Good news and bad news
Discussion
The Chimaera passed it's MOT today 
- advisories on headlamp reflectors (these were coming out anyway prior to respray) and handbrake efficiency (he should have pulled harder!) 
That was the good news, the bad news (this is actually good news timing wise) is that the speedo stopped working on the way home from the testing station.
It started to bounce around a lot whilst at a steady speed and then decided not to play at all. The car starts and runs well in all other aspects, it's just the speedo not functioning.
- advisories on headlamp reflectors (these were coming out anyway prior to respray) and handbrake efficiency (he should have pulled harder!) That was the good news, the bad news (this is actually good news timing wise) is that the speedo stopped working on the way home from the testing station.
It started to bounce around a lot whilst at a steady speed and then decided not to play at all. The car starts and runs well in all other aspects, it's just the speedo not functioning.
On all cars, the most likely issue is that the speed sensor, located close to the diff, has got clogged with gunge or hit by a stone and now isn't at the correct gap from the toothed wheel that is attached by the diff housing. The toothed wheel spins as the car moves and creates I guess a fluctuating magnetic field in the sensor, which ion turn makes the sensor give pulses to the speedo, and the speedo converts the timing of those pulses into speedo readings.
Clean the gap, and reset the sensor position so that it is close to, but not touching, the toothed wheel.
Page 250 of the Steve Heath bible explains all this and has a lovely photo.
The second possible fault is that the speed sensor has simply failed.
The third possible fault is the speedo has failed.
The fourth on later cars is that there is a conversion box under the dash, and that may have failed. Earlier cars don't have it......GKN diff cars?
The fifth possible cause is frayed wiring between the speed sensor and the speedo.
Having had a faulty sensor when I first had the car, which was replaced, I then had wiring rodent damage in October that year. Don't discount this - meeces are coming in for the winter at this time of year, and like to nibble. The cables come through the fibre glass under the little sunglasses triangle in the middle behind the seats, and then run under the driver's left arm rest along the top of the transmission tunnel..
Clean the gap, and reset the sensor position so that it is close to, but not touching, the toothed wheel.
Page 250 of the Steve Heath bible explains all this and has a lovely photo.
The second possible fault is that the speed sensor has simply failed.
The third possible fault is the speedo has failed.
The fourth on later cars is that there is a conversion box under the dash, and that may have failed. Earlier cars don't have it......GKN diff cars?
The fifth possible cause is frayed wiring between the speed sensor and the speedo.
Having had a faulty sensor when I first had the car, which was replaced, I then had wiring rodent damage in October that year. Don't discount this - meeces are coming in for the winter at this time of year, and like to nibble. The cables come through the fibre glass under the little sunglasses triangle in the middle behind the seats, and then run under the driver's left arm rest along the top of the transmission tunnel..
QBee said:
My bet for the most likely cause would be the sender
IF you have the Cearbont gauges, and IF the sensor is faulty, this is the part you needhttp://www.racetechdirect.co.uk/m0631-tvr-car-part...
About the sensor- I went to the effort of pulling one to bits to see how it ticked and why it had failed- its nothing more than a magnetic rod inside a wire coil next to the toothed wheel. The toothed wheels alters the magnetic flux in the coil as it rotates so it produces a very small voltage. The reason for the failure was water had got into the coil, and corrosion had made it go open circuit. A simple test is to unplug it and do a resistance test, and check if its open circuit- then its faulty. The data in the Steve Heath book about nice square waves coming out of it are completely wrong.
blitzracing said:
About the sensor- I went to the effort of pulling one to bits to see how it ticked and why it had failed- its nothing more than a magnetic rod inside a wire coil next to the toothed wheel. The toothed wheels alters the magnetic flux in the coil as it rotates so it produces a very small voltage. The reason for the failure was water had got into the coil, and corrosion had made it go open circuit. A simple test is to unplug it and do a resistance test, and check if its open circuit- then its faulty. The data in the Steve Heath book about nice square waves coming out of it are completely wrong.
I have just carried out this test and checked the resistance across the pins at the plug for the speed sensor. It is open circuit, so it looks like I need a new sensor. J400GED said:
nitrodave said:
how is the speedo stopping working good news, timing wise?
Because it failed after the MOT - with an LSD, the tester checked the brakes on the road with a decelerometer rather than on the rollers - if the speedo wasn't working, the car would have failed the MOT.Best of luck getting it sorted, certainly sounds like it's just not getting a signal
To correct my earlier post, I think I now remember that the early cars don't have a speedo conversion box, the later ones do. The difference is the gauges - the early gauges matched with the speed sensor output, the Caerbont gauges need the signal converting to understand it. Nothing to do with duffs or gearboxes.
J400GED said:
blitzracing said:
About the sensor- I went to the effort of pulling one to bits to see how it ticked and why it had failed- its nothing more than a magnetic rod inside a wire coil next to the toothed wheel. The toothed wheels alters the magnetic flux in the coil as it rotates so it produces a very small voltage. The reason for the failure was water had got into the coil, and corrosion had made it go open circuit. A simple test is to unplug it and do a resistance test, and check if its open circuit- then its faulty. The data in the Steve Heath book about nice square waves coming out of it are completely wrong.
I have just carried out this test and checked the resistance across the pins at the plug for the speed sensor. It is open circuit, so it looks like I need a new sensor. New speedo sensor fitted and the speedo works.
Couple of top tips for anyone else considering/needing to replace their speedo sensor:
1. Before you pull the old wires through the body, attach a trace wire to the sensor wires ( I just used insulating tape) so you can easily pull the new wires back through.
2. Before fitting the new sensor clean the threads in the sensor bracket (I sprayed some brake cleaner in there and wiped it with a cloth).
3. Before fitting the new sensor, wrap the cables round the body of the sensor and then tie wrap them so that they don't get in the way when you are doing up the new sensor (obviously, once the sensor is in place you need to cut these tie wraps prior to attaching the cables to the trace wire).
4. Lubricate the threads of the sensor and the bracket with copper grease or the likes to prevent the brass threads grabbing/binding on the steel threads on the bracket. Don't worry, there is a locking nut to lock it all in place.
5. I used a hacksaw blade like a feeler gauge to set the gap from the sensor to the diff pick up.
Hope this helps.
Couple of top tips for anyone else considering/needing to replace their speedo sensor:
1. Before you pull the old wires through the body, attach a trace wire to the sensor wires ( I just used insulating tape) so you can easily pull the new wires back through.
2. Before fitting the new sensor clean the threads in the sensor bracket (I sprayed some brake cleaner in there and wiped it with a cloth).
3. Before fitting the new sensor, wrap the cables round the body of the sensor and then tie wrap them so that they don't get in the way when you are doing up the new sensor (obviously, once the sensor is in place you need to cut these tie wraps prior to attaching the cables to the trace wire).
4. Lubricate the threads of the sensor and the bracket with copper grease or the likes to prevent the brass threads grabbing/binding on the steel threads on the bracket. Don't worry, there is a locking nut to lock it all in place.
5. I used a hacksaw blade like a feeler gauge to set the gap from the sensor to the diff pick up.
Hope this helps.
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