tach issues

[mtdripo]

I recently installed a zx dist. on my 72 240. I am using the gm modual method. My tach. now only goes up to 3000 rpm. I couldnt find anything related to this problem in the archives. Has this happend to anyone else? What may be causing this problem?

Thanks,

Dan

BTW- some of you may remember me trying to figure out why my dual webbers kept fouling out plugs, it wasnt the carbs, it was the spark-thank god, plugs were getting expencive!-

[Inf]

mtdripo, I had the same problem when I installed a pertronix unit in my 72 240z dist.

There is a diagram floating around telling how to wire it up to work right with the original 4 wire inductive loop tachs (in almost all the 240s). I tried this, but still had a problem that sounds like what yours is doing.

The tach would work fine up to 3000 RPM then it would snap back to zero and bounce around after that. Not sure how it worked at sustained higher RPMs because I chickened out and took it back to my house at low rpm when I was troubleshooting.

I ended up buying a tach from a 280Z and swapping the internals into my 240Z, although I haven't gotten that to work yet (it sort of worked, went to around the right RPMs but it bounced around a lot, even with the correct resistor inline on the negative trip). I think the problem may be a bad ground on the molex plug going into the back of the tach wiring. Some other more urgent stuff came up, and I need to fix that before I can get the tach problems sorted out.

One thing to note about my 240Z tach was I noticed a certain resistor(see attached pic) seemed to have some heat damage, I think I remember reading about some people replacing this resistor and having their problems gone away as a result. I would have tried this, except for the fact that I shattered the PCB in the back of my original tach popping it free from the epoxy stuff holding it into the housing

This thread might be useful to you.

http://www.classiczcars.com/forums/showthread.php?s=&threadid=6814&highlight=zx+tach+diagram

[mtdripo]

Thanks for the help. It seems my best option is to find a later model tach.