Speed Sensor Wire Test

[Green95M3]

I'm in the middle of a complete rear suspension refresh. When I lowered the diff, I stupidly forgot to disconnect the speed sensor wire until I noticed it was being pulled. Before I put everything back together, I want to test the wires and be sure they didn't break, since trying to repair them after the whole rear end is reassembled would be a nightmare. The wiring diagram indicates that the 2 speed sensor wires go directly to pins 10 and 21 of the instrument panel. It would be a quite a bit of work to get access to the connector on the back of the instrument panel to do a continuity test so I'm trying to figure out other ways to test for an open circuit on both of the wires.

One possibility might be to simulate the speed sensor signal and see if the speedometer moves. However, that would probably be tricky since it's a low level pulsed AC signal as far as I know.

I don't have a wire tracer, but I suppose I could get one and touch the transmitter to either of the 2 pins on the connector and test with the receiver further up the wire to see if there is a signal.

I'm fairly sure the 2nd option would work but I'm wondering if there is a better way to do this.

I'm open to all ideas here.

Thanks

[JDStrickland]

First of all, the only wires that need to be checked will be the ones that were stretched. You do not need to check the wires inside of the car, you only care about the wires from the connector to the speed sensor itself. The wheel speed sensors do not go to the speedometer, they go to the ABS controller. There is a Vehicle Speed Sensor lives in the top of the differential. Because the sensor is essentially a magnet that sets up a square wave as the tone ring goes by, you should see a low ohm condition if it works and an open if it does not. Your meter probably does not change fast enough to see the square wave, but if you see an open then you can move the hub a few degrees and observe the meter to go from a high to a low resistance. Replacing a wheel speed sensor is not a huge problem, you can do it with the tire removed. The hard part comes if it has been corroded in place.

[JDStrickland]

Lowered the diff... Read all of the words. Sheesh. Sorry.

POINT OF ORDER
The pin numbers you cite are reference numbers that take you from Page 2 of 2 to the diagram that continues on Page 1 of 2. The actual pin numbers on the Instrument Cluster are Conn C, Pins 3 & 4.

My confusion of what you are doing does not change the description of the sensor, it creates a square wave caused by a magnetic field that collapses as the teeth of the tone ring whiz by. The speed sensor torque value for the mounting screws is 53 - 74 inch pounds.

[Green95M3]

Thanks for the reply. You are absolutely correct!

My assumption that the instrument panel would be difficult to remove was completely wrong. Bentley says the steering wheel has to come off which is not the case. It's the easiest thing I've done on this car! LOL.

There is a connector on the back of the panel (X271) where the speed sensor wires from the diff terminate (Pins 3 & 4) and I was able to test continuity from end to end of both speed sensor wires. Apparently and fortunately they weren't pulled hard enough to break them when I lowered the diff.

Crisis averted.

Thanks again for the input.

[JDStrickland]

POINT OF ORDER...
You can unplug the connector at one end and use a paper clip to short two pins (Pins 3 & 4 in this case), then go to the opposite end of the wire and check those pins for continuity. Two wires tested at once.

When testing for a short, unplug one end of the wire and go to the other end and check continuity to ground.