SMITHS
SERVICE INSTRUCTIONS
IMPULSETACHOMETER
(RVI.1000 SERIES)
SMITHS
M O T O R A C C E S S O R Y D I V I S I O N
— SERVICE—
The Service Technical Department of the Motor Accessory
Division will be pleased to advise on any functional problem
relating to instruments and equipment described in this brochure.
All correspondence should be addressed to: SMITHS MOTOR
ACCESSORY DIVISION,
EXPORT SERVICE,
CRICKLEWOOD WORKS, LONDON NW2,
ENGLAND.
Enquiries will be dealt with promptly.
Printed in England by Roadcraft Press Ltd, London NW2
CONTENTS
Page
List of Illustrations S.2
List of Special Tools & Equipment S.2
General & Technical Description S.3
Testing Procedure S.4
Dismantling Instructions 3.6
Inspection & Cleaning S.6
Assembly Instructions S.6 & S.7
Calibrating Instructions S.7 - S.9
Symptoms, Faults & Treatment S.12 - S.14
S.1
LIST OF ILLUSTRATIONS
Figure Page
1. General arrangement of Impulse Tachometer S.5
2. Circuit Diagram of Impulse Tachometer S.10
3. Printed Circuit Board Diagram. S.11
4. Ignition Pulse Simulator SR/D.347 S.11
LIST OF SPECIAL TOOLS & EQUIPMENT
Description
Code No.
Speedometer Test Apparatus SR/D.221
Adaptor (for use with Apparatus SR/D.221) SR/D.312
OR
Speedometer Test Apparatus
AT.9034
Adaptor (for use with Apparatus AT.9034) SR/D.313
Ignition Pulse Simulator SR/D.347
Generator TV.1100/00
Bridge Spanner SR/D.360
Bezel Fixture SR/D.140
Bezel Spinning Tool SR/D.269
Support Pad SR/D.269/13
Pressure Pad SR/D.269/14
S.2
INPULSE TACHOMETER
GENERAL DESCRIPTION
The equipment consists of an indicator head and a pulse lead. The pulse lead, when
connected in series between the vehicle ignition switch and the
low tension terminal of the engine ignition coil will transmit voltage pulses to
the indicator head.
The tachometer system has been designed to cover a wide range of internal
combustion petrol engines using either 6V or 12V coil ignition electrical systems;
positive or negative ground. This range covers 2, 3 and 4 cylinder 2 stroke
engines and 4, 6 & 8 cylinder 4 stroke engines. Any one of these applications can
be obtained by recalibrating the tachometer. This operation is explained in the
section headed CALIBRATION INSTRUCTIONS on page S.7.
TECHNICAL DESCRIPTION .
(For Component Reference see Circuit Diagram Fig. 2 Page S.10)
The object of the circuit as shown, on Page S.10 is to provide a pulse of constant
height and width to the coil of a meter every time the engine fires. The cycle of
operations consists of a rest period followed by a pulse followed by a further
rest period.
At rest the collector-emitter voltage of transistor TR1 is very low due to the
base current flowing via resistors R8, R5 and R6. Under this condition transistor
TR2 will not be conducting, since its base is effectively shorted to earth by R1
and the conducting TR1. Capacitor Cl is charged to the zener voltage with its
right hand plate negative with respect to earth. No current flows through the
meter M.
The primary of the triggering transformer (T) is connected in series with the
primary of the engine ignition coil, so that when the contact breaker in the
engine distributor closes, the current flowing to feed the ignition coil passes
through the primary of the transformer energising the core. When the contact
breaker opens to provide a spark to the engine, the flux in the transformer core
collapses and appears as a short duration voltage pulse across the secondary of
the triggering transformer.
This pulse causes TR2 to conduct, which effectively brings the right hand plate of
Cl to earth potential so that the left hand plate is positive with respect to
earth. Cl starts discharging through R2, R8, R5 and R6 (driving the base of TR1)
positive making it non-conducting. Simultaneously, the collector voltage of TR1
switches towards the zener voltage and TR2 conducts due to the base current
flowing via R4 and Rl. During this state current flows through the meter via the
conducting TR2 and the temperature compensating circuit
thermistor Thl and R3. This state continues until the charge in Cl is unable to
maintain the positive potential on TR1 and the circuit reverts to the stable
state.
