240 General: Speedometer fault

[Clifford Pope]

A few days ago the speedometer suddenly stopped working. I had been on the motorway at a steady speed for about 40 miles. It was the entire unit - the odometer and trip stopped too.

After disconnecting the connector on the axle casing, I cleaned up the contacts - they looked clean despite the dirt on the outside - and next time it worked again perfectly. However, the same thing happened, and this time after 10 miles it suddenly stopped.

Obviously I need to look again at the sender connection, and also follow the wires back to look for faults. But are there any other likely causes? Can the unit itself develop an intermittant electrical fault? The dash supply seems fine - all the other instruments and lights etc work.

Can the sender become faulty? I tried removing it to look, but after releasing the screw it seems welded into the casing, and looks the kind of plastic that would crack if levered. Has anyone ever known one fail like this, and how do you extract it - remove the axle cover and press from the inside?

Any suggestions anyone, or is it almost certain to be a wiring loom fault at the back somewhere?

Thanks
Cliff Pope

[Penguinistics]

The speed signal comes from the ABS ECU - is the abs light on?

[Clifford Pope]

No, it doesn't have ABS. It gets its signal from a sensor screwed into the rear axle cover plate, sensing the rotation of the crown wheel.

Actually I have now just found the 700 series FAQ, and there is an excellent article on testing the speedo system. Unfortunately mine appears to be an intermittant fault, the worst kind for tracing. So I wondered if anyone could give a "next most common likely fault" after eliminating the usual corroded contact at the axle connector?

[redboy]

for what its worth, I had a similar intermittent speedo fault on a rover SD1- it turned out to be the sensor- but you couldn't tell anything by looking at it. If cleaning it made a difference, its probably the sender on the way out- or there's a broken wire near it that you jiggled just enough to get it working a bit until it shook loose again.
good luck!

[Clifford Pope]

I did the test suggested in the FAQ of rapidly touching test wires together to simulate the action of the sensor. I couldn't get a flicker out of the speedo, so assuming therefore it was faulty, I have swapped in a whole instrument panel known to be working.
It has made no difference - the speedo worked for about a mile this morning, but then went dead as usual. So I conclude that the wire test does not work. (I was a bit dubious that touching wires together rapidly could really mimic what the sensor does.)

So it must be the sensor or the wiring. Anyone any tips on getting the sensor out? Should I lever it, or twist it? I don't want to snap off some locating lug or have the end fall off into the axle.

[Clifford Pope]

Well, as I feared might happen, did. Despite copious soaking in WD40, and carefully trying to rotate the sender while pulling gently, the entire end snapped off flush with the axle casing.
I tried drilling a hole into the plastic and using a screw extractor, then a larger extractor, and finally I drilled a larger hole and epoxied in a coarse threaded coach bolt. That did manage to pull the sender half out, before ripping out again. Finally, after extracting lots of copper wire and bits of metal, I again managed to glue in another bolt, and eventually got the whole thing out.

At an earlier stage I tried to remove the differential cover, but off course the three bolts that were rusted and had rounded heads are at the top, and unreachable except by removing the axle.

What seems to happen is that the O-ring perishes and swells, and the alloy casing corrodes and locks the plastic solid. It's yet another example of one of those maddening things which if you knew was going to seize up after 200,000 miles, you'd have twiddled and greased every 20,000 miles as a precaution. Like the clutch slave cylinder and the Crank Position Sensor.

The good news is the speedo appears to work.

[Clifford Pope]

I put in a new sensor and it didn't work. The explanation is bizare and incredible;
Sensors have two terminals, and the connector can only be put on one way round. The wires are colour coded (green/white, and black on mine, different according to Haynes diagram), and these wire run all the way forward to the speedo, with a junction in the passenger footwell.

But careful examination of the replacement sensor showed that the car it came from had the two wires with polarity reversed, despite having the same two colours.
In other words, a sensor from a 93 car will not work on a 92, unless you cut the wires and reverse the colours, joining green to black and black to green.
This might perhaps explain previous posts from people swapping axles and finding inexplicable incompatibility between sendors and speedos.

This isn't the only example of this Volvo custom I have encountered. I once replaced the locking motor in the tailgate. The motor worked, but back to front. I had to reverse the colour coding, blue and yellow I think.