a rule, installation in the engine compartment
represents sufficient screening. Please contact the
Webasto hotline in the event that radio interference
occurs nevertheless.
4.2To install the heater
The heater must be secured with at least three M8
screws. The screws must be tightened with a torque of
18 Nm. (13.3 lb-ft).
4.3Model plate
The model plate must be positioned so that it cannot be
damaged and must be clearly legible when the heater is
installed (otherwise a duplicate model plate must be
used).
Inapplicable years must be erased from the model plate.
4.4CE mark
The Thermo 90 ST water heater carries the CE mark since
it complies with the regulations in force. The heater
satisfies the requirements of class A. As a result, the
following supplement applies:
WARNING
This is a class A device. This device
may induce radio interference in
residential areas; in this case, the
owner may be required to
undertake appropriate measures.
www.webasto.us7Webasto Product N.A., Inc.
Page 10
Installation Location ExamplesThermo 90 ST
5.Installation Location Examples
Figure 1: Installation locations
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Page 11
Thermo 90 STMounting the Heater to the Vehicle
6.Mounting the Heater to the Vehicle
6.1Allowable Mounting Positions
3
1
2
Coolant Direction
Fuel Connection
Figure 2: Allowable enclosure kit mounting positions, 1 and 2 only. Position 3 is not permissible.
=
Figure 3: Permitted installation positions for the Thermo 90 ST heater - Enclosure and Compact Kits
=
Figure 4: Permitted installation positions for the Thermo 90 ST heater - Compact Kits only
www.webasto.us9Webasto Product N.A., Inc.
Page 12
Mounting the Heater to the VehicleThermo 90 ST
CAUTION
Do not drill holes through top or bottom of vehicle frame flanges!
Do not weld vehicle frame or flanges!
80mm
(3-5/32”)
70mm
(2-3/4”)
Figure 5: Drill frame within the shaded area only!
310mm (12-13/64”)
80mm
(3-5/32”)
70mm
(2-3/4”)
150mm
(5-29/32”)
23mm
(29/32”)
Figure 6: Mounting heater enclosure via back wall (Vertical or horizontal mounting)
Webasto Product N.A., Inc.10www.techwebasto.com
45mm
(25/32”)
Page 13
Thermo 90 STMounting the Heater to the Vehicle
150mm
150mm
484mm
363mm
158mm
500mm
Figure 7: Optional perpendicular mounting with bracket kit P/N 905822 or P/N 905838 (Freightliner style)
Connection to the Vehicle Cooling SystemThermo 90 ST
7.Connection to the Vehicle Cooling System
7.1General Information
The heater is connected to the vehicle cooling system as shown in Figures 10 through 15, depending on the type of
heating system the vehicle is equipped with. The system must contain at least 6 litres (1.6 US Gal.) of coolant. The
system must be filled with an antifreeze and water mixture as recommended by the vehicle manufacturer. Coolant
flow throughout the system must be maintained in all operating conditions.
If desired, shut-off valves can be installed at the engine inlet and outlet connections of the coolant heater circuit,
although, shut-off valves are not mandatory. All valves should remain open throughout the year.
IMPORTANT!
To maintain optimum heater condition, periodically run heater for ten or more minutes monthly
during the off-season. Ensure any shut-off valves are open before operating the coolant heater.
• Use 1/2” or larger NPT pipe fittings for hose connections at engine.
• Use good quality, heavy duty 3/4” ID. coolant hose for the coolant heater circuit.
• Hoses must be installed without kinks and bends which may restrict coolant flow.
• To ensure proper bleeding of air, keep hoses rising if possible. Avoid “goose necks” to prevent trapping air.
• Hoses must be supported where required to prevent chaffing and damage from vehicle components.
• Hose connections must be secured by hose clamps to prevent them from slipping off.
• Coolant heater flow direction should match the flow direction of the engine’s coolant circulation system.
IMPORTANT!
The hose clamps must be tightened with a torque of 4 Nm. (35 lb.-in.)
The use of silicone hose requires special hose clamps.
Separate the heater supply and return
Optimum Cross-Flow Through Engine
connections at the engine as far apart as
is practical for optimum engine heating
and heater performance. See Figure 9.
