924Board.org

[canyonblaster]

bummer....so i sourced a replacment tach that fits exactly, plug-n-play (thanks Ken). I swapped it in and yeah! I have a working tach!. Well, it works great for a couple of short drives today, then it stopped and is now dead/zero signal (light still works). I doubt it is related but I did spray the car clean at local pay-n-spray car wash bay, and that is when it stopped working.....

any ideas anyone?
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PCA Licensed Club Racer & Driving Instructor Current P cars: 1981 931
Past P cars, 1985 944, 1987 951, 1990 911 C4, 2002 Boxster S, 2005 996 GT3, 2013 911 C2S
Daily Driver P cars: 2008 Cayenne GTS, 2013 Cayenne GTS

[fiat22turbo]

I believe that the Tach is fed from one side of the coil via the fuse panel, controlled by the ignition switch.

So that should give you an idea of where you may want to start looking (in other words, another lovely round of find the bad electrical connection).
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[ideola]

The wires that attach to the coil are notorious for getting very brittle. The insulation will crack and break, exposing the raw copper, which will then proceed to deteriorate. This is the first place I would inspect, from the terminal to as far back as you can to the harness wrap.

The three connections on the back inside the dash board are fairly straightforward, and should be easy to inspect, i.e., there is no harness receptacle, each wire attaches directly to spade terminals on the tach. Nevertheless, would still be prudent to inspect these carefully, as rummaging around in the dash can wreak all sorts of havoc with old brittle wiring.

There's also an outside chance that the ignition switch is starting to fail. If you haven't changed yours personally, I can almost guarantee you that it's in need of replacement. This images shows where they always fail.

It's a cheap part, and easy to replace, if not a little tedious. You should replace it as a matter of preventive maintenance whether it is related to your tach problem or not.

If all that checks out, then it's time to break out the wiring diagram and start tracing the circuit.
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erstwhile owner of just about every 924 variant ever made

[canyonblaster]

Thank you Stefan and Dan!
_________________
PCA Licensed Club Racer & Driving Instructor Current P cars: 1981 931
Past P cars, 1985 944, 1987 951, 1990 911 C4, 2002 Boxster S, 2005 996 GT3, 2013 911 C2S
Daily Driver P cars: 2008 Cayenne GTS, 2013 Cayenne GTS