Murphy MT90 User Manual

MT-9004B page 1 of 2

MT90 Model

■

±1 RPM Accuracy

■

Easy Calibration

■

Clear-Read LCD Display

■

Back Light for Night Viewing

(Battery Powered models)

■

Input Source Can Be a Magnetic Pickup
or Engine Alternator

■

Power Supplied by Magnetic Pickup
or 12, 24, or 32 VDC Battery System

Description

The Murphy SELECTRONIC®MT90 is a digital
tachometer. Its high accuracy and dependability
result from use of a quartz crystal time based and
digital, solid-state electronics.

Tachometer power is supplied by either a Murphy
magnetic pickup, mounted at the flywheel ring­gear of an engine, or by a 12, 24 or 32 volt DC
battery system.

RPM data is supplied by either a Murphy magnet­ic pickup or by the alternator in your battery
charging circuit. The MT90 also has back-lighting
for easy readings in low lit areas; this lighting
requires a battery power source.

RPM Calibration

The MT90 is calibrated to engine RPM by setting a
series of calibration rotary switches on the back of
the tachometer. The proper switch sequence for the
engine is determined by (1) the number of ring gear
teeth for the magnetic pickup, or (2) by the ratio of
alternator to engine pulley diameter, and the num­ber of poles of the alternator.

Applications

Typical applications include: Generators,
Compressors, Industrial Engines, Oil Field
Equipment, Marine Vessels, Vehicles, Farm
Equipment, and Construction Equipment.

Specifications

Signal Input Voltage: 4 to 35 Vrms from a mag­netic pickup or alternator

**

.

Pulses per Revolution: 3 to 999.

Power Requirements:

• Pickup Power: 4-35Vrms

**

.

• Battery Power: 8-40 VDC (12, 24, 32 volt).

Current:

• Tach back-light Off, 4mA @ 40 VDC.

• Tach back-light On, 25 mA @ 40 VDC.

Case: 1018 polycarbonate/polyester blend.

Lens: Polycarbonate.

Bezel: #430 Stainless Steel.

Display: LCD, 4-digit, seven segment.

Operating Temperature: -4 to 158°F

(-20 to 70°C).

Storage Temperature: -13 to 185°F (-25 to 85°C).

Mounting Hole: 3-7/16 in. (87 mm).

Shipping Weight: 14 oz. (0.4 kg).

Shipping Dimensions: 5-1/2 x 5-1/2 x 5-1/2 in.

(140 x 140 x 140 mm)

Warranty

A limited warranty on materials and workmanship is
given with this FW Murphy product. A copy of the
warranty may be viewed or printed by going to
www.fwmurphy.com/support/warranty.htm

*

Products covered by this bulletin comply with
EMC Council directive 89/336/EEC regarding
electro-magnetic compatibility except as noted.

**

See “Special Note” for Magnetic Pickup

Powered MT90 applications (back page).

Dimensions

Side View

Back View

MT-9004B

Revised 03-06

Catalog Section 20

(00-02-0068)

Digital Tachometer by Murphy
SELECTRONIC

®

Calibration
Rotary Switches
(located under
removable
rubber seal)

Mounting

Clamp

Terminal

Screws

0

0

0

1

1

1

9

9

9

2

2

2

8

8

8

3

3

3

7

7

7

4

4

4

6

6

6

5

5

5

X 1000 X 100 X 10

3-5/8 in

(92 mm)

3-1/8 in

(80 mm)

MT-9004B page 2 of 2

Magnetic Pickup Powered MT90

(No back-lighting)

Battery Powered MT90

(With back-lighting)

MP3298

Terminal 2: connects to battery (-) or ground.
Terminal 3: connects the back-light to battery (+), (back-light can only be used

when powering from battery).
Terminal 4: connects to battery (+) or power from magnetic pickup or alterna-

tor.

Terminal 5: RPM input signal from magnetic pickup.
Terminal 6: RPM input signal from alternator.

Typical Wiring Diagrams for MT90

Magnetic Pickups

Calibration

To calibrate the MT90 for your engine, remove the rubber seal on the
back of the tachometer and dial in the correct number.

• Magnetic Pickup: dial in the number of teeth on your ring gear.

Set the switches from left to right. For example, if the engine gear
has 125 teeth, dial a 1 on the left switch, a 2 on the center switch,
and a 5 on the right switch. Using this setting, the MT90 will
count the passing teeth and convert them into engine RPM.

