Dead speedo help!

[masterdave]

so i just got a 911sc a few weeks ago, i have been doing some work on it to tune up stuff, oil, trans fluid, etc, the speedo was working fine, then about 2 days it stopped working. I pulled it and checked the connections and they were fine.

Does any one know how to test the speedo on the bench, i.e. what voltage/current should i apply to what terminals to get a reading.

I have heard that on some Porsche's (my 924s) the sensors on the trans-axle went bad some times.
where are they, do they go bad, how do you replace them?

[Grady Clay]

David,

I’m assuming the other instruments and instrument lights work.
Be careful working behind the dash as there are some circuits NOT fused.
For this test, it is useful to have the battery ground ready to be disconnected or fuse the ground.

Pull the speedometer instrument out just enough to get to the wires.
The first step is to check to see if the instrument has 12 V power.
This will be the red/black wire to the 12v terminal (the other terminal is 31b with a brown/red wire).
Test with a voltmeter or light bulb type tester to a suitable chassis ‘ground’.

If the instrument has power, the next step is to disconnect the brown/red wire from terminal 31b.
Connect an ohmmeter continuity tester between the brown/red wire and a suitable chassis ‘ground’.
Move the car a few feet and observe the continuity. It should change from ‘open circuit’ to continuous to ground and back to open eight times per revolution of the differential (both rear wheels moving the same).

If no change, is it open circuit or continuous to ground?

Please report your findings.

Best,
Grady

[masterdave]

The instrument panel works fine, i have a clean 12V at the meter

The interesting thing is there are 2 brown and red wires, one is connected to a terminal, and the other is connected along with 2 solid brown wires, to ground. When measured alone both are continuous to ground. However if the one that is also connected to the brown wires, is connected to them, and measured, it is an open circuit.

thanks
dave

[masterdave]

problem solved, i split a wire when fixing the shifter linkage, soldered the wire and its all good.