Discussion
Got a little further this afternoon trying to solve a dead speedo.
Have a positive and negative on the back of the speedo - so seems ok.
Needed to check the pulse wire so connected a multimeter to the pulse wire and went for a drive. I am getting nothing at the speedo.
Next coupled a multi meter directly to the speed sensor on the diff. (just taped extended wires accross the boot lid and went for another drive. - got a few strange looks anyway!!). I am getting a reading this way - going up to about 4v at 3000revs top gear.
The only thing that i suppose I can do now is continuity check the pule wire.
Any ideas how I do this?
Does anyone kknow where the wire runs to and from as its colour changes (red at sensor and yellow at speedo)?
thanks again
Have a positive and negative on the back of the speedo - so seems ok.
Needed to check the pulse wire so connected a multimeter to the pulse wire and went for a drive. I am getting nothing at the speedo.
Next coupled a multi meter directly to the speed sensor on the diff. (just taped extended wires accross the boot lid and went for another drive. - got a few strange looks anyway!!). I am getting a reading this way - going up to about 4v at 3000revs top gear.
The only thing that i suppose I can do now is continuity check the pule wire.
Any ideas how I do this?
Does anyone kknow where the wire runs to and from as its colour changes (red at sensor and yellow at speedo)?
thanks again
Hi rottweiler, I have the same problem here at the moment. Was going to do it last weekend but weather was cack.
The meter will not show the 12 volts you looking for because the voltage is pulsed and meter won't keep up with the speed of the pulses but will give a sort of average.(complex maths mode switched off)
Reading the bible the sensor also supplies the engine ECU with the same pulses to determine if the engine needs fuel
to idle or if it's on the overrun. This may account for the change in wire colour.
If you have a multimeter, run a bit of old bell wire to the sensor wire (remove it from the sensor first)then connect it to your meter terminal.
The other meter terminal should then be connected to the pulse wire at the speedo end.
Then turn your multimeter to the resistance range and the reading should only be a couple of ohms or so.
If there is no reading or the resistance is high then the pulse wire has a bad connection somewhere.
Hope this helps,
Cheers Paul.
The meter will not show the 12 volts you looking for because the voltage is pulsed and meter won't keep up with the speed of the pulses but will give a sort of average.(complex maths mode switched off)
Reading the bible the sensor also supplies the engine ECU with the same pulses to determine if the engine needs fuel
to idle or if it's on the overrun. This may account for the change in wire colour.
If you have a multimeter, run a bit of old bell wire to the sensor wire (remove it from the sensor first)then connect it to your meter terminal.
The other meter terminal should then be connected to the pulse wire at the speedo end.
Then turn your multimeter to the resistance range and the reading should only be a couple of ohms or so.
If there is no reading or the resistance is high then the pulse wire has a bad connection somewhere.
Hope this helps,
Cheers Paul.
thanks for the advice,
have done as you described and got a very low reading (3 ohms) so guess thats ok.
If this pulse wire is ok, then it is now all pointing to a little black box that the sensor and speedo wires pass through.
I haven't got a wiring diagram so any ideas what this is?
Is it possible to bypass it with the pulse wire just to see if the clock actually works?
thanks again
have done as you described and got a very low reading (3 ohms) so guess thats ok.
If this pulse wire is ok, then it is now all pointing to a little black box that the sensor and speedo wires pass through.
I haven't got a wiring diagram so any ideas what this is?
Is it possible to bypass it with the pulse wire just to see if the clock actually works?
thanks again
I think the box modifies the signal (changes no of pulses e.g. 3 in 2 out) for the speedo, so it reads correctly - I maybe wrong here!
I've also got a problem with my speed sensor - when I first got the car the speedo would say i was doing 150mph when I was doing anything over about 70! Reducing the gap between the sensor and the trigger wheel on the prop shaft sorted things out (moved the sensor so it was touching then backed it off a bit - had to bend one of the sections on the triger wheel slightly as it was higher than the others!).
However I've now got a fault code reader and keep getting the code for low speed seonsor output - maybe I need a new sensor...
I've also got a problem with my speed sensor - when I first got the car the speedo would say i was doing 150mph when I was doing anything over about 70! Reducing the gap between the sensor and the trigger wheel on the prop shaft sorted things out (moved the sensor so it was touching then backed it off a bit - had to bend one of the sections on the triger wheel slightly as it was higher than the others!).
However I've now got a fault code reader and keep getting the code for low speed seonsor output - maybe I need a new sensor...
Rottweiler I have spent this morning trying to get my speedo working.
Looking at the circuit diagram in the bible, it shows the sensor is supplied with 12 volts
on one side from the ignition switch. The other side then goes to the speedo. It does not show anything between the speedo and sensor. (typical TVR)
On my car the 12 volts is not getting to the sensor, although I think the sensor is OK.
(I have an oscilloscope and connected this across the sensor and could see pulses while turning the O/S wheel)
So no wonder the speedo doesn't work. However I can't find where the cable is routed.
Does it come up into the cabin via the boot or behind the seats?
Out of interest where is the little box you are talking about.
Im sure between us we can sort these Bl***y speedos out.
Cheers Paul.
Looking at the circuit diagram in the bible, it shows the sensor is supplied with 12 volts
on one side from the ignition switch. The other side then goes to the speedo. It does not show anything between the speedo and sensor. (typical TVR)
On my car the 12 volts is not getting to the sensor, although I think the sensor is OK.
(I have an oscilloscope and connected this across the sensor and could see pulses while turning the O/S wheel)
So no wonder the speedo doesn't work. However I can't find where the cable is routed.
Does it come up into the cabin via the boot or behind the seats?
Out of interest where is the little box you are talking about.
Im sure between us we can sort these Bl***y speedos out.
Cheers Paul.
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