New tach problem - 240Z

[Arne]

OK, here's a new one on me. The tach in my early '71 240Z (4 wire) suddenly this afternoon started to peg the needle at 8000 RPM any time the key is turned to the Run position, whether the engine is running or not. I suspect a problem with the tach itself, not the wiring as the ignition appears to be working fine, since the car starts and runs normally. Any thoughts?

[a7dz]

Wish I could read this diagram! Maybe you can and then send it to me.

http://www.classiczcars.com/forums/showthread.php?t=21924&highlight=tach

Sounds like a bad tach. Mine is stuck a 1200 all the time.

I have a least one four wire that does not work as well. I did buy one to replace it but, I am not sure is is any good either.

[Arne]

Well, I have a spare tach from the pats car, so if it's dead and can't be repaired easily...

[EScanlon]

Arne:

Swap the tach, the 8k one is dead.

E

[Arne]

Sounds logical, but before I install teh other one, can anyone think of anything in the car's wiring that could cause this failure? No sense in doing the same thing to another one, you know.

[EScanlon]

Age, someone trying to "adjust" and shorting the lower board to the upper, resistor (?) between two boards getting fried. That's just off the top of my head.

Others?

E

[a7dz]

Try this link for the diagram;

http://www.classiczcars.com/forums/attachment.php?attachmentid=12516&d=1148842545

[ktm]

The diode finally went south? I know (from this forum) that the 240z tach is susceptible to heat when the diode starts to 'wear out'. The tach would slowly climb and then peg itself until it cooled down. The fix was to solder in a new diode that is available from radio shack.