Symptoms:
All gauges work except for the speedometer, odometer and trip meter.
The speed shows 0 always.
The speed shows correct some of the time or for a limited time.
The speed jumps around at low numbers even when the car is not moving.
Check:
The first thing to check is the voltage at the connector of the speed sensor. The speed sensor is on the passenger side of the engine bay, in the middle by the engine. It is a cylinder that has a speed cable going to it from the transmission. It will have a 3-pin connector tied to it. Unplug this connector from the sensor. Then start the car, set your voltmeter to DC and check the voltage between pins 1 and 3 on the harness connector. This should be 6.5V when the car is running. Mine was showing 1V and was not steady, jumping around a few millivolts. Looking at the service manual the power unit drops this voltage from 12V to 6.5V and this voltage only goes to the speed sensor. If your voltage is not 6.5V then this fix might work for you.
Parts:
You will need to get a DC buck converter, 12V to 6.5V. I picked an adjustable unit to be able to select the output voltage.
You will also need a way to splice into the wires on the harness. I used Wago splices, easy to use especially for testing.
Wiring:
You will need to take the gauge cluster out to be able to get to the harness that plugs into the back of it. I spliced into the wires by the connector that goes to the Power Unit on the back of the gauge cluster. This is connector 271 in the manual. Pin 19 on the connector is the 12V power wire from the fuse block, this wire is Green. Splice into it and connect it to the 12V positive wire on the Buck Converter. Now splice into the wire that goes to pin 16, this is the ground wire and will be black. You will splice both ground wires from the Buck Converter to this wire. Now you will only have one wire left from the Buck Converter. This is your lower voltage output wire. Do not connect it yet, the voltage will be set high. Start the car and connect the voltmeter to the ground wire and to this output wire. There should be an adjusting screw on the Buck Converter, turn this screw until your output voltage is 6.5V. Once done turn car off. Now the 6.5V signal from the Power Unit is pin 18 which is a Red/White wire. You will need to remove this pin or cut the wire. You don't want to splice into this wire but replace it. If you cut this wire, make sure you tape off or heat shrink the wire from the connector. This wire will still have voltage on it, and you don't want it to short anything out. Now the wire that you cut, going into the harness, you want to connect to your last wire from the Buck Converter. This will send the 6.5V you just set to your speed sensor. Now plug the connectors back into the gauge cluster and set the cluster back in its place. Don't bolt it in yet until you go for a drive to see if the fix worked. If it works, bolt everything back together and you are done.
This was done on a 1986 Turbo model. I don't see a reason it would be any different but please check your manuals to make sure your year has the same pins and wire colors used that are mentioned here. I have been running with this fix for over a week and so far so good. It is nice to have a speedometer and not have to use the phone to see my speed.
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