'78 sc - what should the tach signal read in order to operate the tachometer? Trying to chase down a non-operating tach...recently rebuilt by North Hollywood Speedometer, but still nothing. Only a slight jump of the needle when turning the key on and off. ????????

TIA

 

I put a scope on an early ('70) tach when trying to adapt for my crankfire and found that it had a negative spike for the signal. This was why my porsche tach would not work with the positive signal from the crankfire. I am assuming the 78 has a similar CD system to the earlier one.

I am saying this from memory, and am not an electronics expert. I only took it far enough to decide to create my own tach using an aftermarket movement in the porsche case (cheaper than N Hollywood).

At the time when I was doing this I posted a few questions and got no responses, so I don't think too many people mess around with this stuff.

Art

 

Interested in this also as I am having the same issue with my Tach in my 71. I have checked and cleaned all grounds and fuses. Wire traces out fine. NH Speedo has also checked mine out and deemed it fine.
Crashing Howards question hoping to resolve mine at the same time.

 

Thanks guys. If I had stock SC with CDI I could simply check the output, but with the Electromotive HPX, tach output terminal measures .3 volts. It worked fine for over a year, then I had some overcharging problems and lost the tach, so I sent it out for repair. Not sure how to proceed at this point. I'll have to borrow a known working Tach I suppose..... I hate those damn electrical gremlins.........

 

If you have an aftermarket (pep boys, etc.)tach they generally have a wide range of acceptable inputs. You could check your output that way.

Do you know anyone with an engine analyzer? They usually have some kind of tach input.

Art

 

not an expert, but the CD box should have a pulse shaping circut which provides the signal for the tach.. and I don't know what the specs are. I assume you tested the wiring......... Ron

 

Problem solved. The Electromotive HPX has an putput voltage of 3.5 volts, which is enough to drive the tach. The culprit was resistance in he wire ( the black and violet wire) leading to the tach TD terminal. I ran a new wire from the engine compartment which corrected the problem. Initially I checked the wire via a multi-meter with an audio signal for continuity, but I did not check resistance.
Thanks for the responses, I hope my remedy helps someone out there with the same annoying tach problem !