Figure 9: Ensure optimum cross-flow
Webasto Product N.A., Inc.12www.techwebasto.com
Page 15
Thermo 90 STConnection to the Vehicle Cooling System
The cooling system must be bled carefully before using the heater for the first time or after replacing the coolant.
Proper venting of trapped air can be identified by the circulating pump operating almost silently. Poor bleeding may
cause the resetting temperature limiter to trip whilst the heater is operating.
7.2Various Plumbing Configurations
Select the appropriate plumbing configuration according to the customers requirements and the vehicle system type.
Figure 10 illustrates an engine preheat / boost heat configuration.
Figures 11 through 14 illustrate configurations for bunk heat and engine preheat / boost heat.
Figure 15 illustrates a system for vehicles with continuous fan operation or high amperage heat exchanger fan designs.
In this case, the addition of a low amperage draw, auxiliary heat exchanger system is recommended. Webasto offers a
complete kit under part number 905670 for this application.
1. Coolant supply connection
2
7
1
2. Vehicle heat exchanger
3. Coolant pump
4. Thermo 90 ST heater
5. Coolant return connection
6. Engine coolant pump
7. Thermostat
4
3
Figure 10: System configured for engine pre-heating and boost heating.
22
1
4
3
6
5
1. Coolant supply connection
2. Vehicle heat exchangers
3. Coolant pump
4. Thermo 90 ST heater
5. Coolant return connection
7
6
5
6. Engine coolant pump
7. Thermostat
Figure 11: System configured in a “series” fashion with both heat exchangers.
www.webasto.us13Webasto Product N.A., Inc.
Page 16
Connection to the Vehicle Cooling SystemThermo 90 ST
1. Coolant supply connection
22
7
1
6
4
5
3
Figure 12: System configured in series with rear heat exchanger in a “dual” circuit heating system.
22
7
1
2. Vehicle heat exchangers
3. Coolant pump
4. Thermo 90 ST heater
5. Coolant return connection
6. Engine coolant pump
7. Thermostat
1. Coolant supply connection
2. Vehicle heat exchangers
3. Coolant pump
4. Thermo 90 ST heater
5. Coolant return connection
6. Engine coolant pump
7. Thermostat
4-way back-check valve
6
4
5
3
Figure 13: System configured in the rear heat exchanger circuit using an optional 4-way back-check valve.
1. Coolant supply connection
22
2. Vehicle heat exchangers
3. Coolant pump
4. Thermo 90 ST heater
5. Coolant return connection
6. Engine coolant pump
7. Thermostat
8. Automated control valve
10
8
7
9
1
9. Wye connectors
9
6
4
5
3
10. Bypass hose 13-16mm I.D.
(1/2”- 5/8” I.D.)
Figure 14: System configured with a bypass circuit within the rear heat exchanger circuit where an automated flow
control valve is installed. Bypass is required in the event the valve is closed during heater operation.
Webasto Product N.A., Inc.14www.techwebasto.com
Page 17
Thermo 90 STConnection to the Vehicle Cooling System
1. Coolant supply connection
11
9
4
3
Figure 15: System configured with an additional auxiliary heat exchanger connected in “series / parallel” with rear heat
exchanger circuit. (Recommended for Kenworth AeroCab & Studio Sleeper built before July 1994, Ford
Aeromax, and Navistar CEO & Conventional Cab.)
22
8
7
1
9
6
5
2. Vehicle heat exchangers
3. Coolant pump
4. Thermo 90 ST heater
5. Coolant return connection
6. Engine coolant pump
7. Thermostat
8. Automated control valve
9. Wye connectors
10. N/A
11. Auxiliary heat exchanger
7.3Additional Heating System Accessories
Webasto offers a line of Sleeper Pack Interface Wiring Kits that work with many OEM HVAC systems. These kits
simplify the installation and pairing of the Webasto coolant heater with the vehicle’s own bunk heating system.
The Sleeper pack interface takes control of the bunk heater during coolant heater operation. With the coolant heater
off, the bunk heater system control is returned to the vehicle’s control panel.
