Speedo stopped working…

[Riff Raff]

After some advice. On the way to a local garage to have the oil and filter changed I noticed the speedo wasn’t working. It had been working the day before. When it was in the garage and up on the ramp I asked them to check over the wiring for the sensor at the diff. The wiring looks OK.

So, the question is what to do now? Is it worth pulling the dash top off to check the wiring behind the gauge? Do the instruments themselves ever fail?

Or do I just take the sensor at the diff out and give it a clean?

Suggestions for the quickest and cheapest way forward welcomed.

[blitzracing]

The sensor is just a coil of wire so you can unplug it and ohms test it first off. Its likely to be some k ohms but open circuit means the sensor has failed.

[captain.scarlet]

blitzracing said:

The sensor is just a coil of wire so you can unplug it and ohms test it first off. Its likely to be some k ohms but open circuit means the sensor has failed.

Would a failed sensor not trigger a dashboard light and error code?

I've had two vehicles with faulty speed gauges. No error code or dash light. One registered no speed or mileage, only revs, the other does register revs and mileage, and speed up to a certain point before dropping or being temperamental.

I'm certain I had the sensor changed, thinking that would be the end of it but, but the problem remains.

Sensors aren't dear so replacing that rules that bit out, but if no light or error code is generated then I'd guess from my experience that the speedo needs repair/attention/replacement, hopefully with no mileage discrepancy.

[Steve_D]

Do you have RoverGauge? If so plug it in and go for a drive. It will not show the true speed and will probably read about 35mph.
If you do get a reading then the sensor is working. That leaves the speedo itself or the connection between the speedo and TVRs 'black box'. Or the black box itself which is less likely.
The speed signal from the sensor is not compatible with the ECU so TVR fitted the black box. It just reads the signal and just tells the ECU 'we are stopped' or 'we are moving (35mph)'. I doubt the 'box is an issue as the signal goes straight through.

Steve

[Riff Raff]

Steve_D said:

Do you have RoverGauge? If so plug it in and go for a drive. It will not show the true speed and will probably read about 35mph.
If you do get a reading then the sensor is working. That leaves the speedo itself or the connection between the speedo and TVRs 'black box'. Or the black box itself which is less likely.
The speed signal from the sensor is not compatible with the ECU so TVR fitted the black box. It just reads the signal and just tells the ECU 'we are stopped' or 'we are moving (35mph)'. I doubt the 'box is an issue as the signal goes straight through.

Steve

Thanks Steve. Unfortunately Rover Gauge won’t do me any good, as the car has one of Dom Tricket’s MBE ECUs. Maybe I should give him a call, as the car is partially rewired as part of the conversion process, and I’m not even sure it still has the black box.

Blitz’s suggestion looks like a quick way to tell if the sensor is shagged. I’ll see if I can give it a go getting at it on axle stands.

[Riff Raff]

So, thinking about the MBE, it has an OBD port. I should be able to see a speed signal on that. If I can get hold of a reader!

[Belle427]

The sensor circuit isn’t usually altered with an aftermarket ecu.
Sensors do fail, you can get to the wiring connector under the centre console at the rear and continuity test it.
Bit of a pig to change it though.

[Steve_D]

Normal failure mode is the wires at the sensor get crisp over time and sitting above the exhaust and finally fracture just where they go into the back of the sensor.

Steve

[blitzracing]

captain.scarlet said:

Would a failed sensor not trigger a dashboard light and error code?

I've had two vehicles with faulty speed gauges. No error code or dash light. One registered no speed or mileage, only revs, the other does register revs and mileage, and speed up to a certain point before dropping or being temperamental.

I'm certain I had the sensor changed, thinking that would be the end of it but, but the problem remains.

Sensors aren't dear so replacing that rules that bit out, but if no light or error code is generated then I'd guess from my experience that the speedo needs repair/attention/replacement, hopefully with no mileage discrepancy.

It wont flag anything visable on the dash board as even if the ECU picks up a speed error the MIL is not enabled on the TVR tune in the ECU if you are running a 14cux.

[geordiepingu]

Check the basics first. You can measure the output of the sensor using an oscilloscope, or even a multimeter if you know what you're looking at. You can then test this signal at the back of the speedo gauge, and then before the speedo transducer, to see if that box is causing the problem, or to trace a wiring fault or even a sensor fault.

More often than not though, crud magically works its way in, or the lock nut on the sensor is loose and vibration knocks it back just enough to not read the teeth. Give the sensor a clean and then check the distance from the teeth is adjusted correctly.