Speedo not working - please help

[brubaker19]

So I've had Trevor for 10 years, a 1999 4L Chimaera. The speedo was always a little difficult with the standard jump to 10 mph after moving for a while. The rev counter then started to become more problematic and started dancing whilst driving up and down. I tried to get someone to fix them but no one could take it on. I eventually ended up buying two new ones both from Caerbont.

I left them into my mechanic a year and a half ago to get them fitted. I'll not go into the story but boiled down the TVR is back with me and the speedo still doesn't work. He fitted both dials but couldn't get the speedo to work. Initially he said it was because of the Transducer so I ordered and got a Speedo Transducer - M0631 from TVRSSW and he fitted it. It still does not work, £1,000 later. TBH I feel a little like committing hari-kari in the garage but my wife wouldn't be impressed instead she wants me to get the car fixed so it can be sold. I can get the car MOT'd until the speedo is fixed.

So going up the road the speedo when the entire turns on goes from 0-180 and drops back to 0 (this would appear normal) when driving it says at 0 whilst building up speed and then jumps to 30. When driving at a constant 30 is jumps and drops between 30 and 20, occasionally dropping back to 0. This is more or less constant.

Is there anyone who has experience of this and can suggest a fix for this. I really am at my wits end and it would have been cheaper to drive the car to the mainland and get someone there to do it (we live in Northern Ireland) and the one guy who fixed TVRs here has retired, he was brilliant. If someone can come up with a solution I will try to fix a mechanic here who could implement it.

Really desperate, everything else with the car is 100% but this essentially prevents me driving Trevor and selling him.

All suggestions received.

Peter

[Belle427]

I would first start by checking the air gap at the sensor, it has been known for some sensors to be set a lot closer than others to give a stable signal.
Only other thing i can suggest is checking the compatibility of your new gauges with regards to calibration etc.

[s p a c e m a n]

I've never had a car fail and MOT for a non working Speedo. As long as it is fitted and not visibly broken it should pass, if that is the only reason you are looking to fix it I would just mot it as it is and don't mention it to the tester.

[Daryl357]

How does it drive ? Does it stall at junctions or anything ? I wonder if it's related to the 'speedo interface box'. I think you need an auto electrician, to look at the speedo waveform, rather than a mechanic, and as long as the speedo sensor gap is set correctly.

[8Speed]

Belle427 said:

I would first start by checking the air gap at the sensor, it has been known for some sensors to be set a lot closer than others to give a stable signal.
Only other thing i can suggest is checking the compatibility of your new gauges with regards to calibration etc.

^ This.

My speedo stopped working recently. I bought a new sensor & it only worked sporadically after fitting.
I then found out from helpful people on this forum that the correct sensor gap is different for black bezel gauges & silver ones.
My gauges are silver bezel & once the sensor gap was reduced to 0.3 mm it worked fine.

[M16rky]

Try Jody at Python Racing, I sent my dash in to him from my Tamora and he fixed it, it's been a year so far and no more problems, was only a couple of hundred pounds too - 01206 820044

[brubaker19]

Thanks folks,

I think I will start with reducing the gap in the sensor. I’m not sure what the mechanic set it at but I will see if this fix works and I will get to you hopefully next week,

Many thanks for all the help and useful steers.

Peter

[blaze_away]

Not sure this is helpful but heres how I fixed mine when mileage got stuck at 199,999 and yes I thought it was far fetched but it worked.

Remove dash top
Loosen speedo bracket and swivel speedo 280 degrees so it's upside down.
Jack rear wheels off the ground
Start the engine
Put it in 2nd gear and let rear wheels rotate so speed is about 20mph at idle
Tap the speedo gently with a heavy rubber handled screw driver until it starts working properly again

I ASSURE YOU IT WORKED ON MINE !!

Edited by blaze_away on Tuesday 30th April 15:54

[keeling54]

If you are still having issues, speak with John at ETB Instruments.