• Alternator: Multiply the ratio of alternator to engine pulley dia-

meter, times, the number of poles of the alternator divided by two,
to determine the correct calibration number (also see “Other
Calibration Methods”).

Other Calibration Methods (teeth or ratio unknown)
For setting the calibration switches when the number of pulses per
revolution are not known, set the rotary switches on back of the MT90
to 0-6-0 and read the tachometer at a known engine RPM. This can be
read by a hand-held tachometer or any means which tells actual RPM
The reading is the input frequency in hertz. Multiply the frequency
times 60 and divide the result by the engine RPM. Set this number into
the rotary calibration switches. NOTE: If pulses per revolution are not
a whole number, for example: 21.5, a setting of 021 will read slightly
high and a setting of 022 will read slightly low.

To order a MT90 for your application, use the diagram below.

MT90 – ___ – ___

How to Order

Bezel

Blank = Bright stainless
B = Black stainless

Face Plate

1 = White background

To order a magnetic pickup, specify model number.
Example: MP3298

Pickup Models Total Length Threaded Length Thread Size

MP3298* 3 in. (76 mm) 3 in. (76 mm) 5/8-18 UNF
MP7906

†

3 in. (76 mm) 3 in. (76 mm) 3/4-16 UNF

MP7905

††

4-1/2 in. (114 mm) 4-1/2 in. (114 mm) 3/4-16 UNF

*

Replaces 20-01-0080 and MP100. Lead wire hookup (12 in. [305 mm]).

†

Replaces 20-01-0081. Lead wire hookup (12 in. [305 mm]).

††

Replaces 20-01-0082. Lead wire hookup (12 in. [305 mm]).

In order to consistently bring you the highest quality, full featured products, we reserve the right to change our specifications and designs at any time.

Printed in U.S.A. 0290998

In order to consistently bring you the highest quality, full featured products, we reserve the right to change our
specifications and designs at any time. MURPHY, the Murphy logo, and Selectronic®are registered and/or com­mon law trademarks of Murphy Industries, Inc. This document, including textual matter and illustrations, is copy­right protected by Murphy Industries, Inc., with all rights reserved. (c) 2006 Murphy Industries, Inc.

www.fwmurphy.com

918.317.4100 Email: [e-mail]

MT90

with

customer

wiring

0

9

8

0

0

0

1

1

1

9

9

9

2

2

2

8

8

8

3

3

3

7

7

7

4

4

4

6

6

6

5

5

5

X 1000 X 100 X 10

2

3

4

6

5

7

6

100's 10's 1's

When calibrating for use with a magnetic
pickup, dial in the number of flywheel teeth
or pulses per revolution: for example, if the
flywheel has 125 teeth then dial in125.
See Calibration for alternator installation.

0

1

5

1

9

2

8

3

7

4

6

5

0

1

9

2

3

4

2

8

3

7

4

6

5

Magnetic

Pickup

Locknut

Hole Drilled and

Tapped, Bolt Thread

NOTE: Gap from face of gear tooth must be enough for gear to move. Rotate gear completely

to be sure of minimum, no-touch clearance. See instructions supplied with the magnetic sensors.
To get minimum of 4 VAC RMS, gap tolerance is critical. Turn the pickup in until it stops against
the face of a gear tooth. Back the pickup out only enough to allow rotation of the gear. Rotate the
gear, if any tooth touches the pickup, back it out to clear the tooth. After clear rotation secure the
pickup locking nut.

MT90

with

customer

wiring

Battery

12V-24V-32V

Back-light

ON/OFF

0

0

0

1

1

1

9

9

9

2

2

2

8

8

8

3

3

3

7

7

7

4

4

4

6

6

6

5

5

5

X 1000 X 100 X 10

2

3

4

6

5

Magnetic Pickup

0

9

8

7

6

100's 10's 1's

When calibrating for use with a magnetic
pickup, dial in the number of flywheel teeth
or pulses per revolution: for example, if the
flywheel has 125 teeth then dial in125.
See Installation and Calibration for alternator
installation.

Tachometer

Alternator

1

2

3

4

5

Terminal

0

9

8

7

6

*

0

1

5

1

9

2

3

4

8

7

4

6

5

NOTE:

*

Alternator
must be
equipped
with a Tach.
terminal
(auxiliary
AC terminal).

2

3

Locknut

Hole Drilled and

NOTE: Gap from face of gear tooth must be enough for gear to move. Rotate gear completely

to be sure of minimum, no-touch clearance. See instructions supplied with magnetic sensor.

Tapped, Bolt Thread