Tach Stuck

[Ken C.]

The other day I attempted to fire up my 79 Coupe only find the battery dead. When I tried to fire it up the tach jumped to the far right and stuck there. Even with a new battery the tack is still sitting pegged to full RPM. Has anyone experienced a similiar problem and if so can it be corrected without replacing the entire unit?

Ken

[Greg A.]

Re: Tach Stuck

It's been a while, but I recall a similar problem in my '78. Check the tach filter. I don't believe it was too expensive to replace.

[Tom R.]

Re: Tach Stuck

The other day I attempted to fire up my 79 Coupe only find the battery dead. When I tried to fire it up the tach jumped to the far right and stuck there. Even with a new battery the tack is still sitting pegged to full RPM. Has anyone experienced a similiar problem and if so can it be corrected without replacing the entire unit?

The late models had a tachometer circuit board that most of the times is the culprit. Figure its 30 years old, the copper pathways break, components get old. The fix is to replace the circuit board which is not a bad project on the late models (78-82). I'm including a link to an article that was published in Corvette Enthusiast about a year ago. I've replaced mine in a 78 so familiar with the routine if you have additional questions.

http://www.hunt4cleanair.net/Articles/tachboards.pdf

[Ken C.]

Re: Tach Stuck

It's been a while, but I recall a similar problem in my '78. Check the tach filter. I don't believe it was too expensive to replace.

Greg, I was not aware that the tach's had a filter. I have gone through my manuals and am unable to find any mention of a filter. Can you let me know where it is located and how difficult it is to get to.

Ken

[Greg A.]

Re: Tach Stuck

Ken, follow the wire out of the distributor and you will find the tach filter. Tom may be correct that it is the circuit board. It was soon after I got my '78 11 years ago that the tach quit. I can't recall if it was pegged right like yours or on 0, but the fix was the filter, not the board.

However, I found a good troubleshooting plan in this old thread on the CF: http://forums.corvetteforum.com/c3-t...automatic.html. It describes how to determine if filter or circuit board.

I'm not as knowledgeable on this subject as others here, so you may get some better advise along the way, but it's a start...good luck!

[Ken C.]

Re: Tach Stuck

Hi Greg and Tom,

Thanks to the both of you for the information and leads. Hopefully it is just the filter which is were I will start. If that does'nt fix the problem then on to the circuit board. Tom where would be a good source for a replacement board?

Thanks again guys for your help.

Ken

[Ken C.]

Re: Tach Stuck

Yesterday I checked all tach leads and found a loose wire at the module on the distributor. I also followed the main lead up to the back of the tach and found the filter, which I just jiggled a bit and then countinued on to the back of the tach. Everything appeared to be okay. Fired the car up but the tach was still stuck at 7000 rpm. Later in the day while driving the car to my service tech the tach all of sudden started to bounce around and settled at 2500 rpm which seems to match the speed I was traveling. When I reached my destination and turned the ignition off the tach stopped at about 1500 rpm (is that normal?). I then switched the iginition to the on position and the tach dropped back to 0. I am purplexed on what is happening . Anyone out there with any idea as to what happened. Tom, I think I am going to order a new circuit board just to be on the safe side. Any suggestion as to the best place to source one. I don't want one of those China boards.

Ken

[Greg A.]

Re: Tach Stuck

Ken, the shutdown operation seems normal. The tach will stay at the idle rpm when you turn off the ignition and then will drop back to 0 when you restart, before the engine engages.

Tom should have a better idea if it's the board or the filter. Good luck with the project.