The tach on my '79 L48 is all the way to the right when the car is off. When I start it, the tach will jump up to the 12 o'clock position but returns to the full right spot after it starts. Does this sound like an electrical board problem? If so, I'm guessing Willcox can install a new board and recalibrate? Any help/suggestions appreciated.

 

Wilcox has a video for setting up a new tachometer so that when you turn the key, the needle comes up to the 0. The video shows the tach on the workbench being set up, but I did the tach for my 80 twice. The first is when I got the tach, I set it up on the bench. The second time was when I had the unit back in the car and when I turned on the key it was not exactly where I set it. Anyway, I took the needle off and redid it right in the dash. Now it's right on zero when I turn the key on. With the ignition off, the needle does not have to return to the zero, just when the key is turned on.

 

Hmm.. Maybe that the little relay/resistor has gone bad. It's the small inline block on the tach wire that bolts next to the distributor.

 

 

Is that the tach filter?

My fuel gage is also all the way to the right (always reading FULL). I just noticed that the tach and the other gauges use a printed circuit board. Is it possible both boards are bad?

 

Yeah.. the tach filter. Just bypass it and see if anything changes.

 

Is it possible gravity is bringing the needle over to the full right after it starts? Did this problem just start? If the tach was ok, then started acting up, I don't think the needle position needs resetting. Tach boards do go bad as well as the unit pictured on the intake manifold.
The problem with the fuel gauge may be with the sender. It's been a long time but I think I disconnected the sending wire at the tank and the grounded it to see if the gauge read correctly.

 

The tach was this way when I bought it and the previous owners nephew thought it was a grounding issue. During the cranking phase the needle jumps around in the 12 o'clok position and returns to full right after started.
I'll try grounding the fuel gauge sending wire and see if that has any effect. Thanks for the suggestions and input.

 

I had time today to look around for a tach filter but there isn't one. The spot where yours is bolted to the manifold is empty. There is one white wire coming directly off of the distributer...maybe that's it tach wire?

 

Then I would buy and install the filter first, and see if that clears it up.
(Yes, tach is white)

 

From what you state above the board is the issue.

Willcox

 

If your fuel gauge is all the way to the right... you are missing ohms input to the gauge. Pull the sending unit wire from the sender and ground it out... if the gauge goes to empty then you have a fuel sending unit issue.

There is a video on our site about testing the fuel gauge and how to know what the issue is based on what the gauge does. Plus there is another page on there for helping you find the issue.

http://repairs.willcoxcorvette.com/1...el-gauge-test/

http://repairs.willcoxcorvette.com/w...it-testing.pdf

 

My '79's tach was stuck on 4500 rpm....I ordered the circuit board & filter from Willcox, and life has been GOOD ever since! ($105)

 

Thanks! With your instruction guide I was able to determine a bad sending unit for the fuel gauge.

 

This is what my dist and area around it look like.

 

This is what you should have. By-pass the filter and plug the tach straight to the distributor and see if anything changes.

 

It appears that the PO already did that (no tach filter found). So, my guess is that your first diagnosis of a bad board is correct. What is the cost if I send in my tach for your guys to install a new board and calibrate? Also, would I need a filter?

 

Not necessarily. It is indeed not mounted to the manifold. But It could be hanging loose on the back side our down below. But I will say that it is looking more and more likely to be a "grounding-out" issue.

 

Since the car is a 1979 removal is pretty easy and there are other diagnostic test you can perform before you send in the board.

First if you remove the tach / speedo lens cover from the tach / speedo housing and then remove the bezel. Once completed remove the three tach retaining screws... Here, go read this, it will walk you through the entire process.

1978-1982 Corvette Tachometer Testing, Removal, Board Installation

Once you have the tach out you should verify power and ground on the three tach clips.... Use another ground source to test the power terminal, then use the ground clip to test again. This will verify that you have power and ground at the tachometer. Once you have done this then test continuity from the signal clip to the connector that plugs in to the filter (or distributor if by-passed). This will verify the signal wire is good. You pass all three of these test then you do need a board.



Keep in mind a board is not a fix all and as the tach ages so do the internal analog movements so it is possible for you to purchase a board and not get fixed. If you send the tach in to us for the repair we can test the movement before we install a board and know if the issue is the board or movement.

Tach board installation:

1978-1979 Corvette Tach Board:

 

I checked that. The white signal wire exits the wire harness near the firewall and runs directly to the dist (nothing in between).