On my 1979 vette, while driving last night home from a local cruise in my tach was working fine. At idle it was reading about 600 rpm,while driving down the highway at 60 mph it was reading about 3000 rpm. All of a sudden I noticed that the tach needle has jumped or something, because the needle is reading at 7000 rpm at idle and pegs all the way out at highway speed.

Can I just reposition the tach needle to zero or do I have to do something else to fix this problem?

 

JT.

Re-zeroing the tach needle will not solve your problem. Something has gone wrong for the tach to act this way.
Since the tach is obviously working and has power, ground and signal I think your board is a problem.

Un-plug the tach from the distributor and see if the tach goes to zero with the key turned on.

It's pretty hard for the tach needle to come loose from the spindle, I guess it is possible if the tach was rebuilt and the needle was not pressed on hard enough.

Willcox

 

Willcox,
Thanks for your reply. I will unhook tach at the dist. like you suggested. I just replaced the curcuit board about three weeks ago due to the dash lights not working. Some time it seems that when you fix one problem another one happens.

 

You may have a wire pinched or one of the fingers hitting another one on the circuit. I would check this and also make sure the connector is plugged in good. The reason I told you to power it up with no signal was to see if it was a signal problem or something else.

I posted a picture of what wires control the tach on the board. I think I'll re-post this for you too! It will help you when you go trouble shooting.

 

On your tach circuit, you will need to verify the following:

Make sure you have 12 volts to the tach

Make sure you verify the Tach ground

Make sure you verify continuity from the tach wire on the connector to the distributor.

Inline on our car there is a tach filter, this filter is held in place with one bolt to the intake manifold. If you do not have continuity from the connector to the distributor, un-plug the tach filter from the system. See if you have continuity from the dash connector to the plug where the filter plugged in. If you do have continuity to this connector then plug this connector to the “tach” connector on the distributor bypassing the tach filter. Now plug in the connector and see if it works. If this works inspect the filter to see if the wire is broke at the capacitor, if it is you can try to place a drop of solder in the capacitor to get the filter working again. This can be hit or miss. If the filter is bad we do carry a replacement style on the web site.

Testing without the filter will not hurt the tach in any way shape or form. If the tach works when you do this you need to replace or repair the filter.

If you have power at the tach and a verified ground with good continuity from the distributor to the tach and it is not working you have a board problem more than likely. If you want to test the tach by doing a hard wire test the help page listed below will tell you exactly how to do this. It is the same test we suggest you do before installing a tach with a new board in the car just to verify the tach is working properly.

I have a page on my web site that will help you in this repair: (and has the hard wire test). http://willcoxcorvette.com/repairand...lp.php?hID=252

I also have a page on the site on how to remove the tach from the car at this link:
http://willcoxcorvette.com/repairand...lp.php?hID=146

Should you need the tach board, I have them listed in the Forum Special section under this link and these boards are pre-tested and calibrated before we send them out. http://willcoxcorvette.com/advanced_...&keywords=tach If you enter the current sale discount code on check out, you will receive an additional discount on this item too. This sale code is DRWH3A and must be entered when checking out on the site.

On your printed circuit, the top wire on the tach side is your ground, the second wire down is the signal wire from the distributor and the third wire from the top is the power wire. Not this is if you were looking at the back of the tach with it sitting on our lap. So the wires in the car if you were facing the connector when un-plugged go to the passenger side top wire. This is the tach ground. The passenger side wire second from the top is the square signal wire from the distributor, and the third one down on the passenger side of the plug is the 12 volts to the tach.

I hope this make sense. . .

Now pull the tach, test your wires and if this is not enough info either post again or email me direct at Willcoxcustomerservice@willcoxcorvette.c om.

Willcox Inc.

Here is the printed circuit board for the tach and speedo showing the wires above.

 

Willcox,
I have unpluged the white wire at the dist. like you suggested and when I turn the key on the needle does not go to zero.
With the key off the needle is pointed at 7000 rpm when the key is turned on the needle jumps to about 5500 rpm.
I will pull the tach out like you suggested and see if there is a problem with the circuit board. Just a little info on the curcuit board. When I installed it the first time the tach did not work so I removed it and I noticed that the little metal clip on the back was not making contact with the board so I shimed the back off the board with cardboard strips so that they would make contact and when I put it back the tach worked fine until now. Could this have caused my problem?

 

Are you talking about the three clips that hold the printed circuit board to the plastic housing? The clips are what transfer power to the studs on the back of the tach.

I think I would pull this out of the car and on your car it is pretty easy. Then I think I would hard wire the tach to the car and see what you get. Starting first with only 12 volts and ground to see if the tach zeros out! Then add the signal wire from the distributor and see if it works. This will take the printed circuit out of the picture and tell you if you fried the tach board.

If the tach works this way, I would then look for a reason in the wiring or the printed circuit. I would also look for something burned on the printed circuit.

Now if the tach fails, you're probably going to need a new board. I'm not sure why you had to shim the thing with cardboard unless the clips were not touching the studs.

 

Willcox,
I had to shim the circute board because the clips did not lay flat. Some of the clips was flat the other was stiking up, I could not bend them all the way down flat as I was sure that I would break them.

As I was removing the tach from the dash I reach behind and unpulged the harness, I repluged up the harness cut the key on and the tach went to zero. I must not have had the harness plug it good but everything is working fine now.

Thank you so much for your help in finding out the problem. This board is a wonderful place to come to when you have a lot of people willing to help others out.

 

JT! Thanks you and I'm glad it's working now!

You really should think about pulling this out and inspect the circuit tabs. Make sure one is not loose and touching the other one!

If one of the fingers is loose it could short something out and cause more damage. You may have what we call a finger on the printed circuit board that has come un-bonded and is flopping around. If it was just a simple issue of the connector not being plugged in all the way then I would leave it alone. .. .and enjoy the summer!

But if you are not sure about what caused the problem it is surely worth pulling out to inspect it to see!

Willcox Inc.