The time taken for Cl to discharge is a function of Cl, R6, R5, R8 and R2. Hence
current pulses of constant charge are applied to the meter at a frequency
depending on engine speed. The voltage applied to the circuit is stabilised by the
zener diode Dz, C3 and R7. The capacitor C2 prevents the circuit being triggered
by spurious voltages generated by auxilliary equipment or faulty contact breaker
points.
TESTING PROCEDURE
(Using Speedometer Test Apparatus SR/D.221 or AT.9034)
PREPARATION OF THE TEST EQUIPMENT
Using the appropriate adaptor SR/D.312 or SR/D.313 couple a known good
generator TV.1100/00 to the speedometer test apparatus SR/D.221 or AT.9034.
Using a suitable electrical cable connect the output terminals of the generator
to the two left hand terminals of the ignition pulse simulator SR/D.347.
(it is unimportant which way round these connections are made).
Refer to the white pulse lead on the tachometer and connect the lead with the red marker
to the red terminal on the simulator and connect the other lead with the black marker
to the black terminal on the simulator. (It is important to connect these leads in the
correct order).
Connect a well stabilised D.C. supply of 12 volts to the two right hand supply
terminals on the simulator marked + and -. It is imperative that the correct polarity
be observed while making this connection. Wrong connections will result in the
transistors used in the simulator being instantly destroyed.
Adjustment of the terminal locking nuts may also result in damage to the simulator.
Connect the tachometer to a 14.5 volt. D.C. supply, so that it is wired
for a negative earth. It is important to use 14.5 volts. Voltages less than this
figure can result in errors of up to 400 r.p.m. being indicated on the tachometer.
TESTING
Switch ON the supply to the ignition pulse simulator and the impulse tachometer. Wait
for two minutes and allow the units to warm up. Start the speedometer test apparatus
and operate the tachometer throughout its range.
Check the calibration by running the speedometer test apparatus at 3000 r.p.m., the
tachometer should read as follows:
6000 r.p.m. when calibrated for 4 cylinder 4 stroke application
4000 r.p.m. “ “ 6
3000 r.p.m. “ “ 8
If the tachometer is found to be out of calibration it should be recalibrated as
described on page S.7.
It should be noted that for all service purposes 2 cylinder 2 stroke application
can be considered as being the same as 4 cylinder 4 stroke and 3 cylinder 2 stroke
as 6 cylinder 4 stroke.
Set the tachometer to read 3000 r.p.m. and then switch the ignition pulse simulator
OFF and ON. When the simulator is switched OFF check that the tachometer reading
falls to zero and when switched ON the tachometer reads 3000 r.p.m. Finally, operate
the tachometer throughout its range.
During the complete testing procedure it is most important to observe the behaviour
of the pointer. The tachometer, when satisfactory, should read with little or no
fluctuation between 500 r.p.m. and maximum; in this range there should be no tendency
for the pointer to drop to zero.
If any unusual movement or fluctuation is observed, the tachometer should be regarded
as suspect and in need of more detailed inspection.
S.4
S.5
DISMANTLING INSTRUCTIONS
Numbers in the parentheses refer to Fig. 1 on Page S.5.
Remove the pulse lead (34) complete with plastic former (29) and iron
core (30). Remove the fixing strap (47), the terminal insulator (46) and
the rubber grommet (33). Remove the nuts and washers securing the three
point terminal block (39) and lift the block clear. The external rubber ring
(28) can then be removed. Using pliers or other suitable tool remove and
discard the bezel (1) taking care to avoid breaking the glass (2). It is
now possible to remove the glass, sealing washer (3) and slip ring (4).
Remove the pointer (5) using suitable pointer lifters which will not damage
the dial (7). Supporting the dial and movement assembly (10) unscrew the
movement fixing screws (38). Lift clear the complete movement assembly and
unscrew the base plate to movement screws (26) and remove the magnet (11).
If necessary the foam washer (14) can be detached from the pole piece (13).
The remainder of the components can be removed as required.
INSPECTION AND CLEANING
All parts of the tachometer should be checked for wear or damage and
be renewed as necessary. All screws should be checked for tightness. The
movement assembly (10), which is available only as a sub-assembly, should
be inspected with care. It is important to check that the hairspring tails
are securely soldered to the tags.
All soldered connections throughout the tachometer should be inspected
for dry joints. If it is necessary to remove components from the printed
circuit board the minimum of heat should be used. Excessive temperature will
cause distortion of the board and damage the electronic components. Finally,
prior to assembly, all parts must be carefully cleaned and any foreign matter
removed using a fine paint brush where necessary.
