The tach on my 82 does not work. I've replaced the circuit board and the tach filter. What else usually goes bad on these that I can look in to? Thanks.

 

Any thoughts from anyone? I'd really like to get this fixed. Thanks.

 

There is a cluster circuit board back there too. What where the symptoms before and after you replaced the tach board?

 

The symptoms were no movement. It was stuck at 2K. The needle moves freely and the contacts on the tach side are good. It is possible that the contacts on the cluster side are not making good contact or whatever plugs in the back of the cluster has issues. I'm also not ruling out that the tach itself could be bad. I'm just looking for suggestions before I arbitrarily start throwing money at parts. Thanks.

 

Make sure that you have power and ground to the tach board. The tach needle should go to 0 when power is applied and it just stops wherever it happens to be when you shut the key off. You need to re-set the needle anyway after replacing the board.

If the needle moves freely I'm guessing that the mechanical assembly is OK unless a wire is broken.

Was the tach ever working or has it always been stuck at 2K?



Rick B.

 

Some simple test can help you diagnosis what the problem is.

First.. do you have the tach filter in place... if so disconnect it and plug up your tach wire to the distributor.

But.. here is a testing help page for you... following this should help you figure out your issue.

http://willcoxcorvette.com/repairand...lp.php?hID=290

Willcox

This is copied from another thread, I hope this helps you too!



The tachometer in your car works off 12 volts, a ground and what is called a square wave signal generated by the distributor. The 12 volts and the ground are supplied to the tachometer by the printed circuit on the back of the speedometer and tachometer housing. The signal is supplied from the distributor through a filter and then straight to the tach.

Before you pull the tach out of the car lets test some things real quick.

un-plug the tach filter from the system and plug the tach in direct! This will take the filter out of the system.. Start the car and see if the tach works.

Here is where you'll find the filter.


If the gauge works from by-passing the filter then you should replace the filter.

If the gauge fails then pull the tach/speedo housing from the car and go to this step by step page on how to figure out the problem. It will tell you how to remove the tach from the car in a matter of minutes and also take you step by step on how to determine if you have a wire issue or a board problem. This was updated recently with new pictures and details!

http://willcoxcorvette.com/repairand...lp.php?hID=290

If you get to the point where you need to replace the board, we have a video on youtube which will show you step by step on installation as well as how to do the zero set.

18243


Willcox

 

Willcox - Thanks for the link. I can use that to check the signal on the cluster. The tach PC board is new from you guys. The tach filter is still a possibility as the replacement was bought used.

The tach was inop when I bought the car so it has been stuck at 2K since I picked it up. The first time I did the tach filter bypass check there was no change in needle position but my connection with the jumper was questionable. I now have sufficient wire length and proper connectors to try it again.

 

is there any movement in the tach at all?

 

None under power. I can physically move the needle freely by hand with the tach removed. Once installed it stays wherever it was pointed when installed unless you physically move it by hand. It does not jump or flicker when the key is turned on or when the car is started.

 

Your not getting power to the tach.. Pull it back out.. go to the repair page above and follow the testing instructions for the power clips. http://willcoxcorvette.com/repairand...lp.php?hID=290

When you turn on the key.. the needle should zero out.. if it has no power it will store the last signal received.

check this out.. post again..

Happy Easter..

Willcox

 

Thanks. I'm taking a break from any and all maintenance today. I'll try out your suggestion tomorrow and let you know what I found. I hope your Easter Sunday is going well.

 

If you don't have a volt/ohm meter I would strongly suggest that you buy one. You will need it many times when trying to diagnose issues with these old cars. You can verify power to your tach with a test light but for troubleshooting shorted or open circuits you will need a meter.



Rick B.

 

A good tip for anyone working on any car. A meter has been a part of my toolbox since 1981. My job in the USAF involved avionics and ground radar. I learned how to use it from them.

 

Agree 100 percent.. very handy tool.. and not to expensive.

 

I have 3 meters floating around. Use them all.

Sounds stupid but do you have the fiber washers installed on the prongs on the tach. I did not on the 82 I was working on and Ernie clued me in on this. I did not buy the board from Ernie and it did not have the washers but he was very helpful in this for me. The tach now works great after Iinstalled the washers in insulate them. Just a thought.

 

Gordon - I did not see any washers on the old board or for the new one. All I had was the aluminum nuts. Do you have a link or pic as what and where these washers should be? I did some troubleshooting tonight and here's what I found using the instructions Willcox sent.

I have 8.7 volts between the 12V post and ground. I have 8 ohms between the signal post and the tach wire. I did some research and found there is also a signal coming from the 7 pin ignition module inside the distributor that drives the tach. I have not checked this but I do have a new module on hand.

As always I appreciate all of the help I've received not only on this task but my entire ordeal on this resto.

 

Not stupid at all Gordon.. The only board that needs them is the imported boards and you had no way to know this.. I think I read your post and realized it... not sure.. but something triggered my old crusty mind..

Our boards are made like the original boards.. they don't need washers..

But.. I'm not sure where Talley got the board from he used and so this may be the issue...

 

Talley..

Your car is to easy to pull the tach out of.. I think at this point..

Pull the tach out of the car.. and hard wire it!

Just run a jumper from the alternator to the positive post, and a ground from the engine to the ground post. Then run a jumper to the coil terminal on the distributor bypassing the filter and put this on the input signal stud on the tach.

This will tell you if the tach is the problem... it will eliminate either the tach or the car...

The board in most cases is the cure.. but there are cases where the issue is the movement.. I can test the tach for you and there is no charge..

There should be aprox. 12 volts between the power and ground clip..

And we put filters in the board to keep down spike issues from car problems but if there is a dead short it is possible to blow the board.

Do a hard wire and let me know what you find.

Willcox

 

Wilcox,

are you saying that when you turn the ignition key to "on" the tach should read zero before starting?

I think i have a problem with my tach (a US shop replaced the circuit board for me) in that it sits below zero when at idle, but seems to increase faster than the engine does..

looking at your video, it seems as though I need to set my zero and adjust the potentiometer using your signal generator...

sorry for the Hi-jack....

 

The PC board is new from you. I'll try the hard wire next, possibly tonight if the weather is good. I don't like working during thunderstorms.