Consult to your Webasto Sales Representative for Sleeper Pack availability by vehicle make, model and year.
www.webasto.us15Webasto Product N.A., Inc.
Page 18
Fuel SupplyThermo 90 ST
8.Fuel Supply
8.1General Information
The values for the maximum pressure at the fuel
extraction point are shown in the table below and in
Figure 17.
Permissible fuel inflow
height H
0.00 m (0.00 in.)0.2 bar (2.9 PSI)
1.00 m (39.4 in.)0.11 bar (1.6 PSI)
2.00 m (78.7 in.)0.03 bar (0.44 PSI)
Maximum fuel intake
height S
0.00 m (0.00 in.)-0.10 bar (-1.45 PSI)
0.50 m (19.7 in.)-0.06 bar (-0.87 PSI)
1.00 m (39.4 in.)-0.02 bar (-0.29 PSI)
At maximum pressure
in fuel line
At maximum negative
pressure in the fuel tank
The fuel is taken from the vehicle fuel tank or from a
separate fuel tank. The exploded view in Figure 16
Illustrates the progression of fuel system components.
A sign must be affixed to the fuel tank’s filler neck
warning that the heater must be switched off before
refuelling.
l1 + l2 ≤ 10 m
l
≤ 1. 2 m
2
≤ 8. 8 m
l
2
Figure 17: Fuel system parameters
B
A
A. Fuel standpipe
B. Connector hose
C. Fuel filter
D. Hose clamp
Figure 16: Fuel system components
F
E
D
°
C
E. Fuel pump
F. Fuel line (Mecanyl)
G. 90° Connector hose
0
G
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Page 19
Thermo 90 STFuel Supply
8.2Fuel Standpipe Installation
To install the standpipe:
This separate fuel pickup precludes any effect of
pressure.
Where available, a spare NPT threaded port can be
utilized for the standpipe. If a spare port is not present,
proceed with the following instructions under “To install
the standpipe”.
A
B
C
D
Ø 25mm (1”)
E
F
1. bore a 25mm (1”) hole through top of fuel tank
(item G, Figure 18).
2. remove sharp burrs and smooth edges with emery
cloth.
3. determine length of standpipe when installed. End
should sit at least 25mm (1”) above tank bottom.
Cut off excess standpipe at a 45 degree angle.
Remove burrs.
4. loosely assemble items C, D, E, and F (Figure 18).
5. place sealing compound on threads of item B and
thread into item F. Tighten item B completely.
6. slide standpipe into hole at angle. Slip one shoulder
of item F, Figure 18 inside the hole as shown in
Figure 19.
Only steel, copper and plastic lines made of plasticized,
light and temperature-stabilized PA 11 or PA 12 (e.g.
Mecanyl RWTL) pursuant to DIN 73378 may be used for
the fuel lines.
Since the lines normally cannot be routed with a constant
rising gradient, the internal diameter must not be
allowed to exceed a certain size. Air or gas bubbles will
accumulate in lines with an internal diameter of more
than 4 mm (5/32 in.) and these will cause malfunctions
whilst the heater is operating if the lines sag or are
routed downwards. The diameters specified in Figure 17
will ensure that bubbles do not form.
The lines should not be routed downwards from the
metering pump to the heater.
Unsupported fuel lines must be secured to prevent them
from sagging. They must be installed in such a way that
they cannot be damaged by flying road debris and high
temperatures, i.e. close to the exhaust line.
The fuel lines must be secure at the connections using
hose clips to prevent their slipping.
8.4Fuel Metering Pump
The fuel metering pump is a combined delivery, metering
and shut-off system and is subject to certain installation
criteria (see Figures 24 and 25).
IMPORTANT!
Cut lines without burr and do not crush
them. Do not cut fuel line with side cutting
pliers!
8.3.1Connecting two fuel lines with a
coupler hose
The correct procedure for connecting fuel lines with
hosing is shown in Figure 23. Ensure that there are no
leaks.
Correct
Clamp
Wrong
Bubble
Figure 23: Fuel line / coupler hose connection
Bubble
Figure 24: Fuel metering pump DP2 - installation position
12 V and 24 V – Diesel Only (with damper)
Horizontal Only!
