964 Speedo not working solved..

[rattlsnak]

I posted this in the 964 forum, but someone already asked me about putting a 91+ eng/trans into an earlier car and as far as i can tell, it would be as simple as installing the senor ring and sensor in the diff and the wiring and it all should work..

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I'm going to post this to help someone else who might have this problem down the road one day..

Car is a 1990 C2, eng and trans came out of a 1991 C2. Installed fine, runs great, but speedo does not work. Did major research and everybody had different opinions on why and what car has what electronics.

Here are the facts as I know them: 89-90 cars with manual trans used a magnectic sensor ring that bolts to the back of the ring gear and a sensor that mounts in a hole in the diff cover which send a pulse signal to the speedo.

91+ cars uses the left front ABS sensor for input, which goes directly to the ABS module where an output is sent to Relay #52 in the the central electronics panel where it is converted to a pulse and then sent to the speedometer.

Besides that confusion, my 91 trans had the hole in the diff cover with a plug in it, so I figured, why can't i just mount the sensor, run two wires to the engine connector (since the 90 model was wired for it) and the speedo should work.

But, as I found out, the sensor ring is not installed on the 91. SO the next step was to compare the wiring diagrams and part numbers of speedos and modules between 90 and 91. Hmmm, all are the same except for one wire which controls the additional spoiler relay on the 90 model. So I figured, I must be able to run the wires from the ABS module to a relay connector and get the missing relay and it should all work. Well, as i was checking that, I noticed that my relay panel has the spot for relay # 52 and it had all the terminals already installed. A quick check with an OHM meter confirmed it. I simply bought a relay#52, (blue color and NOT cheap! @ $210.00!) and presto, speedo works fine.

What has happened, is all the Tiptronic cars used this system from day one but for some reason the factory still used the mag sensor in 89 and 90 manual cars and so all the cars should be prewired already, so its a simple as installing a relay and everything works.

The 89-90 manual versus the Tips and 91+ cars reminds me of the engine fan issue as well, but thats another story!!

Hope that helps someone in the future!