No Tach

[jlwelding]

1st start up of the morning and no tach. 4-5 min. warm up and it starts working again. Do I have a problem in the making?

[W-T]

TACHOMETER
DESCRIPTION
A tachometer is standard equipment on all instrument
clusters. The tachometer is located just to the
left of the speedometer near the center of the instru-
8J - 30 INSTRUMENT CLUSTER BR/BE
SHIFT INDICATOR (TRANSFER CASE) (Continued)
ment cluster. The tachometer consists of a movable
gauge needle or pointer controlled by the instrument
cluster circuitry and a fixed 210 degree scale on the
gauge dial face that reads left-to-right either from 0
to 6 for gasoline engines, or from 0 to 4 for diesel
engines. The text “RPM X 1000” imprinted on the
cluster overlay directly below the hub of the tachometer
needle identifies that each number on the
tachometer scale is to be multiplied times 1000 rpm.
The gauge scale of the gasoline engine tachometer is
red lined at 5000 rpm, while the diesel engine
tachometer is red lined at 3375 rpm. The diesel
engine tachometer also includes text that specifies
“DIESEL FUEL ONLY” located just above the hub of
the tachometer needle. The tachometer graphics are
white and red against a black field, making them
clearly visible within the instrument cluster in daylight.
When illuminated from behind by the panel
lamps dimmer controlled cluster illumination lighting
with the exterior lamps turned On, the white graphics
appear blue-green, while the red graphics still
appear red. The orange gauge needle is internally
illuminated. Gauge illumination is provided by
replaceable incandescent bulb and bulb holder units
located on the instrument cluster electronic circuit
board. The tachometer is serviced as a unit with the
instrument cluster.
OPERATION
The tachometer gives an indication to the vehicle
operator of the engine speed. This gauge is controlled
by the instrument cluster circuit board based upon
the cluster programming and electronic messages
received by the cluster from the Powertrain Control
Module (PCM) over the Chrysler Collision Detection
(CCD) data bus. The tachometer is an air core magnetic
unit that receives battery current on the instrument
cluster electronic circuit board through the
fused ignition switch output (st-run) circuit whenever
the ignition switch is in the On or Start positions.
The cluster is programmed to move the gauge needle
back to the low end of the scale after the ignition
switch is turned to the Off position. The instrument
cluster circuitry controls the gauge needle position
and provides the following features:
² Message Failure - If the cluster fails to receive
an engine speed message, it will hold the gauge needle
at the last indication for about four seconds, or
until the ignition switch is turned to the Off position,
whichever occurs first. If a new engine speed message
is not received after about four seconds, the
gauge needle will return to the far left (low) end of
the scale.
² Actuator Test - Each time the cluster is put
through the actuator test, the gauge needle will be
swept to several calibration points on the gauge scale
in a prescribed sequence in order to confirm the functionality
of the gauge and the cluster control circuitry.
The PCM continually monitors the crankshaft position
sensor to determine the engine speed, then
sends the proper engine speed messages to the
instrument cluster. For further diagnosis of the
tachometer or the instrument cluster circuitry that
controls the gauge, (Refer to 8 - ELECTRICAL/INSTRUMENT
CLUSTER - DIAGNOSIS AND TESTING).
For proper diagnosis of the crankshaft position
sensor, the PCM, the CCD data bus, or the message
inputs to the instrument cluster that control the
tachometer, a DRBIIIt scan tool is required. Refer to
the appropriate diagnostic information.

 

This information is from the FSM section 8J - 31 Instrument Cluster TACHOMETER.

 

From your description and the conditions being momentarily non-functional, I'm going to guess this is an intermittent error.

 

I'd suspect dirty contacts at connection point(s) to begin with. Remove the multi-pin connectors at the PCM fluid flush the connectors with WD-40 and purge with compressed air. WD-40 works very well as a cleaner.

 

This system is entirely electronic in design and 20 plus years of "under the hood" conditions along with lead-acid batteries fuming sulfuric acid gasses can lead to contamination of low current sensing circuits.

 

I'd be interested in knowing what you discover?

 

W-T

 

 

[Mopar1973Man]

On 1/3/2023 at 12:49 PM, jlwelding said:

1st start up of the morning and no tach. 4-5 min. warm up and it starts working again. Do I have a problem in the making?

More like a bad cam sensor it's about a 15 minute job to change. I just changed mine last for the first time for lack of tach signal. Be aware if the tach is dead then the alternator is too.