Tach dies when hot. Why?

[Fidalgo911S]

My tachometer will not run when the guage itself gets hot. This happens when sitting in direct sunlight for a period of time. When I start the car - nothing, it lies at 0. When I get driving and I run cool air through the blower it comes back to life.

As a control, I parked the car so that the dash was not directly exposed to sunlight. After a couple of hours it worked fine. Also this is not a problem on cloudy days or in the winter.

What would cause that? Is there a DIY repair? If it's an easy fix, I'll do it to avoid sending the guage off for $$ repairs. Thanks.

[Mattlock18]

I posted a thread on this a few months back and had a few others said they have the same problem. Unfortunately, there weren't any real solutions offered to me. Hope you have better luck. If your tach is like mine, it will start working a couple miles down the road once the needle unsticks after sitting in the sun.

[Fidalgo911S]

Exactly right. Sorry, didn't find your thread when I did a search. Maybe the answer will come this time around.

[T77911S]

could be a "cold" solder joint inside the tach. temp change can make the joint expand and contract resulting in the solder point making and breaking contact. if you want, you may be able to take it apart, use a good light and viually check all the solder joints. you are looking for a very fine line around one of the connections. if you cant find it, try taking a solder iron and just heat up all the joints and hope you fix it. i have never taken one apart, so i dont know what to expect inside, i imagine there is a chip in there, that is where i would start. also check around any wire connections, resolder them just to make sure.
i had "cold" solder joints in my TV and in my stereo, they are quite common.

[Porsche_monkey]

Mine was a cold solder joint. I don't know which joint, I just touched all the joints with the soldering iron and it has worked fine since then.

[kimlangley7]

My olny experience with a "wild" tach was when my alternator / voltage regulator was "over -volting"... the tach would swing wildly.... //80 SC

[Paulporsche]

I don't know the answer, but I think I would look into things that may expand and bind in the heat/sunlight, like the needle itself, or the case, the glass, etc. Have you removed the unit and looked it over carefully? Maybe under different heat conditions?

[Fidalgo911S]

Thanks for the good leads. I'm going to try the cold solder fix and then check any expanding components. I haven't had it apart yet but I think I have a good idea of where to start now.

[304065]

Nate, I would save yourself the aggravation of prying off the bezel ring, which despite the best efforts of anyone outside of two specialty shops in California, tends to look like a pit bull chewed it off. Once inside, you'll be looking at an old brown circuit board filled with components so old even Early_S_Man can't find a cross-reference for modern equivalents. Then you'll be faced with looking for a bad solder connection or failed component. But what then? Since the tach works intermittently you might be able to press the iron onto one spot and get it to work again, but it's far more likely that one of the discrete components has failed.

If it were me, I would look out ahead and determine what ignition system to use, whether Bosch CDI, MSD, Perma-Tune, M&W, etc., and have North Hollywood or Palo Alto fit a new board to accomodate signals from that kind of box. It's not super expensive and you can have the face re-screened if necessary.

If you need an old '74 tach I can dig one up, I have one from the race car I am no longer using, yours for a handshake if you want it. Here's a bad picture of it but you can see the black bezel and the black center with red trapezoidal needle, should match what you have. It will take me a few weeks to dig it up, though.

[RoninLB]

cramer's probably correct..

a few years ago a pelicanhead posted that his lens would expand into the needle when hot.

[KNS]

This would happen to me too. As the guage heats up, the needle presses up against the glass and it sticks. I sent it to North Hollywood Speedometer and they installed a thin spacer ring behind the glass (it's not an uncommon problem as you have found). I'm in Arizona and it's been fine ever since.

[RarlyL8]

2nd on the needle hitting the glass (in my case).
I figured it out by tapping the glass when it froze. The needle moved then stuck every time I tapped it. I could also hear a "pop" which sounded appropriate for plastic expansion every now and then when the car sat in the heat and then began to cool as I drove.

[Fidalgo911S]

Originally Posted by RoninLB

cramer's probably correct..

When isn't he? Sounds like sage advice. It'll save me a few hours of aggravating work. John thanks for the kind offer but there's no need to send your tach out here, I'll go ahead and use mine.