Speedometer and Tach
Thread Starter
Senior Member
Joined: Sep 2000
Posts: 700
Likes: 32
From: McDonough, GA
Car: 1991 Camaro RS
Engine: 305 TBI
Transmission: 700R4
Axle/Gears: 3.42
Speedometer and Tach
So after nine years I have gotten my 1991 RS running again. She had shorted out fusable link which we think shorted the fuel pump, distributor, ignition coil, and ECM. All have been replace with new pieces. She is alive and running but the speedometer is not working at all and the tach reads 2,000 rpm at idle and the motor is nowhere near that engine speed. It's closer to 1,250. Looking for ideas that may be causing each problem. Any insight is appreciated.
Supreme Member
Joined: Jan 2006
Posts: 3,341
Likes: 10
From: Mooresville NC
Car: LOWERED ♦ CRIMSON METALFLAKE
Engine: ► 400 KUBES ◄
Transmission: 765R4
Axle/Gears: EATON POSI 4.56
A Tach The Problem
There is circuitry in the dash near the tach.
It’s contained on a small plug-in PCB or integrated in the main board.
On or near the PCB you’ll see a capacitor, replace it.
Also on the PCB you’ll see a resistor chip.
Desolder pins 4 & 10.
Use a 500K potentiometer.
Solder it across traces 4 & 10.
Move the potentiometer to half way.
Start the engine and compare the tach reading with a test equipment quality tach.
Dial it in, unsolder the potentiometer, measure it and install a matching resistor.
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SPEEDO:
Gain access to the buffer located behind the instrument panel at the right most VINS.
Connect an audio amplifier to terminal A yellow wire to make sure it’s getting a steady signal.
Ground to ground, the signal wire to terminal A yellow wire of the VSS.
With the car on jackstands listen for a hum from the amplifier when the rear wheels are rotating.
If you do, then find out if you get a hum from terminal C tan wire.
The tan wire then goes to the speedometer unit.
If it gets to the speedometer but the speedometer does not work, have it repaired or replace the unit.
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Happy Racing !
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If You Think You Need Brighter Headlights ▬ You Have A Vision Problem.
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