c4tonezone

Hello All,

I have a 90 c4 coupe that was running perfect until yesterday (Thanksgiving day) when I took her on the fwy for a cruise. By accident, I hit one of those famous California pot holes on the freeway and felt a violent jolt. At that point I noticed my tachometer had gone nuts showing that I was red lining even though I was only driving 65mph. As i exited the freeway and coming to a full stop, my tach need was pressing over 4000 rpm even though the engine was at a perfect idle at the stop.

Has any one experienced this and if so any solution or pointers on where to start looking would be much appreciated.

Thanks all in advance...c4tonezone

Cliff Harris

This thread has info on the tach circuit:

https://www.corvetteforum.com/forums...-problems.html

c4tonezone

Thanks for the heads up and links. After reading all of them, I see that there could be a number of ways to approach this tach problem. I'm leaning toward a short or ground in the distributor.
I had a chance to take a peak under the hood and all looked normal. I attached a error coder reader and got a #42 error code. Since these error readers are not supposed to provide a accurate reading, I'm going with the old fashion way of testing which includes a jump wire and some grounding . Turning the key to the on position to access the diagnostics, I noticed that my secondary cooling fans come on now, where they triggered to turn on after the motor had reached a certain temperature.
So my day will be filled with lots of electrical testing and trying to run this thing down. Manual suggest the Ignition Module inside the distributor is the likely suspect, so once I have cleared the errors, I will start there. Hopefully with any luck I'll be back on the road soon with a fully functional tach . And if that happens, I'm staying away from those confounded pot holes. Any and all are invited to chime in if with any suggestions or any helpful /positive solutions. And I thank you for any info that maybe coming my way.

WVZR-1

If your car has C68/electronic AC you can check RPM using it OR use a hand held device/scanner to check. If either of those check OK your problem is the cluster as mentioned. Your chase will be "shortened" if you check the RPM doing that and if they seem correct you'll save much time and money.

I doubt you were victimized by the pot hole. When the key is turned to RUN but NOT started where does the needle park? Where does it park with KEY OFF?

c4tonezone

Thank you for the suggestions. With the key in the "ON" position (engine off) the needle rises slightly but still below zero mark on the tach.
With the key in the "OFF" position, the needle drops all the way down in the rest position.

The RPM when the AC is on reads 3500 rpm - which is high.
When I shut the AC of , the RPM dipped slightly but then back to 3500 .

This all started (Thanksgiving Day). The following day I noticed with the key in the "ON" position - engine had not been started, my secondary cooling fans went on and remained on.
Today when getting my error code (42) with the key in the "ON" position - engine off, the secondary fans were still on. As per the diagnostic procedures from the Chevy manual, I cut the power from the ECM to erase the error codes. I had to move my car so I started the car and no secondary fans. Shut the car off, rechecked the error code (key on- engine off) and the fans were back on.

Maybe a short? Thank you...