Figure 25: Fuel metering pump 30.2 - installation position
8.4.1Installation Location
It is advisable to install the metering pump in a cool
place. The maximum ambient temperature must not
exceed +20 °C (68 °F) for gasoline heaters at any time
during operation.
The metering pump and fuel lines must not be installed
within range of the radiated heat from hot vehicle parts.
A heat shield must be used if necessary.
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Page 22
Fuel SupplyThermo 90 ST
8.4.2Attachment
The metering pump must be secured with a vibrationdamping mounting. Its installation position is limited as
shown in Figure 24 and 25 in order to ensure effective
automatic bleeding.
As a result of the risk of corrosion, only genuine Webasto
parts may be used for the plug connections between the
metering pump and the metering pump wiring harness.
8.5Fuel Filter
Only a Webasto filter is allowed to be used if the fuel is
expected to be contaminated. Install vertically if possible,
however at least horizontally (check flow direction).
IMPORTANT!
Do not substitute the Webasto supplied
fuel filter with a non Webasto replacement.
Irregular heater operation may result.
Figure 26: Fuel filter
0° - 90°
A
Ø 5mm
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Page 23
Thermo 90 STCombustion Air Supply
9.Combustion Air Supply
Under no circumstances may the combustion air be
taken from areas occupied by people. The combustion
air intake opening must not point in the direction of
travel. It must be located in that it cannot become
clogged with dirt or snow and cannot suck in splashing
water.
IMPORTANT!
The combustion air must be extracted using
a combustion air line from a position that is
as cool as possible and protected from
splashing water.
The combustion air intake line (internal diameter at least
30 mm) may be 0.5 m to 5 m (1.6 ft to 16 ft) long with
several bends totalling 360°. The minimum bending
radius is 45 mm (1.75 inches).
The combustion air intake must not be routed above the
exhaust outlet.
IMPORTANT!
If the combustion air intake line cannot be
installed so that it slopes downwards, a
water drain hole with a diameter of 4 mm
(5/32”) is to be made at its lowest point.
If the heater is installed in a general installation space
near the vehicle’s fuel tank, the combustion air must be
taken in from the outside and the exhaust fumes
discharged into the atmosphere. The openings must be
splash-proof.
A ventilation opening measuring at least 6 cm² (1 inch²)
is required if the heater is installed in an enclosed box.
The size of the ventilation opening must be increased
accordingly if the temperature in the box exceeds the
permitted ambient temperature of the heater (see
Technical data).
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Page 24
Exhaust SystemThermo 90 ST
10. Exhaust System
The exhaust pipe (internal diameter 38 mm) can be installed with a length of 0.5 m to 5 m (1.6 ft to 16 ft) and several
bends totalling a maximum of 360° altogether. The minimum bending radius is 85 mm (3.35 inches).
The exhaust muffler is essential and must be installed near the heater.
The opening of the exhaust pipe must not point towards the front of the vehicle (see Figure 28).
The exhaust pipe opening must be located so that it cannot become clogged with snow and mud.
Rigid pipes of unalloyed or alloyed steel with a minimum wall thickness of 1.0 mm or flexible piping of alloyed steel
only must be used as exhaust line. The exhaust pipe is secured to the muffler using an approved muffler clamp. See
Figure 28 for details. See the statutory regulations for other requirements.
Heater removed for
clarity of illustration
B
C
AA
Figure 27: Exhaust muffler installation - enclosure kit only
A. Muffler
B. coupler (between heater and muffler)
C. “L” bracket
The exhaust muffler is essential and must be installed near the heater. The exhaust muffler is secured to the heater
using a coupler between the heater and muffler and a bracket fastened to the underside of the enclosure. See Figure
27 for details.
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Page 25
Thermo 90 STExhaust System
The exhaust pipe must be securely attached
no further than 150 mm (6 in.) from the end
of the exhaust pipe to ensure that the angle
of 90° ± 10° verticle-down is maintained.
