Speedo not working

[Belle427]

My speedo stopped working last week, cables to back of it look intact.
I've got the car in the air at the moment for something else and the sensor looks to be in a pig of a place.
Any advice on where to start?

[Bristol ave fag]

I know on some not all there is a black box under the dash top that receives the signal from the transducer then passes it on to the speedo. You could run the car up on stands and check for the signal there.

[Steve_D]

Do you have rovergauge?
If so plug it in and go for a drive.
RG will not show an actual speed but if it comes up from zero then a speed signal is getting to the speed module behind the dash.
The module and the speedo see the same signal so there is either a wire break behind the dash or the speedo is dead.

Steve

[Belle427]

Thanks, I’m Megasquirt now so not able to rovergauge it.
Will do some basic wiring checks first, car is a 1995 model and I’ve not noticed any black boxes under the dash top.

[Steve_D]

Belle427 said:

Thanks, I’m Megasquirt now so not able to rovergauge it.
Will do some basic wiring checks first, car is a 1995 model and I’ve not noticed any black boxes under the dash top.

The 14CUX needs a speed signal but the speedo signal from the diff is not compatible so TVR built the black box to convert the speed signal into something it could understand. Whoever converted to MS may have removed the box.

Steve

[Belle427]

Steve_D said:

Belle427 said:

Thanks, I’m Megasquirt now so not able to rovergauge it.
Will do some basic wiring checks first, car is a 1995 model and I’ve not noticed any black boxes under the dash top.

The 14CUX needs a speed signal but the speedo signal from the diff is not compatible so TVR built the black box to convert the speed signal into something it could understand. Whoever converted to MS may have removed the box.

Steve

Thanks, I converted it and didn’t remove it. I think I found it last night so will go ahead with some basic checks, maybe it can be removed now the 14cux is gone but will need to get some info first.

[ChimpOnGas]

Belle427 said:

Thanks, I converted it and didn’t remove it. I think I found it last night so will go ahead with some basic checks, maybe it can be removed now the 14cux is gone but will need to get some info first.

I wouldn't remove it

Also, you really need an oscilloscope to check the waveform from transducer to speedo and all the connections between the two.

[Dougal9887]

Same problem here
After rebuilding the chassis it hasn't worked except briefly after I twiddled the wires in the back of it, but no luck since. The connector above it seems fine and I'll check the connector under the centre console, but don't expect to find a problem there. Looks like I'm in for a new sensor.
Dougal

[Belle427]

I looked at mine yesterday, one of the metal tabs of the trigger wheel was slightly bent allowing it to contact the sensor so I’ve ordered a new one.
There are some Upgraded ones for sale in the classifieds by a member here at a very reasonable cost.
I ended up removing the exhaust and prop shaft for easier access but it helped me with my battery relocation job anyway.

[Dalamar]

Belle427 said:

I looked at mine yesterday, one of the metal tabs of the trigger wheel was slightly bent allowing it to contact the sensor so I’ve ordered a new one.
There are some Upgraded ones for sale in the classifieds by a member here at a very reasonable cost.
I ended up removing the exhaust and prop shaft for easier access but it helped me with my battery relocation job anyway.

You can actually test the output of the speedo transducer attached to the diff using a digital voltmeter set to AC. The output from the transducer is quite low and if I remember correctly I was getting around 1.3V AC at around 30 mph.

I got this tip from Speedycables a few years ago when I was diagnosing my fault speedo.

[blitzracing]

The sensor is just a wire coil with a magnet inside, and not much to go wrong- although I have seen water ingress corrode the joints inside and it goes open circuit. You can do a basic continuity test on ohms, and see if you get a reading- I dont remember the value, but chances are if its in the k ohms region or below its fine. Open circuit its not OK. The two wires feed under the dash to the speedo calibration unit and if you unplug this you can continuity check it hear as well. From memory the signal is in parallel to the back of the speedo, so the box does not do anything to the speedo signal, only to create the fake signal the ECU sees.

[Steve_D]

The normal failure mode for the sender is broken wire just as it enters the back of the sender. I have repaired one by grinding out the epoxy on the back of the sender and making new solder connection. Very time consuming so not a valid option on a customers car.

Steve

[Belle427]

This is the one I used, not tested yet but half the cost of a Tvr supplied one.
https://www.pistonheads.com/classifieds/parts-and-...

[blitzracing]

I bet its a bog standard CPC or RS components part.

[Belle427]

So new sensor fitted and all is well.

[laracer]

Do you have the black bezeled or the silver bezeled instrument dials?

[Belle427]

Neither mine are a cream colour.!