924Board.org

[S4m_T]

Hello guys !
It's my first post here, but so far this forum helped me a lot to troubleshoot some problems on my 924S

Since I bought the car, the tach never worked (the previous owner told me it was working fine before, but after 4 years of storage it came out not working anymore).

So long story short, I'm trying to figure out if the problem comes from the wiring or the tach itself... I have the model with the 8-pin connector behind. I could figure out from other discussions that wiring is as follow :

1 - signal on pin 11 from DME (not sure what exactly is this signal)
2 - road speed output from speedometer (not sure of its exact use)
3 - 12V from lights on signal
4 - some people say it's a ground, but not so sure since on wiring diagram it seems to be hooked up on the coolant temperature sender unit
5 - 12V from the fuel pump relay
6 - ignition coil signal

So, pins 2 and 3 are irrelevant for tach itselft. I hooked up an oscilloscope to pin 1 and could measure a 12V square wave signal to the tach, seemed to be about at the same frequency than the engine rev, but unsure if it's driving the tach. pin 4 I didn't measure any voltage, but no direct continuity to ground either (had maybe 40 Ohm of resistance with chassis ground). Pin 5 I had +12V. Pin 6 on the tach I could measure also a square wave matching what looked like the engine rpm.

Is there anything I'm missing before ruling out the wiring and ordering another tach ? They are selling on ebay for about $200, so I don't want to spend that money if I'm not sure it's really the problem.

Thanks for your help !

Here's a photo of my 924S since I'm a new member here

https://photos.app.goo.gl/rozfvoHNN1NBH9VP8
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Porsche 924S 1987