10°
10°
Discharge direction almost vertical 90° ± 10°
Figure 28: Exhaust pipe termination
IMPORTANT!
If the exhaust line cannot be installed so that it slopes downwards at all points, a water drain hole with a
diameter of 4 mm (5/32”) is to be made at its lowest point to allow condensate to drain.
www.webasto.us23Webasto Product N.A., Inc.
Page 26
Electrical ConnectionsThermo 90 ST
11. Electrical Connections
11.1 Power / Control Harness Connections
Ideally, the heater power supply should be directly from the vehicle’s batteries or main buss-bar.
The heater control unit is equipped with low voltage protection. It is imperative that the vehicle batteries be kept in
good condition for optimal heater operation.
For sleeper and engine heating applications, Webasto recommends a four battery system for best results.
A weather sealed fuse holder is to be fitted to protect the heater (supplied with the power / control
harness).
1
1. Switch / Timer Harness
2. Heater Harness (Pre-installed)
3. Main Harness Connector
4. Power Supply Harness
5. Blower Interface Connector (3-pin)
4
2
5
3
Main Fuse
+
+
+
–
–
–
Vehicle System Power
+
–
To Heater Control Unit
Figure 29: Power and control harness assembly
A. Route item 1 into the vehicle cab in the area where the switch or timer is to be installed.
B. Route item 4 to vehicle battery box and connect to batteries as shown in Figure 29. Brown wire to ground, red wire with fuse
holder to positive. Ensure battery connections are clean and protected with an anti-corrosive compound.
C. Item 3 (main connector) should be left unconnected until all electrical components are installed and heater is ready for the
initial start-up.
Webasto Product N.A., Inc.24www.techwebasto.com
Frame Ground
Page 27
Thermo 90 STElectrical Connections
11.2 Connecting the Controls
The heater can be switched on and off using the following Webasto controls:
• Switch, see circuit diagrams in Figures 30 and 32.
• Timer, see circuit diagrams in Figures 31 and 33.
Heater Control Harness Connection
Figure 30: Toggle switch
X3
1
2
4
3
12
56
(MATING END SHOWN)
Using a 12 or 24 Volt Switch
1. White (Ground)
2. Red (Power)
3. Black (On signal)
4. Green (Operation indication)
CONNECTOR X1
34
78
Heater Control Harness Connection
Using 1531 Comfort Timer
1. Red (Power)
2. Not Used
3. Green (Operation indication)
4. Optional (Dash lights)
5. White (Ground)
6. Black (On signal)
Connector X2 (Violet)
For truck operation, connect to red
jumper of heater control harness.
For Coach, Transit and School Bus
operation, connect to ignition source.
Figure 31: 7-Day, 3 Program Comfort Timer model 1531
All the cables and wires that are not required must be insulated against accidental
CAUTION
shorting or grounding.
X1
X2
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Page 28
Circuit DiagramsThermo 90 ST
12. Circuit Diagrams
12.1 Thermo 90 ST Circuit Diagram Legend
c Digital timer P2:
Connection X14 to Red jumper of switch harness results
in continuous operation with immediate heating
(recommended for trucks requiring sleeper heating)
Connection X14 to vehicle ignition connection results in
variable heating duration that can be programmed (10
min. to 120 min.); default setting 120 min. when ignition
key is switched off. (Recommended for School bus,
Coach and Transit vehicles.)
d Optional connection to vehicle instrumentation
panel lighting circuit.
The following Table applies to Figures 32 and 33.
