oazix

maybe someone out there can shine a light on this for me,
ever since swapping a 968 engine into my 85.5 i haven't had a tach.
when feeding the new harness threw the firewall the plug exploded into a million little brittle pieces of plastic, and i haven't a clue which one is the tach.
the one i think is the signal to the tach reads -.785 DCV and decreases to -1.2 DCV with more throttle, anyway this is the line i have hooded up to the tach on the red plug near the DME. The Negative signal kinda has me thinking either I'm totally on the wrong wire or something is reversed on the engine side and the tach is not registering the neg voltage.

does anybody have an idea what a normal tach signal looks like?
I'm guessing the voltage would increase with the RPM's

oazix

ok there is also a 12VDC signal coming in when i start the engine could this be the wire I'm looking for?
I've been searching this and best thing i could find was that the signal is a 12v with pulses from ground, which i cant see on the DMM I'm using. but that being said i don't really want to plug it in and fry the tach if I'm not sure.

Bri Bro

The tach signal consists of pulses that are related to the engine speed. Your DMM will average the pulses and make them into a DC voltage so it will not be of much use in finding the signal. The signal is a 0 to 10V signal coming out of the DME on pin 21 and goes to a small connector (6 pin?) near the DME and this connector goes to the tach. At least that is how it is done on a 944T.

The scope picture below is the tach signal at idle.

mopar bob

You need to jump P2 to P2 On the DME connectors in the footwell. I have a 968 motor in a S2

oazix

yea, i have p2 on the car side figured out but, like i said the 968 side plug blew to smithereens, do you know what color wire the p2 on the 68 side is or what the signal would look like w a DMM?

Bri Bro

All you will see with a DMM is a DC voltage with doesn't tell you much. . Get one of these.
https://rennlist.com/forums/944-turb...ighlight=scope

oazix

that's pretty cool, we have lots of oscilloscope at work. I'm just being impatient and want to see my tach move this weekend

Bri Bro

One though, some DMM can measure frequency. At idle it is a low frequency, around 33Hz.

oazix

hmm ill check that out not sure if this one does its a fluke pretty decent model