tach reads wrong when hot outside

[Pir0San]

I have experienced some weird things with my tach, but only when it's really hot outside. For some reason, my tach will read incorrectly at times, where when the engine is not running, but the key is in the on position, the tach will point at 1k, although the engine is not running. I do know that the tach gets it's signal from the coil, so I checked all of my wiring, and everything is fine, but the tach would still read incorrectly. I allowed the engine to cool, and drove it later that night (it was 100* yesterday when this happened) and the tach read correctly again.

What is wrong with my Z?

[EScanlon]

Somebody got you the "optional" Temperature / Tach combo?

Honestly, this one is new to me. But the reason's may be due more to the age of the vehicle and it's components than anything else. Temperature can increase resistance in wiring and therefore affect electronic items.

I presume you've already eliminated the resistor at the coil. You are dealing with the E12-80 Electronic Dizzy Module, I presume.

I don't know, anyone else?

E

[Goose52]

The original tach on my '72 has been doing the exact same thing for years - I just figured the heat was affecting one of the components inside the tach. I've lived with it, figuring the tach would eventually die and I'd worry about it then. In any event, when it's that hot (I have no AC), I usually don't feel very skippy and don't usually come close to redlining the engine ... so don't need the tach anyway ....

Early this year, I did upgrade to the '79ZX dist. w/E12-80 module, along with a new coil. It is just now getting hot enough here in SoCal to bring on the symptom so it will be interesting to see if I have the same problem with the tach this summer or whether it is now gone. If it doesn't come back this year, perhaps the problem had something to do with the coil???

Goose

[Dtsnlvrs]

I had the same issue with my 70 when I bought it. I replaced the tach with a new one at that time and the issue remained. Then I started researching. Follow me here. The tach in a 240 is of an inductive loop type. That means that power comes FROM the ignition, THRU the tach, TO the coil. The tach reads the electrical pulses of the coil via an inductive loop. British cars of the same vintage used the same setup. I chased down every part of the harness I could and never resolved the issue until I installed a MSD 6A with a tach adapter. Now my tach reads perfect. The tach adapter provides a false grounding signal for the tach.

[Goose52]

I'm familiar with the tach wiring (I put a kill switch under the dash back in '72 by tying into the power going into the tach) but what seems to be key about your situation is that changing the tach did not fix the problem (eliminates that from consideration?) and that your problem went away after adding the MSD and tach adapter - implying that my problem and Jeremy's problem perhaps is not the tach but something to do with the circuit after it gets to the coil.

So, what is heat sensitive with the coil or primary/secondary circuit that would cause the tach to read incorrectly, and the other symptom - the tach reading about 1000 to 1500 with the ignition turned on, but the engine not running. All that's out at that end is the coil, ballast resistor, and then of course the points & condenser.

For folks with these tach symptoms, still having a points type ignition, I'm not sure an answer has been found yet ...

Goose

[Dtsnlvrs]

Yeah I replaced the coil too, as well as the points with a pertronix unit. It baffles me still because when I wired the MSD, I did not cut any part of the harness. I made a harness for the MSD that was "plug n play" The only thing I can figure is as the tach ages, it becomes sensitive to variances in the inductive signal it gets from a hot coil. Remember I replaced mine with a new one...NOS been sitting on a shelf for 25+ years

[Goose52]

Yep - a stumper for me too.

Well, bring on the hot weather and I'll see what happens this summer!

Goose

[Pir0San]

My setup is the stock '72 distributor with Pertronix electronic points. I've had this problem with both points style and my Pertronix setup. It was hot today, around 95*, and no issue with the tach. The tach issue just baffles me.

EScalon, when you talk about a resistor, are we talking about the ballast resistor? That's the only resistor on my coil.

[EScanlon]

Yes, that is the resistor I was referring to.

You don't need it with the electronic ignition. Simply connect the wiring at each end (one Green/White wire and a Black/White wire) together. By eliminating the resistor you are basically providing 12v to the dizzy at all times not just when you're starting the engine.

It's original purpose was to reduce the voltage to the coil and points to 6v or so when the car was RUNNING (while providing 12v at START) in an effort to reduce pitting of the points. With the Electronic ignition, you may actually be causing it to not operate properly because you're "starving" the circuit.

FWIW

E

[hls30.com]

Maybe the capacitor discharge in the EI unit is not bleeding down when the car is shut down/loose connections-get and install spark checker in place of the coil wire and open the hood at night in the dark after a hot run(to see if the ign is still trying to make the car go-but with no gas, it won't) As Enrique said you have 12v ready even when ther starter isn't

WIll

[ZaPete]

I have a 78 280Z, 140,000 m. The tach seems to work just fine. last week it was over 90degrees out and I took her on the road and ran the gears almost to red line a few runs. After the 3rd time the tach never moved off the '0' peg. I though she up'd and died. Drove it last night, tach works fine, temp was 75 degress... Just one of those "Z" misteries. I had similar issue with all the tachs in my 1970 & 1972 240's and my old 1976 280..

[Pir0San]

alright, I feel kind of stupid, but I know I have to ask. I was told that the ballast resistor was used to reduce static interference on the car stereo. Is this correct?

This is the way that my pertronix is wired now, so how would I rewire it without the ballast resistor, and still have the tach work?

Sorry for the lame questions, I just don't want to wire the car wrong, since she's my daily driver for the moment.