Tach doesn't..... ???

DDS · 07-25-2003, 06:31 PM

Hello Gentlemen:

Just wrapping up my nearly 3 year resto and am down to troubleshooting a few problems, firts, my tach is dead.

I installed a mallory unilite distributor and mallory Hyfire 6AL. Originally I conencted the tach per the instruction to the white wire from the hyfire box. Nothing. Then I took John at Aircooled.net's advice (I bought the stuff from him) and connected the tach to the green wire (coil trigger) frim the distributor itself. Still no dice, tach does nothing. I tried a second tach, nothing. 12V is getting to the tach and the wire I'm connected to in the engine bay measures zero resistance to the one in the dash, so it seems right, though there is only 2Kohms reistance to ground on that one. The ground to the tach is good.

Any thoughts?

Many thanks, Dave

Biggy72 · Biggy72's Garage

All of the tach's I've hooked up have gone to the negative post on the coil. I don't know if that's how it is on all cars but it worked on my bug.

Howard H · 07-26-2003, 07:08 AM

Dave:

I ran into a similar situation when I recently installed a Crane XR700 ignition system. The instructions say to connect the tach wire (black w/ purple) to the "-" terminal on the coil. No tach. I also hooked up my dwell/tach meter to the coil to see if I could get an RPM reading, nada.

During a series of calls to Crane's tech support line, I got the following suggestions --- install a 10K ohm resistor between the "-" coil terminal and the tach wire --- increase the spark plug gap up to .045" --- make sure that there is at least 10.5 volts at the "-" coil terminal. Unfortunately, non of these suggestions cured the problem, and it looks like the final solution may be to purchase a tach adapter that they sell (naturally).

The bottom line here is that the OEM tachs don't seem to interact well with the newer high energy ignitions systems. I would suggest that you disconnect the wire to the OEM tach and hook up a dwell/tach meter. If you're not getting a reading on the meter, you can rule out problems with the OEM tach and the related wiring. Then, pump the Mallory people for all the suggestions you can get. If none of them work, you may be forced into an aftermarket tach. I think that some other guys here on the bbs could make some suggestions on VDO and other tachs.

Best of luck, and congratulations on your just about completed restoration.

Howard

DDS · 09-11-2003, 05:58 PM

Done. I ordered a tach adaptor from Mallory - what a joke, a little air-cored inductor, $10 item cost me $80.00 (Canadian) Like they don't charge enough for the ignition and distributor. Grrrrrr. Anyway, the instructions with the adaptor were incomplete and were for an earlier unit, (useless) and the instructions for the ignition were wrong too in this case (didn't work), and the emailed instructions I got from the vendor didn't work (contradicted those in the unit and didn't work).
All were different.
However, two wrongs made a right and by combining two of the three I got it to work I wired the tach to trigger off the coil, then shunted 12VDC (switched by the ignition key) to the tach across the inductor and voila - tach. The nice thing about doing it this way is you can wire it under the dash and get 12V from the tach itself rather than adding to the clutter in the engine bay. One more item off the list....

Dave

DAve