ASSEMBLY INSTRUCTIONS
ASSEMBLY OF MOVEMENT
Holding the movement by the two side members replace the magnet with
its south pole facing away from the moving coil. Correct assembly of the
magnet, from the polarity point of view, is most important to ensure
satisfactory operation of the tachometer and for this reason the south pole
faces of all magnets are marked on the factory production line with yellow
crayon. If the crayon mark has become erased the south pole of the magnet
can be determined by means of a separate magnet of known polarity.
Alternatively the polarity of the magnet can be determined by using a compass.
Having correctly located the magnet, replace the pole piece (13) and
fit the pole bridge piece (12). It will be observed that the pole bridge
piece is rectangular in shape and if incorrectly fitted will overlap the
front pole piece by 3/16" (4.7mm).
ASSEMBLY OF THE MOVEMENT TO THE BASE BOARD
In a central position secure the foam washer to the rear pole piece,
using Paripan Locking Varnish or a suitable equivalent. Using the base
board to movement fixing screws, dipped in Paripan Locking Varnish or a
suitable equivalent, secure the base board to the movement. Should any
wires between the base board and movement assembly have been removed
during repair or dismantling re-solder them to the correct terminals.
S.
6
RE POISING THE MOVEMENT
Replace the dial and pointer, locking the dial screws with Paripan Locking
Varnish or a suitable equivalent. Fit the iron core, former and pulse lead
and then proceed to poise the movement and set the basic calibration as
described below.
Connect the impulse tachometer to the ignition pulse simulator and
speedometer test apparatus as described in the Testing Procedure on page S.4.
Switch on the supply to all these units and set the tachometer so that the
pointer rests at the lower calibration mark at the 1750 r.p.m. position on the
dial.
Check the poising by holding the tachometer in a vertical plane and rotating
it at 90°intervals, observing the position of the pointer after each turn.
In all four. positions the pointer should be within the limits of the lower
calibration mark. If the pointer does not lie within the
mark the movement is unbalanced and should be re-poised in the following
manner.
On the movement balance arm is fitted a moveable brass counter weight and
at 90 to the counter weight are two extension arms on which is wired two
small moveable coils. With the tachometer reading oppositethe lower
calibration mark proceed to check the tachometer at 90 intervals
in the vertical plane. If the pointer lies to the right or left of the
calibration mark it is possible by moving the counter weight and/or a
coil to bring the pointer into line. Care should be taken to avoid causing
a short circuit as the adjustment is made. This procedure should be carried
out until the pointer shows correct balance in each of the four positions.
If it is found that correct poising cannot be obtained the magnetic circuit
of the tachometer should be carefully checked for correct assembly, and if
further difficulty is experienced the magnet should be changed.
S.7
BASIC CALIBRATION
This calibration is necessary when any component affecting the
electric or magnetic circuits of the impulse tachometer is altered in any
way.
The figures quoted in the following instructions will only apply to
instruments which have a range of 0-8,000 rpm. For instruments having
alternative ranges, eg. 0-6,000 rpm or 0-10,000 rpm the values of the
calibration points on the dial should be noted and the speed of the test
apparatus calculated by using the following ratios:-
Test Apparatus Tachometer
4
cylinder engine 1 2
6
cylinder engine
3 4
8 cylinder engine 1 1
Calibration of the impulse tachometer must be carried out with the
movement mounted in the instrument case. It is therefore necessary to use a
dummy case with a section cut from the side to allow the bridge spanner
SR/D.360 to be fitted to the anchor plate.
4
CYLINDER4STROKE
Connect the impulse tachometer to the ignition pulse simulator and
speedometer test apparatus as previously described in the Testing Procedure
on Page
4.
Switch on the supply to these units and run the speedometer test
apparatus at 1,500 rpm. Using the potentiometer "L" (See Fig.l. on Page S.5)
set the impulse tachometer to read 3,000 rpm and wait for two minutes to
allow the equipment to stabilise. Reduce the r.p.m. of the speedometer test
apparatus
to 875 rpm and check that the impulse tachometer reads opposite the low
calibration mark 1,750 rpm. If the impulse tachometer does not read 1,750 rpm
fit the bridge spanner SR/D.360 to the hairspring anchor plate and rotate
the plate in the desired direction until the pointer is in line with the
calibration
mark. Care should be take to avoid causing an electrical short circuit during
this adjustment.
