Speedo stopped working

[Colin RedGriff]

After my over winter work and having got the car back on the road the speedo has stopped working. Checked out the threads on here and as this is a T5 with the sensor mounted on the diff I thought I must have disturbed the connector at the sensor while messing around with the handbrake cable.

Had a check yesterday and it all looks fine, the wire seems to be firmly attached to the sensor. However I can only see 1 cable going into the sensor, is this a two core cable or should there be another wire connected somewhere (I can't see anywhere to connect it though)?

It's difficult to tell where the cable goes to from the diff. Anyone know the route the cables goes and if there are any connectors in it that I might have disturbed? I presume that the cable runs up onto the transmission tunnel and then behind the dash from there.

I see a fuse in the bible which also feeds the reversing lights, my reversing lights are working so I presume that the feed for the sensor is OK as well.

Anyone got a simplified diagram of the wiring for the sensor as I'm struggling to see how the circuit works and can't reconcile the Bible wiring diagram with what is on my car.

[Ballistic Banana]

Should be two wires , mine shoot up through the bodywork under the center console and then along and under the dash. There are some conectors inline under the center console.
You van do some basic checks with a multi meter to check wiring and sensor is still working.

BB

[HRG]

If it's the sensor gone I'd look at Steve Heath's one, works from about twenty feet away and has a built in diagnostic light.

[Colin RedGriff]

Yeah looked at that - if I need one then that'll be the one I buy.

Would be a bit miffed though if I came to fit it and found a wire on the current one disconnected.

[Colin RedGriff]

I checked under the transmission tunnel and found this connector



These are the only wires under there apart from some others towards the front where the meta alarm unit is. Wire colours are red and black going down under the car, then grey and black going forward. Is this the speedo sensor wire? They don't match with the wiring diagram which refers to green and green/yellow, but this is TVR and also the diagram is for a pre cat anf I think those used a different sensor, so I'm not phased if the colours are completely wrong.

I pulled the joint apart and stuck a multimeter on what I assume to be the sensor side (red/black wires) using a resistance setting to check for a circuit. It showed an open circuit - does this mean its fubar'd or can you not test them like this. The other way (forward) is a resistance of about 13k ohms.

One of the things that is niggling at me is where does the power come from? I am assuming it would somehow be hooked into the reverse light feed as they are fed from the same fuse. But from what I can see it looks to be completely unpowered.

[Ballistic Banana]

open that connector and put the multimeter on it's two wires towards sensors. With rear wheels lifted spin them and you should get a low high Reading as you spin them slowely as each tooth goes past the sensor, ideally you'd use an oscilloscope . If you get nothing then I would check the gap on the sensor .

[shpub]

The sensor is not powered at all. It generates a voltage when the tooth goes past the sensor tip. A multimeter may not see this and the problem is made worse because the speed needed to generate the voltage is very high. Simply turning the wheel by hand is unlikely to do anything. Any multimeter tests of this type are therefore inconclusive. If you see a reading change then good. If you don't it still doesn't mean that the sensor is at fault.

There is normally a connector a few inches from the sensor. Don't forget that could be at fault as well. It is exposed to the elements.

[Colin RedGriff]

shpub said:

There is normally a connector a few inches from the sensor. Don't forget that could be at fault as well. It is exposed to the elements.

I didn't see any other connector when I was under the car - not sure what I am looking for though - some sort of spade connection on the side of the sensor?

[Ballistic Banana]

No The connector will probably be a plastic push type one.

I can understand where Steve is coming from and as I said earlier an oscilliscope is the best way to test them.
But I have tested mine in the past using a multimeter reducing/enlarging the gap from the sensor and the twigger wheel and found differences.

The wheels move real slow from 0-10mph and your speedo still reads it so I cant see where the comment about just turning them buy hand would be good enough. After the Transducer sends a pulse and this is reliant on the speed the wheels are going. So slow would give a pulse all the same.

BB

[shpub]

The sensor is an analogue type and the wave form is not a pulse as such but an AC waveform. The voltage amplitude is determined by the speed of the trigger wheel teeth moving past the sensor. The sensor does not send a digital type waveform and this is where the testing problems come from. If the car is moving very slowly, the speedo doesn't register the mileage because the pulses are too small. Not a problem in the real world but one when you are testing system to see what the problem is. Ideally you want the wheels to be moving at least 10 mph to get a good signal that you can rely on. This is very difficult to do by turning the wheels by hand and I certainly would not advocate getting under the car with it jacked up and wheels turning. Recipe for a big accident.

I spent days trying to track down a similar fault on my Cerbera. I thought it might be the sensor but couldn't prove it one way or the other. There was a signal from the sensor on my scope but it wasn't big enough to trigger the speedo. Was this due to the limited rotation speed or a speedo fault? Did I buy a new sensor and risk it not being the problem. I had no way of knowing. Hence the intelligent speedo development.

[spend]

Ballistic Banana said:

...reducing/enlarging the gap from the sensor and the twigger wheel and found differences...

Ith that a gay lithp in your spelling now BB?

HS166 UJ's btw if I forget to do TGP

[Colin RedGriff]

Ballistic Banana said:

No The connector will probably be a plastic push type one.

Where should I look for this connector?

Inline in the sensor wire? Sounds a bit odd to have a connector in the wire under the car and then another one a foot further along under the transmission cover.................What AM I thinking of this is a TVR.

I'm not going to bother trying to test it - I just want to make sure I've not dislodged something while working on the car. If I can't see anything obvious then I'll be contacting you Steve.

How easy are the intelligent speedo sensors to fit? If I need to get to the back of the speedo to fit the adaptor then it sounds like a dash out job.

[shpub]

Find sensor and follow the wire back.



Yes the dash has to come out to fit the adapter but equally you typically have to do this to ensure that there isn't a fault at the speedo end.

[Ballistic Banana]

spend said:

Ballistic Banana said:

...reducing/enlarging the gap from the sensor and the twigger wheel and found differences...

Ith that a gay lithp in your spelling now BB?

HS166 UJ's btw if I forget to do TGP

Bollocths and Thanks

BB

[Ballistic Banana]

shpub said:

The sensor is an analogue type and the wave form is not a pulse as such but an AC waveform. The voltage amplitude is determined by the speed of the trigger wheel teeth moving past the sensor. The sensor does not send a digital type waveform and this is where the testing problems come from. If the car is moving very slowly, the speedo doesn't register the mileage because the pulses are too small. Not a problem in the real world but one when you are testing system to see what the problem is. Ideally you want the wheels to be moving at least 10 mph to get a good signal that you can rely on. This is very difficult to do by turning the wheels by hand and I certainly would not advocate getting under the car with it jacked up and wheels turning. Recipe for a big accident.

I spent days trying to track down a similar fault on my Cerbera. I thought it might be the sensor but couldn't prove it one way or the other. There was a signal from the sensor on my scope but it wasn't big enough to trigger the speedo. Was this due to the limited rotation speed or a speedo fault? Did I buy a new sensor and risk it not being the problem. I had no way of knowing. Hence the intelligent speedo development.

Cheers for the explanation Steve I was under the impression the pulse was created from a the magnetic field when the trigger ( just for Spendy ) wheel passed the Sensor.

BB

[Ballistic Banana]

spend said:

HS166 UJ's btw if I forget to do TGP

Spendy is that the lower or upper one? and where did you source them from?

Cheers

BB