Can you test a speedo transducer?

[Mags]

Before I fork out on a new one, can I test the existing one has failed as I assume its the most common reason for the speedo to give up with no other problems?

Mags

[Steve_T]

Steve Heath does something to help with testing the speedo iirc.

[Mags]

Steve, You are correct, however:

Steve Heath Engineering Ltd has ceased trading from 31st May 2012.

My remaining stock of ARB drop links, bolts and poly bushes etc., will be transferred to another company, which intends to continue to stock and sell these products. Contact details will appear here when they are up and running with these products.

[Tanguero]

The easiest way to test the speedo transducer is to hook it up to an oscilloscope in situ with the back of the car on stands so the wheels are both off the ground. If you run the engine in 1st gear you will be able to see and time the pulse train from it. The output is too fast and to low a level to be measurable by any other means.

Steve Heaths tester is a pulse generator that you can plug in in place of the sensor in order to confirm that the rest of the circuit is working correctly.

Almost all speedo problems are because of either crud on the sensor or too large a gap to the wheel on the diff. Try removing it, cleaning it and resetting it as close to the toothed wheel as you can safely get it without touching.

[Mags]

Thanks Tanguero, I don't have access to an oscilloscope so will start with the simple option of a clean and re-fit this weekend.

Mags

[Mags]

I took the sensor off, cleaned it up and put it back on set to 0.1mm gap and no joy. After driving for a while the speedo seems to jump into life over 40mph and works for a bit, then stops working again. I have no idea where I can borrow a 'scope from.
Does it matter if the trigger wheel (CV joint) has some surface rust? I cleaned it up best I could.
I'm guessing I need a new sensor?

[Tanguero]

If it works sometimes then the sensor is probably ok. If they fail it tends to be permanent, its just a coil of wire round a piece of iron at the end of the day. As it is an intermittent problem I would be looking at the wiring in between the sensor and ECU and the ECU and the speedo its self.

[ukkid35]

I bought a USB scope off eBay and separately a set of probes for about £25 total in order to try to trace the speedo fault on mine. I found no signal from the replacement transducer, which I'd set too close so now there's evidence of rubbing. Not happy.

[gruffalo]

Mags said:

I took the sensor off, cleaned it up and put it back on set to 0.1mm gap and no joy. After driving for a while the speedo seems to jump into life over 40mph and works for a bit, then stops working again. I have no idea where I can borrow a 'scope from.
Does it matter if the trigger wheel (CV joint) has some surface rust? I cleaned it up best I could.
I'm guessing I need a new sensor?

I had exactly this issue, fitted a new sensor, problem sorted.

[x 7usc]

Check the plug and socket connection of the sensor (sensor to loom) before buying a new one.

[Mags]

Thanks. I should have mentioned that it looked like the sensor had been touching at some point in the past as the end of it was clearly marked where it had touched the trigger wheel.
Was looking at the USB scopes, need to understand how to work it, I'm erring towards a new sensor though, being lazy....

[Rufus Roughcut]

I've looked at the schematics and from what I can see there is a slate coloured wire from the transducer going to splice joint 16, from there it either goes to the Door lock ECU or multiway 1 pin 26 and then into the speedo.

I'm assuming you could check the sensor by trying to open the door when the car is moving, obviously not to be tested on the public highway

[Mags]

Rufus Roughcut said:

I've looked at the schematics and from what I can see there is a slate coloured wire from the transducer going to splice joint 16, from there it either goes to the Door lock ECU or multiway 1 pin 26 and then into the speedo.

I'm assuming you could check the sensor by trying to open the door when the car is moving, obviously not to be tested on the public highway

I like your thinking, so if the signal from the transducer isn't reaching the ECU then the door will open? narrowing it down to the cable or transducer?

[Mags]

New sensor fitted, problem solved.
Thanks for all the answers.