After the impulse tachometer has been checked at 1,750 rpm the
speedometer test apparatus should be set at
3,475
rpm and the impulse
tachometer should now read 6,950 rpm. If any adjustment is necessary at this
point it can be made
by adjusting the potentiometer "L" (See Fig 1 on Page S.5.) in the
required direction, but, any adjustment of the potentiometer will make
it necessary to re-check the calibration at the 1,750 rpm position. This
cross checking
should be carried out until accurate calibration is obtained at both positions.
Finally, set the speedometer test apparatus to read 2,300 rpm and check
the mid-calibration point on the impulse tachometer. Switch off the supply
and remove the iron core, former and pulse lead.
The tachometer is now calibrated for4cylinder4stroke application.
S.8
6 CYLINDER 4 STROKE
First calibrate the instrument for4cylinder4stroke
application to ensure that the movement is poised correctly
and that the pointer reads opposite each
of the calibration marks.
Set the speedometer test apparatus to run at 3,000
rpm and the impulse tachometer should be reading 6,000
rpm. Adjust the potentiometer "L" (See
Fig. 1 on Page S.5) to reduce the speed of the impulse
tachometer until it reads 4,000 rpm and then switch off the
supply and remove the iron core, former and pulse lead.
The impulse tachometer is now calibrated for6cylinder
4
stroke application.
8 CYLINDER 4 STROKE
First calibrate the instruments for4cylinder4stroke
application to ensure that the movement is poised correctly and
that the pointer reads opposite each
of the calibration marks.
Set the speedometer test apparatus to run at 3,000
rpm and the impulse tachometer should be reading 6,000 rpm.
Adjust the potentiometer "L" (See Fig.
1 on Page S.5) to reduce the speed of the impulse
tachometer until it reads 3,000 rpm and then switch off
the supply and remove the iron core, former
and pulse lead.
The tachometer is now calibrated for 8 cylinder4stroke
application.
FINAL ASSEMBLY
Carefully remove all dust particles from the
assembled movement and base board, then place this sub
assembly into the case securing it in position with the
shakeproof washers and screws. Replace the three point
terminal block, terminal insulator and rubber grommet.
Carefully clean the dial, replace the slip ring and
sealing washer and then fit a clean dial glass. Over the
glass place a replacement bezel and spin in the manner
described below.
S.
9
INSTRUCTIONS FOR SPINNING THE BEZEL
Set up the bezel fixture SR/D.140 on a bench with the steel plate and rubber
pad to suit the tachometer; fitted in the ratchet table. Slacken the centre
clamp screw and fit the wooden block over the back of the instrument. Place
the instrument face down on the rubber pad, ensuring that the V bezel is seating
down in the groove. The centre clamp screw can now be tightened down, so that
the instrument is gripped firmly.
Set the ratchet to turn the table clockwise and loosely assemble the spinning
attachment to the left hand pillar.
Slacken the feed screw right back and adjust the height of the spinning
attachment, the wheel should be just above the level of the case flange.
Clamp the spinning attachment tightly to the support, so that the wheel is
almost touching the bezel.
Turn the table by means of the ratchet arm, slowly tightening the feed screw at
the same time. This should be continued until the bezel has been spun over,
if necessary adjusting the height of the spinning attachment.
Slacken the feed screw and centre clamp screw, then remove the instrument. After
spinning proceed to complete the assembly of the tachometer.
FITTING THE PULSE LEAD
Particular care should be taken when fitting the pulse lead assembly. It is
essential that a good electrical connection be obtained at the iron core. Poor
connection is liable to result in intermittent operation of the tachometer,
particularly during high speed motoring when the tachometer is subject to
increased vibration.
Fig.2 Circuit Diagram of Impulse Tachometer
S.10
R. Resistor
C. Capacitor
TR. Transistor
Th. Thermistor
Dz. Zener Diode
M. Meter
a-e Reference Points
Fig.3 Printed Circuit Board Diagram
Fig.4 Ignition Pulse Simulator
SR/D.347
S.11
SYMPTOMS, FAULTS AND TREATMENT
It is possible by using these instructions to eliminate any
single defective component in the Tachometer. To assist this a
schematic circuit diagram showing points letter A - H is shown.
In addition to the special recalibrating equipment a good
quality voltmeter suitable for reading voltage and resistance
values will be necessary to locate most faults. When taking
voltage checks, always take them with respect to the positive
line.
