Tach Issues

[Tarmac]

The Car: 1970 240Z w/ automatic transmission

The problem: Sometimes the tach works fine, some times it reads 2-3k higher then what the motor sounds like it is running at, sometimes it it is just pegged.

The answer: <fill-in-the-blank>.

Thanks, any advice or even a good place to start would be helpfull.

BTW: I did search for "tach" and came up with 4 pages of stuff. Didn't really find what I was looking for.

[Tarmac]

Anyone? Bueller?

Is this a symptom of the tach failing mechanically, or does it sound like a electrical problem? Can the tach be rebuilt/fixed or do I need a replacement?

[EScanlon]

Electrical failure internally.

I've not been able to get with my electronic hobbyist friend to determine how and if one can be fixed... our schedules just haven't coincided.

At this point, it might be easiest to replace it, although the supply is starting to get thin.

FWIW

E

[moritz55]

I have had my Tach refurbished from the following company: http://www.gaugeguys.com/contact.htm . They will restore a Tach provided you give them specific guidance and instructions so call them before they start anything. They are not cheap - it cost $186, but they did update a 240z Tach for me with new electronics and cleaned the needle and refinished the exterior and dial gauge. With the restored Tach it now is triggered correctly from Electronic Ignition systems like newer tachs do. Only thing I had to do is leave Inductive Loop plugged into my harness so my ignition and car runs once it's started to complete the ON circuit.

Good luck ... Mark.

[Tarmac]

Thanks to both of you!

EScanlon: I did read your thread, i'd love to see how it ends

It looks like I can get a tach on Ebay for cheaper then $185. Much cheaper, but I guess I'd be taking a chance as to it's working condition. Gotta think about this....

[JerseyJoe]

Does it stick in position after the car is turned off? I had one like that, and I was able to remove the needle, and loosen the bushing on the front of the shaft to free it up to return to zero.

[Tarmac]

No sticking. 80% of the time it seems to be working properly. Once in a blue moon it reads high, only 1 or 2 times has it been pegged while the motor is at idle. As soon as I shut it off it goes back to zero.

[EScanlon]

How does it relate to ambient temperature?

I'll wager that the higher the temp is, the more prone it is to failure.

That points to one of the capacitors beginning to fail.

E

[m4xwellmurd3r]

hey EScanlon would those high temps happen to cause an electrical problem with sticky tachs? or is that just a mechanical stickyness. My 78's tach will stick if it's hot out, and i have to tap/hit the face plate to get it to go back down. but after cooling off a bit it starts working perfectly again. I'm thinking it's just some grime that get's gummy when it get's hot and needs to be cleaned.

[Tarmac]

How does it relate to ambient temperature?

I'll wager that the higher the temp is, the more prone it is to failure.

That points to one of the capacitors beginning to fail.

E

Well, I can't say that I was paying too much attention, but I do live in Tucson AZ, and it is summer. Im guessing 100+ F. I'll pay more attention next time it happens.

Can I replace the (flux) capacitor?

What happens if I hit 88mph?

[EScanlon]

To answer you Max, since the 78's tach starts using different sensing circuitry than the early tachs, it is plausible to think that you're not suffering from the same heat induced problems that can and have failed other tachs.

Capacitors (if 25 year old electronics course doesn't fail me) are devices to both store and control current by filtering it. They are in essence two pieces of conducting material separated by a thin permeable NON-conducting material. That the separating material is non-conducting and permeable is what allows capacitors to filter certain frequencies from the current. As they age, whether due to natural chemical breakdown or due to heat induced breakdown, that property of filtering or storing current breaks down as well.

Commonly, people have reported that their tach suddenly "pegs out" and won't return to normal until after a period of cooling. Other times, it's intermittent operation with increasing periods of non-function while still wavering all over the scale.

My electronics wiz friend, on being told the symptoms, summarily mentioned "failing capacitor" and promised to look at one that had failed in the manner mentioned, in the hopes of repairing it. (You'd be surprised how long it took to get a tach whose failure cause could be ascertained.) That's where the project stands.

And Tar, if you hit 88mph, make sure you're buckled in and doing it on a stretch of highway that isn't densely packed, and look out for the smokeys.

FWIW

E

[m4xwellmurd3r]

Ah. well I'm curious to know if anyone else has the high heat sticking issue as well. I could take the tach out and take it apart to clean it, but I'm not sure exactly what I should be looking for in terms of what to clean. AZ get's really hot, so my tach sticks quite a bit if it's not totally in the shade. and I'd rather not have to wait a good 10+ minutes of reving and hitting my dash to get the tach to work smoothly.