Hi All:

I've got a 1978, L48, 4-spd. I bought a new circuit board for my tach from Willcox, followed the video and all instructions accordingly. I thought I was good to go (and maybe still am).

The tach is slightly below zero when the ignition is off. When I start it (in neutral) it goes to zero but when I rev the motor the tach doesn't move. Is it supposed to? The car is not road ready at this point so I can't take it out. Any suggestions??

 

Frank,

When did you purchase this board? We had some issues earlier this year but your symptoms don't match the criteria for this issue.

Here is a thread about it.
http://forums.corvetteforum.com/c3-g...-question.html

You are more than welcome to send in the tach assembly to me. I will put it on our tester as an assembly and calibrate it to match the tach no charge!

Doing this will either point to the tach or the board.. or it will eliminate it completely.

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. If not.. go to the help and repair page I've listed.... it will show you how to test the other wires in the dash. If the tach is pulled out you might check these before shipping the tach.

One thing thats bothering me is you said it did not go to zero when you turn the key on. The tach has to be powered up when you install the needle or do what we call a zero set! This is basically the tach getting power and ground with no signal thus the term zero set. You said you needle didn't go to zero and it should as soon as you turn the key on or the needle is not set at zero.

Another thing you might do is watch the video again. In the video you'll see that after I pulled the needle of, I had to push the spindle back in the gauge. This does not happen all the time, but it can and will happen. If the spindle is jambed in the housing that supports it the needle and tach will operate exactly as you have said. It ill barely move... So this might be the problem as well.

Either way.. you are welcome to send me the tach and I'd be happy to check it out for you.

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

Here is the video again.

18243

Willcox

 

Is the tach wire (brown) connected to the HEI distributor?

The filter between the HEI and tach can also be bad. If you follow the brown wire from the HEI (assuming it is connected) you will find the filter. You can bypass it and see if the tach works. If so, just get a new filter.

 

Thanks Ernie and Steve.

Ernie, I bought this board ~July. Followed your video (excellent!). I'll send the whole tach back if you don't mind. I appreciate all your assistance and echo the compliments that everyone else posted previously. A real pleasure to work with Willcox!!!!!!!!!!!!!

Thanks again,

Frank

 

Frank..

Thank you.. All boards produced after May have to pass two test before shipped and one is a ramping test to be assured we never have the problem again.

I'll look forward to it when it arrives.

 

Just my $.02. I had the same problem but it turned out the base of the tach was in contact with the board. Added a couple of fiber washers under each of the screws to make sure it didn't touch and it fixed the problem.

 

Interesting

 

With the China boards you have to use the washers.. The boards are made wrong and will ground out when installed.

 

Ernie:

I mailed the tach today. Thanks again for all your help.

Frank