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Instrument cluster - stepper motor inside that controls gauge . could be the problem , GM instrument cluster gauges stop working all the time . Plus could be other thing's , wiring , module's etc.... Just depends where the tach gauge info. comes from . Hooking up a factory scanner an checking data to the cluster from the PCM/ECM -engine computer . The instrument cluster has a microprocessor inside an it's hooked up to the class 2 serial data buss .
Tachometer
The instrument panel cluster (IPC) displays the engine speed based on the information from the engine control module (ECM). The ECM converts the data from the engine speed sensor to a 2 pulses/engine revolution signal. The IPC uses the engine speed signal circuit (2 pulses/engine revolution) from the ECM in order to calculate the engine speed. If the IPC detects a malfunction in the engine speed signal circuit, the IPC uses the serial data message from the ECM indicating the engine speed.
You may wan't to take it to a qualified repair shop.
What tach . factory in the instrument custer , Aftermarket ? The tach signal comes from terminal E at the ignition control module and the wiring diagram for a 1988 pontiac bonneville 3.8 L is a white wire .
This is likely either due to a faulty instrument cluster, the wiring to the cluster, or the camshaft or crankshaft position sensor. Does the car run well? Do you have access to a scan tool that can monitor the datastream of the ECM to confirm if the computer is getting a tach signal?
Most vehicles power the tach from the same circuit as the other instruments. So if the speedo and fuel gauge work the tach probably has power. May not have a ground signal from the engine computer.
If it has an ignition control module there will be a "tach out" wire it is usually white in color, you can check that wire for tach signal. If no signal replace Ignition control module. If it has proper signal it is likely a bad gauge in the instrument cluster.
If both the speedometer and Tach quit working there is a high probability that the instrument cluster has checked out. The speedometer takes it's signal from the VSS (vehicle speed sensor) via the computer, and the tach comes from either the distributor or coils (tach pin) to the tachometer. Both these items would have to go out to cause your symptoms.
Tachometer failures are very common on this brand. The tach will tend to hop around first and then just stop. The most common failure for this concern is a faulty tach in the instrument cluster. This can be checked by using a scan tool to observe the instrument cluster data for the r.p.m. signal, if the scan tool shows a tach signal (RPM) signal then the instrument cluster isn't displaying this signal and is faulty and needs replaced or repaired.
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