Speedo malfunction, cure?
Discussion
phazed said:
Thanks for that.
I will put it up on the two post lift tomorrow and check out the wiring from the sender. Hopefully it is something simple.
I just wondered if there was some sort of electrical test that I could do with the sender?
As I am sure you know, the sender has to be a precise distance from the toothed wheel on the diff.I will put it up on the two post lift tomorrow and check out the wiring from the sender. Hopefully it is something simple.
I just wondered if there was some sort of electrical test that I could do with the sender?
If the little blighter has been knocked by a stone thrown up from our less than perfect roads, or has come loose and moved away from the toothed wheel, that would cause there to be no signal. I do remember that the sender has to sit very close to, but not touching, the toothed wheel.
I cannot remember if there is one wire or two coming from the sender, but I would guess that you ought to be able to see the signal pulses on your multimeter if you put the earth lead to the chassis and the live one to the sender output, and rotate the rear wheels by hand. Not sure.
phazed said:
I'm all ears. How would I test it?
Put the multimeter on the two wires and turn the wheels?
Exactly that. The same as testing ABS sensors. It's just a little 'generator', the toothed wheel induces a current in the windings of the sensor. You might not be able to get enough speed hand spinning the wheels, but run it in gear with the engine ticking over and you should get something. Also check for continuity of the sensor's windings (plug disconnected). Put the multimeter on the two wires and turn the wheels?
If both speedo and odometer have stopped , that points to a faulty transducer etc. If either the speedo or the odometer stop but the other works, points to a defective speedo. So days the Bible, but logical when you think about it
They do seem to have a habit of just suddenly stopping working. Two types IIRC.
Bible also mentions a fuse?
They do seem to have a habit of just suddenly stopping working. Two types IIRC.
Bible also mentions a fuse?
Edited by TarquinMX5 on Monday 18th February 14:12
Not sure of year of your car, but looks like a facelift. There are a couple of fuse boxes, not helped by a different versions fitted to some 97/98 cars due to supplier problems.
Having said that, fuse for the speedo generally, but not always, also supplies reverse lights / brake lights on 93-95 cars and 98 cars (with two separate cooling fans). On interim 97/98 fusebox (identified by 6 relays on top line, 5 on bottom, no spare fuses alongside relays) fuse controls speedo/reverse lights and door lock.
It's a 10A fuse. Unfortunately, there are 7 different fusebox layouts in the Bible, so depends what year/layout you have.
Should be a layout in the handbook, but my fusebox was different to that listed in the handbook.
Some early 95 cars don't appear to have 'speedo' listed on the fusebox list, so possibly the 15A fuse that feeds 'instruments and heater'.
Sorry, difficult to be more precise without knowing which fusebox you have.
My gut feeling is that the transducer has packed up, as it seems to have been a not-uncommon fault, but it's an expensive 'try it' way of diagnosing. That's what mine was, fine one day, absolutely nothing the next.
Having said that, fuse for the speedo generally, but not always, also supplies reverse lights / brake lights on 93-95 cars and 98 cars (with two separate cooling fans). On interim 97/98 fusebox (identified by 6 relays on top line, 5 on bottom, no spare fuses alongside relays) fuse controls speedo/reverse lights and door lock.
It's a 10A fuse. Unfortunately, there are 7 different fusebox layouts in the Bible, so depends what year/layout you have.
Should be a layout in the handbook, but my fusebox was different to that listed in the handbook.
Some early 95 cars don't appear to have 'speedo' listed on the fusebox list, so possibly the 15A fuse that feeds 'instruments and heater'.
Sorry, difficult to be more precise without knowing which fusebox you have.
My gut feeling is that the transducer has packed up, as it seems to have been a not-uncommon fault, but it's an expensive 'try it' way of diagnosing. That's what mine was, fine one day, absolutely nothing the next.
It doesn't sound like the fuse if it's still working intermittently.
On a 1999 car, the fuse is 'probably' the 4th in from the right (if they're 25A,15A, 25A, 15A, 10A) working in from the right.
If the transducer isn't mounted close enough to the toothed wheel it can lead to erratic readings. However, if it used to be okay, and it hasn't been touched, it probably hasn't moved. It might be worth cleaning the sensor 'reader' end and cleaning the multiplug fitting it's wired into.
I forget the correct gap fitting, but **think*** it was about 10thou. Don't take my word for it though!
However, if there's run out on the toothed wheel, you could destroy your new transducer if you set it too close.
If it was me, I'd clean the sensor, clean the multi-plug and check the gap. If no change, I'd take a punt on a new sensor. Easy fit if you can get under the car.
TVR parts normally stock them.
On a 1999 car, the fuse is 'probably' the 4th in from the right (if they're 25A,15A, 25A, 15A, 10A) working in from the right.
If the transducer isn't mounted close enough to the toothed wheel it can lead to erratic readings. However, if it used to be okay, and it hasn't been touched, it probably hasn't moved. It might be worth cleaning the sensor 'reader' end and cleaning the multiplug fitting it's wired into.
I forget the correct gap fitting, but **think*** it was about 10thou. Don't take my word for it though!
However, if there's run out on the toothed wheel, you could destroy your new transducer if you set it too close.
If it was me, I'd clean the sensor, clean the multi-plug and check the gap. If no change, I'd take a punt on a new sensor. Easy fit if you can get under the car.
TVR parts normally stock them.
The information in the "bible" is incorrect, there is NO voltage supply to the transducer. Its a passive device with a magnetic core and coil around it, and as the tooth wheel goes past the tip the shift of magnetic field cause it tiny voltage to be induced into the coil, but nothing you can measure with a DVM, it needs a 'scope. The good news is as its just a coil, a simple resistance test across the wires to the transducer (unplugged near the diff) should show some resistance, and not be open circuit. They do get water and salt in them and go open circuit. If you have RoverGauge, up can check to see if the ECU is getting a speed reading above 3mph as this will show the transducer signal is present. Note that the signal is processed by the speedo calibration box found under the dash, and the ECU gets a "fake" signal of between 30 and 50 mph once the car is moving. The signal from the transducer feeds the speedo calibration box, but is also feed to the back of the speedo in parallel and is not modified so you get a valid speed reading.
I fitted an active sensor (RS Stock No. 700-1460) when I did my GEMS conversion which allows the speedo to work at a much lower value. I took the power from the fuel pump to power it. As has been stated the speedo takes the signal direct from the sensor so I can confirm it works fine. But I do have a different conversion box to feed the signal to my GEMS ECU, but don't see why it would not work with the TVR one. The active sensor just gives a constant amplitude square wave output, unlike the original which the amplitude is related to the speed (Low speed small signal which is why the speedo does not work so well).
Another benefit is that the sensor can be placed further from the trigger ring so less likelihood of contact. It also has an LED on the end so easy to setup.
Another benefit is that the sensor can be placed further from the trigger ring so less likelihood of contact. It also has an LED on the end so easy to setup.
Just see this Peter I wish I see it earlier there is a Cerb guy that sells better quality ones , got one from him around 4 years ago about £40 must admit your sounds txt book failing inductive sensor 
found him straight away in TVR parts on here 
found him straight away in TVR parts on here Edited by Sardonicus on Wednesday 20th February 10:26
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