Tach Repair

Caution: The following is intended for educational purposes only. Neither the author nor TPI PARTS assume any responsibility for any injuries or damages that may arise out of following these instructions. This repair procedure was devised by an Fbody enthusiast, not an electrical engineer. If you choose to follow this repair procedure, you must do so at your own risk.

If you own a 90-92 Camaro, chances are you have a factory tachometer which reads too high. As these cars get older, they tend to read higher, and higher. The problem is caused by a faulty resistor that gradually changes resistance. The solution is rather simple and very inexpensive.

How A Tachometer Works

A tachometer works by converting pulses received from the vehicle's ignition system into needle deflection on the gauge. A pulse is generated each time a spark plug fires. On an 8 cylinder engine, there will be 4 pulses per revolution. On a 6 cylinder, there will be 3 pulses per revolution. Inside the gauge cluster, there is a small circuit board which is responsible for controlling the tachometer's needle deflection. Naturally, there are some differences between a 6 cylinder tach board and an 8 cylinder tach board. The 6 cylinder board needs to consider 3 pulses to be equal to one revolution, while the 8 cylinder board needs to consider 4 pulses to equal one revolution.

There is one resistor and one capacitor on the tach board which control the needle deflection. Basically, the resistor is responsible for the calibration, and the capacitor is responsible for how quickly the tachometer responds to change in RPM.

This resistor in question is going to be part of what is called a "thick film resistor chip", or "resistor network". This will be a 14 pin white and black chip mounted on the tachometer circuit board. This resistor network contains several other resistors necessary to operate the tach. The resistor which is important here is across pins 4 and 10. The picture below shows the resistor network on the left, and the previously mentioned capacitor on the right of the tachometer circuit board.

Grab a voltmeter, and set the scale so that you can read over 100k ohms. Check the resistance across pins 4 and 10. If you have infinite resistance (no continuity), then the fix will be easier.

Infinite Resistance:
All you will need to do is solder a resistor across pins 4 and 10. The proper resistance will be about 190k ohms for a V8 engine (More resistance makes the tach read higher, less resistance makes the tach read lower). Trim the resistor ends a bit to shorten the length, and then bend it in a U shape. Be sure not to get solder onto the adjacent pins. If you are not comfortable soldering, practice a little first before trying it on the tach board. Once you solder the resistor on the board, you can reinstall it into your gauge cluster. You might need to bend the resistor slightly to the side for adequate clearance once you go install it. Start your car, and you should notice your tach reading properly now.

Finite Resistance:
If you have resistance across pins 4 and 10, you will first need to isolate a portion of the resistor network. Using nippy cutters (available at Radio Shack), cut pins 4 and 10 very close to the circuit board. Carefully pry the pins upwards, away from the board. At this point, you can solder a resistor (about 190k ohm is the proper resistance) onto the contacts on the board which used to attach to pins 4 and 10. Be very careful not to get solder onto the adjacent pins. Also, the pins that were cut/bent out of the way must not touch the resistor that you installed. Once you soldered in your resistor, you are good to go. Install the tach board back in the car, and enjoy having a tach that actually works.

Note: The resistor values mentioned will work for a V8 application. I don't know what the proper values will be for a V6, but the fix should be the same.

You can buy resistors at your local RadioShack. Chances are you won't find a resistor with 190k ohms, so you may have to solder in more than one resistor in series or in parallel to reach the desired resistance. Soldering in resistors in series will have an additive effect on resistance. For example, a 100 ohm resistor, a 50 ohm resistor, and a 25 ohm resistor soldered in series will provide 175 ohms of resistance. The formular would be

Rtotal = R1 + R2 + R3
Rtotal = 100 + 50 + 25 = 175 ohms

Soldering them in parallel will diminish resistance. If we soldered a 100 ohm resistor, a 50 ohm resistor, and a 25 ohm resistor in parallel, we would have just over 14 ohms. The formula would be

Rtotal = 1 / ( 1 / R1 + 1 / R2 + 1 / R3)
Rtotal = 1 / (1 / 100 + 1 / 50 + 1 / 25)
Rtotal = 1/ (.01 + .02 + .04)
Rtotal = 1/.07
Rtotal = 14.28 ohms

If you want to dial in your tach as close as possible, you may want to consider soldering in a potentiometer instead of a resistor, and you can adjust the resistance by hand until its dead on accurate. You can leave it as it is at that point, or remove the potentiometer and install a permanent resistor with the potentiometer's value.