I got a replacement speedo and rev counter from them as my originals had failed.

[Colin L]

I had my speedo repaired by JDO Instruments in the past month

Great service, one week turn around and works perfectly

They are in Keighley, West Yorkshire.

Colin L

[blaze_away]

Colin L said:

I had my speedo repaired by JDO Instruments in the past month

Great service, one week turn around and works perfectly

They are in Keighley, West Yorkshire.

Colin L

Very useful info !
Thanks

[blitzracing]

A little science behind it all. The diff sensor is just a coil of wire around a magnet. As the toothed wheel goes past it , it alters the magnet field enough to produce a tiny voltage. This voltage goes up a bit as the seed increases which is why Speedos suddenly burst into life.The Speedo has to be sensitive enough to trigger on this small voltage but it has a long cable run from the back plus the car is glass fibre. This means earth's are super critical or electrical noise gets on the signal and it won't trigger the Speedo as it should. There is also a piggy back box that sits in parallel to the Speedo input and grabs the signal to feed into the ECU. This produces a fake speed signal of between 30 and 50 mph to allow the idle control to work correctly so if you have RoverGauge you can see if the ECU is getting the speed signal as it should. In terms of trouble shooting, get the sensor and close to the toothed wheel as practical and make sure the connection at the back are clean and corrosion free to maximise the voltage.

[8Speed]

blitzracing said:

A little science behind it all. The diff sensor is just a coil of wire around a magnet. As the toothed wheel goes past it , it alters the magnet field enough to produce a tiny voltage. This voltage goes up a bit as the seed increases which is why Speedos suddenly burst into life.The Speedo has to be sensitive enough to trigger on this small voltage but it has a long cable run from the back plus the car is glass fibre. This means earth's are super critical or electrical noise gets on the signal and it won't trigger the Speedo as it should. There is also a piggy back box that sits in parallel to the Speedo input and grabs the signal to feed into the ECU. This produces a fake speed signal of between 30 and 50 mph to allow the idle control to work correctly so if you have RoverGauge you can see if the ECU is getting the speed signal as it should. In terms of trouble shooting, get the sensor and close to the toothed wheel as practical and make sure the connection at the back are clean and corrosion free to maximise the voltage.

Thank you - this sort of expert advice is invaluable.

[brubaker19]

So these are my dials. This is with the car off. Perhaps a stupid question but shouldn't the rev counter be a 0 when the car isnt even on?

[brubaker19]

8Speed said:

Belle427 said:

I would first start by checking the air gap at the sensor, it has been known for some sensors to be set a lot closer than others to give a stable signal.
Only other thing i can suggest is checking the compatibility of your new gauges with regards to calibration etc.

^ This.

My speedo stopped working recently. I bought a new sensor & it only worked sporadically after fitting.
I then found out from helpful people on this forum that the correct sensor gap is different for black bezel gauges & silver ones.
My gauges are silver bezel & once the sensor gap was reduced to 0.3 mm it worked fine.

I' ve stuck up a picture of the dials there are the standard replacements from caerbont, I'm assuming these are the silver bezels. I'm hoping to get it to the mechanic this week to see if reducing the gap will work.

[phil430]

Hi, I had exactly the same problem with a caebont speedo, did your mechanic fit the frequency booster as per the instructions? If so remove it, I did this after much messing around and now it works fine, hope this helps

[sixor8]

brubaker19 said:

I' ve stuck up a picture of the dials there are the standard replacements from caerbont, I'm assuming these are the silver bezels. I'm hoping to get it to the mechanic this week to see if reducing the gap will work.

They have become the standard because the analogue ones can no longer be repaired due to lack of spares. The Chimaera only got digital ones from the factory for the last 2 years (2001-2002), but since the digital speedo / odometer has no warning lamp apertures, you have to change both gauges if the analogue one fails completely. Connections at the rear are different too.

N.B. Yes, the very first ones in 1993 had digital speedos but they looked very different and are know for internal battery failure losing the mileage display.