TACH acting very funny

[Zedyone_kenobi]

WHen I first joined this forum, I noticed my tach was never really happy anywhere above 5k. I quickly determined that it was in fact the tach that was not happy, rather than the engine.

Today, it was the first really hot day out, and after being in the sun all day and then getting stuck in traffic, I noticed my tack was hovering around 4k. Which in itself is not that unusual, but I was idling at a stop light. I noticed it then began to bounce slowly back and forth as it made it way to peg the needle past 8k. Now since I was at idle, I was pretty sure that this was not really happening :stupid:. The car ran just fine, so I was really not worried, figured the tach was just borked.

Now when I got it home I turned off the key and it immediately flopped back to zero. Flopped is pretty much the right word. When I turned the key back on, but did not start the engine, it slowly rose to peg the tach again. I went inside to mill it over.

I went back outside in the garage after grabbing some water and fired her up again, about 10 minutes had passed. The tach was working.

Ahhhhh Haaaaaa the plot thickens!

I think this has the signs of a tach thats on its last legs. THe thing is though when it operates its dead smooth and pretty accurate. Well up to about the 5500 rpm area anyway.

So, my question, is, could this be just a bad connection somewhere, or is the fact that the heat may have caused this just a sign that the tachs internal electronics are just too old and somewhere the Ohm (ie resistance) load is way to high. Or it could be short...So...........

I am going to start by cleaning off the terminal ends around the coil and resistor on the firewall. Then I will look for any frayed wiring.

I have an extra tach in the box I can replace it with, but the thought of pulling my tach back through my full dash cap is less than exciting...

THanks for any answers..

[moritz55]

Sorry to give you the bad news but my guess is it's the Tach. Mine acts very flakey in extreme heat. It stays above 2k all the time and bounces once it's hot. If it's the original I'm sure either a capacitor and/or resistor has aged and even cracked such that "leakage" occurs under high heat and humidity.

Good to check all your connections though, also double check your ballast resistor connections if you have one. Your ballast resistor should measure around 1.5ohms.

Let us know what you find.

I have an extra Tach on hand also - but haven't gotten around to replacing my original one. If you tackle this - I'd welcome advice on how to do it.

Good luck and keep us posted ..

Edited June 5, 2009 by moritz55

[Arne]

I agree. I had a bad one in my yellow 240Z - it was fine when the temp was moderate, but would do many funky things when it got warm outside (85°F and up). Replacing the tach with a different one fixed it.

[a7dz]

Changed mine out for the same reason.

[Zedyone_kenobi]

THen my mission should I choose to accept it is to change my tach out without removing my full dash cap!

This thread will self distruct in 5 seconds

[Arne]

Ugh! As always, should you or any of your team be caught or injured, the Secretary will disavow all knowledge. Good luck!

[a7dz]

It should not be a problem I have had a tach through a dash cap more then once with out hurting the cap or the tach. Just take your time.

[Zedyone_kenobi]

Just out of curiosity, did you get to it from under the dash, and did you have to remove the steering wheel? I remember having my arm pretty far over there when I did the three guages in the middle through the glove box.

By the way..

Update.

I put an Ohmmeter on the coil terminal to terminal. Right at about 4 Ohms. When I checked one side of the large square resistor I was getting 0 Ohm, which of course is wrong. THen I noticed a small jumper that ran from one side of the resistor to the other (mind you I am running the pertronix in my car) WHen I removed the jumper and just measured the resistance across the resistor, it was 1.6-1.7 Ohms.

I think all that may sound right, so I may just try to pull the tach ASAP to swap it out. But if something is fishy, let me know folks! THanks as usual.

Here is pick of my set up. By the way, the red wire goes to the dizzy.

You can see the jumper I am referring to that spans the resistor. The thing is, the resistor is basically useless with that in place...I never had the instructions on installing the pertronix, I have to assume this is what was instructed.

Edited June 6, 2009 by Zedyone_kenobi

[DeesZ]

My Pertronix installation has no jump wire, nor do I recall it being required by the instructions. IIRC, the Pertronix installation instructions included keeping the ballast resistor in the circuit (at least for a 'stock' coil).

[Zedyone_kenobi]

interesting... I will have to go download the instructions from the Pertonix website then, no worries!

[Zedyone_kenobi]

UPDATE:

Just to cap this thread off for future use..

I did some research, and my Bosch coil is the blue type, which does not require a 1.5 Ohm resistor, the red Bosch coil does. So instead of removing it, the PO just bypassed the resistor with a jumper.

By the way, drove the car all day yesterday in the cool morning, acted perfectly normal. I concur its heat related.

[Seppi72]

When I got my second '72 with a Rebello L28 and Pertronix, it didn't even have a resistor and the car runs just fine, thank you. For years, I've wondered why that was. Obviously, I must have one of those "bypassable" coils too.