Item DesignationComment
A1Heater
A2Control module
B1Flame sensor
B2Temperature sensor
B3Temperature limiter/
Overheating guard
B4Room thermostat
BTBatteryVehicle battery
DSIlluminated toggleOperating indicator (in item S1)
X16 5-pin relay socketVehicle fan interface harness
Y1Metering pumpFuel pump for heater
Operating indicator (in item P2)
ready indicator, switch-on control
(in item P2)
option
To pin-10 of item X9 (see
systems (Sleeper Pak™)
option
c)
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Page 29
Thermo 90 STCircuit Diagrams
X11
A2
12
11
2
1222241
Brown
12
Blue
Green
Orange
Orange
Gray
Violet/Blue
VioletViolet
Green
Green
Gray
Gray
Violet
Y1
A1
B2
B1
B3
White/Blue
Blue
Black
Brown
Red
Yellow
12568
21
X2X4X1X5X6X7X3
4
10
11
3
1
9
12
7
X8
Gray
Brown
Red
Red
Red
White
Red
Red/Black
Red
E
RedRed
Red
F
Red
Brown
Brown
GHJ
Brown
Brown
Brown
White
F5
Brown
BT
WhiteWhite
Black
Violet
Brown
X10
Yellow/Red
1
2
F3
F2
CDE F
B
F1
A
White
Violet
White
Brown
White
Black
Green
ABC
X15
Black
Green
85
Green
White
White
87
Black
87a
1
1
0
0
B4
5
1
3
2
4
Black
BDC
X12
Black
Violet
Green
Red
A
Red
Red
Brown
M2
Blue
Black
MM
M1
Brown
Brown
E
Yellow
M2
A
B
X15
X12
1 2
X11
X10
X8
X4/X5/X6/X7
2
1
2
X3
C
1 2
1
2
1
3
5
7
9
11
1
K1
30
86
Optional Blower Interface Harness P/N 905613B
White
12V +
X16
Black
GND
DS
White
Black
S1
X2
1
EB2B3
M1
B1
X1
2
1
4
3
2
Figure 32: Thermo 90 ST with On/Off switch - connection diagram (Includes optional blower interface harness)
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Page 30
Circuit DiagramsThermo 90 ST
X15
X13
4
3
2
1
X12
X11
X10
X9
X8
X4/X5/X6/X7
X3
X2
X1
1 2
A
C
B
8
7
6
5
1 2
A2
12
11
2
1222241
21
X2X4X1X5X6X7X3
4
1
2
5
6
8
10
11
3
1
9
12
7
X11
Brown
12
Blue
Green
Orange
Orange
Gray
Violet/Blue
VioletViolet
White/Blue
Blue
Black
Brown
Red
Yellow
Y1
Green
Green
Gray
Gray
Violet
Brown
Blue
Black
Brown
Brown
Yellow
A1
B2
B1
B3
M2
MM
M1
E
X8
1
2
Yellow/Red
1
2
X10
12
9
6
3
1
3
5
7
9
11
1
2
1
2
Violet
ABC
X15
Brown
White
Violet
Black
BDC
X12
Black
Black
Violet
WhiteWhite
Green
Brown
F3
Red
Red
Red
F2
CDE F
White
B
F1
Red
A
White
Red
Red
A
EFGHJ
RedRed
Red
F5
c
Red
Red
Red/Black
Brown
Brown
Brown
Brown
Brown
Brown
Gray
Brown
White
BT
Black
1
2
1
4
3
2
X14
c
X13
Violet
634
Black
Gray
Red
Yellow
d
1
Red
White
2
7
5
8
Brown
Brown
M2
EB2B3
B1
M1
Figure 33: Thermo 90 ST with 7-day digital timer model 1531 - connection diagram
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2
11
8
H6
4
H1
101
P2
X9
7
12
H3
Page 31
Thermo 90 STInitial Start-up
13. Initial Start-up
IMPORTANT!
Refer to the safety instructions in the
operating and maintenance instructions.
The operating and maintenance
instructions must be read through without
fail before starting the heater.
NOTE:
As a result of the low fuel consumption the heater must
be switched on several times to fill the fuel line and prime
the system.
After you have installed the heater, bleed the water
system and the fuel supply system carefully. Follow the
instructions supplied by the vehicle manufacturer for this
purpose.
Conduct a trial of the heater to check all the water and
fuel connections for leaks and to ensure that they are
secure.
If the heater suffers a fault during operation, the fault
must be located and remedied.
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Page 32
TroubleshootingThermo 90 ST
14. Troubleshooting
14.1 General Information
The control unit continuously monitors the heater
operation. The control unit identifies errors on individual
heater components and faults during operation. Should
the control unit experience component errors and
operational faults, the heater will be shut down.