SYMPTOM FAULTS TREATMENT
Incorrect
Reading
Poor Connections
Incorrect Calibration
Out of Poise
Pointer Sticking
Movement Assembly
C2 Faulty
Check all connections for dry joints
and also the pulse lead and iron
core assembly.
Check the calibration of the
Tachometer as described under
calibration instructions on Page
S.7.
Check poising as described on Page
S.7.
Check positioning of pointer on
movement spindle and refit if
necessary. Poise and recalibrate.
Remove suspect movement and replace
with a good one and check the
Tachometer. Poise and recalibrate.
If C2 has a low capacitance replace
component.
No Reading Poor Connections
Pointer Loose
Component defective
R9, C3, DZ,
R5, R89R7,
R6, TR1, TR2,
R3, R2, Cl,
C2.
Check all connections and in
particular the pulse lead and iron
core assembly.
Refit the pointer. Poise and
recalibrate the movement.
Switch on the supply to the
Tachometer but do not feed in a
pulse. Check the voltage across DZ
and if it is between 4.7V
and 5.1V the meter circuit is
satisfactory. If voltage is more than
5.1V replace DZ and check the
Tachometer again. If the voltage is
less than 4.7V check the rest of the
components.
S.12
SYMPTOM FAULTS TREATMENT
No Reading
(contd.)
Resistor R9
Capacitor C3
Diode Dz
R5 and R8
C1
C2
R2
R7
R6
Component defective M,
Th1, R3, TR1, TR2
Meter M
With the supply OFF check resistance
value -6v and -12v terminals. This
should be 82 ohms.
Disconnect one side of C3 and switch
the supply ON. If the voltage across
Dz is between 4.7V and 5.1V, C3 is
faulty. If the voltage is still less
than 4.7V, reconnect and test Dz.
Disconnect one side of Dz and switch
the supply ON. Measure the voltage
across C3. If it is more than 4.7V
change Dz, if not proceed to test
R5, R8, C1, R2, R7 and R6.
With the supply OFF check the
resistance between A & G. On
adjusting R8 the resistance should
vary between 6K and 22K. The value
of R5 should read 5K.
With the supply ON check the voltage
across C1. This should be at zener
potential approximately.
Check for short circuit. Replace if
necessary.
With the supply OFF check the
resistance between A and the left
hand of C1. This should be 1320 ohms
– 1080 ohms.
With the supply OFF check R7 between
H & G. Resistance should be 3.3
ohms.
With the supply OFF check R6 between
G & 6v terminal. The resistance
should be 18 ohms.
With the supply ON and D shorted to
earth. The Tachometer should read
8000 r.p.m. If it does the
components are satisfactory, if not
proceed as follows:
With the supply ON, short point C to
earth. If the meter reads, it is
satisfactory and the fault lies in
R3 and it should be replaced.
S.13
SYMPTOM FAULTS TREATMENT
Permanent reading
on tachometer without a pulse being
fed in.
Transistors TR1 & TR2
Triggering Coil T
Resistor R1
TR1, R4, R5, R6 and R8
R4
R8 & R5
R6
TR2
With the supply ON short point D to
earth and check that the meter reads.
The meter should not read when point A
is shorted to earth. When A is
earthed measure the voltage at E. If
it is less than 1.5V change T11, if
more than 1.5V change TR2.
Measure resistance of the coil
between + terminal and B. The
correct value is 120 ohms. If
Correct proceed to Rl.
With the supply OFF check the
resistance between E & B. Should be
between 517 ohms and 423 ohms.
With the supply ON measure the
voltage at E. If the voltage is
less than 0.2 volts proceed to
check TR2. If it is greater
than 0.2V the fault lies in TR1, R4,
R5, R6 or R8.
With the supply OFF check the
resistance between E & -6V terminal.
This should be 750 -900 ohms.
With the supply OFF check the
resistance between A & G. On
adjusting the potentiometer the
resistance should vary between 6K and
22K. The value of R5 should. read
5.1K.
With the supply OFF check resistance
between G & -6V terminal. This
should be between 16 ohms and 20
ohms. If R4, R5, R6 and R8 are
satisfactory change TR1.
With the supply ON short B to earth. If
the meter still reads,
replace TR2 and then check that the
meter reading falls to zero. If
after shorting B to earth the meter
does not fall to zero, recheck TR1,
R4, R8, R5 and R6.
S.14
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