mjt911rs

I have a 1991 C2 targa with a non working Tachometer. I sent it off to PaloAlto Speedo thinking it was internal. They told me it was functioning properly. I checked the wires to the back and there is power. All lights and Both turn signal lamps in the tach work. But the needle doesn't move. Where and how do I check for a pulse or signal? Anyone having similar problems?

Chokingfai

I am having the same problem on my 90 c4, lamps work but needle doesnt move! How did you solve it?

marty964

I am having the same problem on a 1990 C2. Anyone find a solution? Thanks!

Spyerx

I posted this in the other forum you posted in. Does your tach test good? That's where you start. If it does, then you need an oscilloscope or frequency reading volt meter to check the signal wire at the back of the tach. if that is't working, check the continuity from the DME to the tach. If that is OK then it's a good chance you have a DME that needs to be serviced. The C2 receives the tach signal from the DME.

But you'll want to start with testing the tach first. For that you'll need specialty equipment, swap to another c2, or send it off to Palo Alto speedometer or north Hollywood speedometer.

ToreB

Working turn signal/high beam/back light bulbs in the instrument proves that it has GND connection only, and not that the positive power supply is OK.
Non-OBC tachos have pin numbers and functions printed at the back. Use a voltmeter and check for (igniton) switched 12V at Terminal 15, pin 7. (orange wire) GND is pin 2, brown wire. If there is no 12V, check fuse 18. This fuse gets power from relay 11, Terminal 15E.
Cheers,
Tore

marty964

Thank you ToreB and Spyerx! Appreciate the patience while I figure this out. I took out the tachometer and tested the plug behind it (pictured below) with the ignition switched on.

1. Voltmeter between pin 7 (positive) & pin 2 (ground) and received a reading of +(-) 12.40 V. Test light also showed power.
2. I checked the same pin 7 but used ground as a bolt in the door - same reading.
3. At fuse 18 - the same reading, grounded at battery terminal
3. At the battery terminals +(-) I got a reading of slightly above 12.4

I don’t have an oscilloscope. I read another post ToreB has commented on, but not sure if this test can be run without an oscilloscope (between DME and the plug behind my tachometer)

is there a continuity test I should run between the DME and the tach to make sure I don’t have a fault in those wires?

At this point if I have power at the fuse and the the tachometer plug, am I looking at a faulty DME? Is there any other way to rule out the faulty tach before I send that out for repair? Assuming the tachometer repair will be less expensive than the DME repair.

A friend who’s opened up numerous DME’s from the 80’s (non Porsche) told me I might have luck just opening and looking for obvious faults (smell, sight, solder points)