Edtay

So, when I got the 1986 944N/A out of storage I noticed my speedometer isn't moving. Since its been back on the road, I've swapped out the trans sensor and also got another cluster to see if swapping those fixed my problem but no dice. My first speedometer looked like it got burnt on the circuit board and I tried to repair it but decided to just get another working cluster. What else would stop the late speedometer from working? I'm under the impression that there's a short in the wiring but I don't want to go tracing just yet.

PS: The speedometer worked before it went in the locker.

Tom M'Guinn

I'd probably start by confirming that the speedometer signal is making it's way from the sensor to the cluster. It's on a blue wire with red strip -- pin 14 of the middle edge connector behind the cluster. If that has continuity, then see of the sensor is putting out ground pulses by hooking up a multimeter to that blue/red wire and seeing it you get ground pulses when the car moves forward very slowly. It should pulse between ground and an open circuit 8 times per tire revolution -- so just let the car roll very slowly to see if it's changing back and forth on. If it isn't, and if the speedometer, sensor and harness is good, the problem might be inside the transaxle? The burned circuit board is also curious -- have you checked for burned out fuses in case there is a power short somewhere?

Edtay

Hey Tom, how would you go about checking for continuity? I put a voltage meter on pin 14 and to the metal around the vents and slowly moved the car. With the voltage in the range of 0-10V, I was getting a slight variance/fluctuation (sub 0.5V) but not enough to rule out just a bad meter (it was cheap).

In regards to the burnt up board, I did a little bit of digging and supposedly this happens on VDO clusters. I replaced some of the components (a capacitor seems to be the cause but its been 2-3 months since I looked at that board). The resolder job didn't fix my issue and I also reflowed some of the joints too. I didn't find anything fuse wise for the failure. If a fuse popped, wouldn't it take out lights to the dash as well?

Turbodan

a few years ago the speed sensor on the transmission went on my 87 951. It is easy to see/ change on these cars.

Tom M'Guinn

Continuity means no, or almost no, ohms of resistance. I was saying to test the resistance on that wire from the sensor to the cluster, just to make sure there is no break. Worth getting a good meter. To test the pulse itself, set the meter to Ohms, and put one probe on ground and the other on pin 14. If you then roll very slowly, you 'should' see it toggle between no (or a few) ohms and something else 8 times per tire rotation. If the wiring is good and yet you get no pulse, then the sensor is probably bad.

FRporscheman

How fast did you drive? Did you try tapping the cluster while moving?

My 951 was sitting for 4 years, and now the speedo sticks at 0 until I get to about 25-30 mph. Unless I gently tap the cluster, which wakes up the speedo at any speed.

Tom M'Guinn

Mine did this too. I didn't mention it because the OP said he swapped clusters (and sensors) but if they had both been inactive for a while it's a good point -- there's a decent chance they could both be sticky. If swapping the cluster and sensor with known good units didn't/doesn't fix it, then the only things left (that I can think of) are to make sure the wiring is good and that there isn't something wrong inside the transaxle where it screws in...