14.2 Fault Lock-out
Fuel is supplied for max. 240 seconds if the flame does
not start to burn.
Fuel is supplied for max. 240 seconds if the flame goes
out during operation.
The fuel supply is shut off if the system overheats
(temperature limiter is tripped). If the system overheats
the button on the temperature limiter must be reset.
Once the cause of the fault has been eliminated, the
fault lock-out is cancelled by switching the heater off and
on again.
If the under-voltage protection switches off the system
Thermo 90 ST
12 Volt10.V – 0.5V
14.3 Diagnostic after a Fault Lock-out for
the Thermo 90 ST
Check the fuses and plug connectors.
14.3.1 Version with switch
If the system is operated with a switch, the nature of the
fault is indicated by a flashing code on an indicator light
(toggle of switch) during the run-on time of the heater.
After five short signals, count the long flashes:
1xNo start (after 2 attempts to start)
2xFlame failure
3xUnder-voltage or over-voltage
4xPremature flame recognition
5xFlame sensor interrupt or flame sensor short-circuit
6xTemperature sensor interrupt or temperature sensor
short-circuit
7xMetering pump interrupt or metering pump short-
circuit
8xBlower motor interrupt or blower motor short-
circuit or blower motor incorrect speed
9xGlow plug interrupt or glow plug short-circuit
10x Overheating
11x Circulating pump interrupt or circulating pump
short-circuit
24 Volt21V – 1V
for longer than 20 seconds, the fuel supply is interrupted.
14.3.2 Version with timer
If the system is equipped with a digital timer, a fault
message appears on the display of the timer after a fault
occurs:
F 01No start (after 2 attempts to start)
F 02Flame failure
F 03Under-voltage or over-voltage
F04Premature flame recognition
F 05Flame sensor interrupt or flame sensor short-
circuit
F 06Temperature sensor interrupt or temperature
sensor short-circuit
F 07Metering pump interrupt or metering pump
short-circuit
F 08Blower motor interrupt or blower motor short-
circuit or blower motor incorrect speed
F 09Glow plug interrupt or glow plug short-circuit
F 10Overheating
F 11Circulating pump interrupt or circulating pump
short-circuit
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Page 33
Thermo 90 STTechnical Data
15. Technical Data
Except where limit values are specified, the technical data
refer to the usual heater tolerances of
ambient temperature of +20 °C (68 °F) and at the
nominal voltage and conditions.
±10% at an
15.3 Fuel for Thermo 90 ST (Diesel):
The diesel fuel specified by the vehicle manufacturer
must be used. Heating oil may also be used as long as it
complies to the normal quality available on the North
American market.
We know of no negative influences due to additives.
15.1 Electrical Components:
If fuel is extracted from the vehicle’s tank, follow the
additive instructions issued by the vehicle manufacturer.
The control module, motors for combustion air blower
and circulating pump, ceramic glow pin, metering pump,
switch and timer are designed for either 12 V or 24 V.
The temperature limiter, temperature sensor and flame
sensor are identical on 12V and 24V heaters.
If you change to low-temperature fuel, the heater must
be operated for approximately 15 minutes so that the
fuel system is filled with the new fuel.
The Thermo 90 ST heater is also licensed for use with
PME (bio-diesel), which complies with DIN EN 14214.
15.2 Fuel for Thermo 90 ST (Gasoline):
The fuel specified by the vehicle manufacturer must be
used. Both leaded and unleaded fuel may be used.
HeaterOperationThermo 90 ST GasolineThermo 90 ST Diesel
P/N 906558A - Kenworth AeroCab built after September 1996
P/N 906668 - Kenworth Studio Sleeper built after September 1996
P/N 906582 - Peterbilt with Unibilt Sleepers except model 387
P/N 906543 - Freightliner Century Class, Freightliner Classic-FLD, Kenworth Modular Sleeper Box, Kenworth T-2000,
Kenworth AeroCab and Studio Sleeper built before September 1996, Mack CL, CH, Vision, Peterbilt model 387,
Sterling - all, Volvo VN, WIA, Western Star - all.
Call your Webasto representative for information or questions regarding “Sleeper Pack” systems.