Hi. The speedometer doesn't work in my 1993 945T. 145k miles. I did the diagnostic test as described in the FAQ (below) by shorting the wires as indicated. No reading was measured on the speedometer. I therefore ordered a used 1994 940t instrument cluster (125 k miles on the clock). Swapped them today but this one did not register any speed either. So either the new speedometer is also bad or the problem is elsewhere. I tested the wires between the connector in the trunk area and the instrument cluster for continuity and they are okay. I have not tested the rear sender unit in any way yet. But, the short test procedure doesn't seem consistent with the other info in the FAQ saying that someone recorded an AC voltage from the sender with the car moving. (This is also pasted below.) So what should I do next? I tried disconnecting the sender and driving the car, but the ABS (brake) light did not come on (the light does work). Does the sender unit put out a pulsed closed signal (like the short test would suggest) or a pulsed voltage signal (like the AC voltage reading would suggest)?

Thanks
Steve


" C. Fault location.
REAR SENDER TEST:
Faults are usually either in the gage itself or the rear sender.
A very effective and easy way to diagnose the rear sender is to disconnect the rear connector on the differential case and connect two long pieces of wires to the plug coming from the vehicle wiring, bring the “extension” wires inside the car, start the car, and start shorting the wires at a pace of 4-5 shorts per second. If the gage is OK you should start to read a speed over 15MPH. The faster you short, the higher the speed.
If you have a reading, then STOP HERE, and instead of tearing your dash apart, remove the rear sender unit, clean it using DeOxIt electronics cleaner (available at Radio Shack and good electronics stores) or eventually replace it (and while you are at it change your rear differential oil!). See the notes in the Instruments FAQ File on replacement of the rear sensor connectors, which become corroded. Volvo sells replacements."


"[Tip] I had a dead speedo and odometer. Using my digital volt meter, I switched it to 10 volts AC range. I had a friend hold the leads where the wires go into the speedometer. I drove up to approximately 15mph, and as I did, the needle smoothly moved up towards 1 volt. As I slowed down, the voltage dropped smoothly as well. As far as I can tell, this means that the pulse generator in the rear end is working fine, as is the wiring that comes from it. If I had known this beforehand, it would have saved me major headaches. Now I can just concentrate on getting the speedometer fixed. [Editor] Make sure that there is no corrosion at the rear sensor connector. If there is, spray with electronic deoxidizer and clean it. You can take off the old plug and rewire it with new connectors that you can seal up with silicone sealer.[W Jepsen] After my speedo failed, I tried testing the sensor signals with an oscilloscope. They appeared fine. After installing a new speedo which failed to fix the problem, I installed a used sender and wire pigtail from Revolvostore ($30). That did the trick. Seems that the sender can show up with an oscilloscope test but not a good enough signal to power the speedometer. See the FAQ File from Davide D. on testing the rear sensor and isolating faults. [A Buxton] If you still have the tamper proof wire on the connector at the rear axle, cut it off. Over time it cuts into the sensor wire. [Rick Zimmerman] A really easy way to eliminate the speed sensor as a suspect in a speedometer failure is to disconnect its connector in the trunk. The signal from the speed sensor also goes to the ABS circuit. If your ABS light does not light up until you disconnect the speed sensor and goes out as soon as you reconnect it, then your speed sensor signal must be OK."