TCS says right rear speed sensor, but I don't think that's the problem. Help

[carlosinseattle]

I have a 94 Q45a which I bought around 18 months ago. It was a project car from the beginning, and I'm finally starting to make some progress. the car is TCS equipped, which is what's causing my problem. I'm getting code 7 from the TCS CU. I can't understand why. I've Ohm checked the speed sensor and it checks out fine. I've checked the connection between the harness and the TCU CU and all is well there too. Here's what happens. When I first start the car it's fine, but after driving for about 30 seconds the ABS, Slip, and "TCS off" lights all light up at the same time. I've checked the rotor for cracks or deformities and it looks even better than the one on the rear left. This problem all began about a year ago when I was stuck in some snow. I was spinning the trires trying to melt the snow directly under the rear end. The TCS was kicking in, so after a few tries I hit the switch to turn it off. The next day when I drove the car is when I began having the problem. Can anyone help me figure out what to do next? The sensors are expensive, pretty difficult and time consuming to replace, so I'm trying to eliminate any other potential problems. And I think the sensor is fine because when I first start the car and accelerate hard I can cause the rear wheels to lose traction, and the TCS system kicks in flawlessly to stop wheel spin.I checked battery voltage both with the vehicle running 14.18 volts, and with the motor off 12.01. I replaced the alternator a year ago too, and, had it tested before installation to be sure it was good. I also replaced the battery around 6 months ago. Anything else I should check????

[jimbyjimb]

Maybe the control unit is the problem. Any debris clogging up the sensor? I'm sure you would have noticed it if there were, but alot of time crap gets sandwiched in there and that's the problem. It's strange that it still works seemingly correct. But if the sensors are cool and there isn't anything clogging them up maybe the computer is having an internal struggle, or a ground problem, bla, bla. I can't say I'm a pro at TCS systems on Q's however.

[elwesso]

I would throw a VOM on the VSS (on the actual sensor and the sensor via the control unit) and see if voltage drops off at all.. Once it sees that the sensor drops off to an unreasonably high or low voltage (short or open circuit) then it throws a code and goes into failsafe.

At least confirm that the TCS CU is correct in its assumption of the sensor failing.

[carlosinseattle]

I would throw a VOM on the VSS (on the actual sensor and the sensor via the control unit) and see if voltage drops off at all.. Once it sees that the sensor drops off to an unreasonably high or low voltage (short or open circuit) then it throws a code and goes into failsafe.

At least confirm that the TCS CU is correct in its assumption of the sensor failing.

Wes, I understand part of what you're saying, but not all of it. I can put a VOM on the sensor directly and spin the wheels with the car jacked up to see what the sensor says, but I don't understand how I can power the sensor with the connector unplugged. And I definitely don;t know how to check the voltage at the TCS CU, unless I attach small wires to the back of the CU while spinning the wheels with the car jacked up. Is that what you're suggesting? Help me understand the process you're describing.

Also, I read some really old post and they suggested it might be the TCS or ABS relay causing this roblem. I'll check those this weekend and see what happens.

Anything else I can do?

[elwesso]

I thought the sensor was just a pulse generator? you can always probe the terminal... I would still try testing at the control unit first, because if voltage is dropping off at the CU you know its somewhere in there...

What I would do is take some straight pins and shove them in the harness side. They make them so you can do that. Then just hook some leads up and ideally you'd like to put the VOM in the car and drive.