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1990 964 C2 Erratic Speedometer Update

Electrical and Electronic

Vehicle Information: Model: 911; Year: 1990; Body Type: Cabriolet; Total Mileage: 108900;

I hope you don't mind posting what my problem ended up being; maybe it will help someone else down the road.The symptoms were a very erratic speedometer needle...sometimes jumping from 55 to 0 then back to 40, then back to 0, etc. There were no warning lights, and all of the other trip functions in the computer worked fine, except for the digital speedometer display in the tachometer; it would mirror what the speedometer needle was doing. My spoiler was getting quite the work out as well; it was doing the same tap-dance that the speedometer needle was doing. Since I was not getting any kind of ABS errors (which I would expect to see if one of the wheel speed sensors was faulty) I didn't feel like the problem originated from there. You were 100% correct when you said that Porsche's documentation was not complete in that time period. As it turns out, the 1990 Tiptronic cars speedometer pulse sender is located in the transmission control unit and not one of the wheel speed sensors as I had believed. The transmission gets it speed feedback directly from the pulse sensor, and since my car was shifting ok, the problem had to be elsewhere.Once the speed signal reaches the speedometer, it is conditioned and sent on along its merry way downstream to whatever other components rely on the signals output. (the spoiler for one) It's too late to make a long story short, but the problem was a cracked circuit board in the speedometer itself. I removed the speedo and sent it to Nichols Speedometer Inc. in Greenville, NC. where they repaired it and it has worked fine ever since.

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