Speedo Fault Fix
Discussion
A post that might help others based on my recent speedo troubleshooting.
A quick test you can do with no real effort is to find a quiet bit of (private) road, and whilst driving 20-30mph, press the door open button. If you're getting no signal from the speed sensor for some reason, the door will pop open. If the door doesn't pop open, your problem will be car forwards of the big 36 pin connector (J30/J33) that's behind the helmet holder. Likely to be the speedo or a connector to it.
If it does pop open, it's likely to be something to do with the sensor or connector behind the diff.
Previously it's been said that most speedo faults are due to the speed sensor on the diff, and as a recap on that point, it's worth checking the sensor is clean and correctly gapped. The correct gap is pretty much no gap, so wind it in until it touches and then back off enough that it doesn't touch when you rotate the rear wheel.
Clean the connector lots, use contact cleaner, maybe even remove and solder it and you stand a chance of fixing the problem.
To check it's working as expected, unplug the connector and wedge a couple of lengths of wire in either side of the connector. Handily they should be red and black from the sensor so it's easy to work out which length of wire goes where in a multimeter. With both rear wheels in the air, start the car and drive it. With a multimeter on AC volts, look for a steady rise of voltage the more you rev, which in 2nd gear without revving too much will be between about 1.8 to 3.3v, rising towards 5v if you go a bit more nuts with your speed.
If you get no voltage or tiny amounts, replace the sensor.
Having established the sensor is good, and the connection is good, test the speedo by removing the dash cowl, and unplugging the round-pinned 7 pin plug hanging from the back of the speedo. Check then clean the connection pins and plug it back in. The yellow wire from the rear of the speedo is the one that needs the signal from the sensor by the diff. The other side of the plug the yellow wire goes to is grey, and this fat grey wire runs the length of the tunnel to behind the helmet holder. It's easy to spot because it's on its own at the bottom of the plug.
There's nothing between the speed sensor signal and the speedo (It doesn't go through any boxes or the ECU). Take a length of wire from the red side of the speed sensor to the yellow or grey side of the plug behind the speedo, and a length of wire from the black side to an earth somewhere (or the black wire in the plug behind the speedo. Run the car in the air as before. You must have the speedo 7 pin plug plugged in to its counterpart for this test to work because the speedo takes a 12v live signal via the green wire that goes on to the back of the speedo itself as well, and it won't work without that feed. Lighting is also from that connector, but a separate 12v feed.
If the speedo is fine, it'll work perfectly with this temporary direct feed from the sensor, meaning you know there's an issue in the loom somewhere.
The connector you're interested in is J30/J33 and it's the massive 36 pin connector behind the helmet holder, and you want Pin 26. It should be a grey wire from the speedo plug to J30/33 along the tunnel. Check for continuity of this section.
Rearward of the connector, it from J30/33 it splices to go to the door control ECU pin B9 as a one way connection to deactivate the door buttons (the biggest plug, 2nd row, last pin), and then of course to the speed sensor. Check for continuity from J30/J33 to these places to track down where the problem is.
Hopefully this gets you to track down the problem.
If it is the speedo, the lead time for a new one from Caerbont can be less than a fix, and not much more expensive. I tried JDO instruments who were very quick but unable to properly test my 220mph speedometer, which it turned out was perfectly functional thankfully.
Hopefully this helps someone, and saves a load of time searching Google for odd posts here and there or following wiring diagrams to work out what goes where.
A quick test you can do with no real effort is to find a quiet bit of (private) road, and whilst driving 20-30mph, press the door open button. If you're getting no signal from the speed sensor for some reason, the door will pop open. If the door doesn't pop open, your problem will be car forwards of the big 36 pin connector (J30/J33) that's behind the helmet holder. Likely to be the speedo or a connector to it.
If it does pop open, it's likely to be something to do with the sensor or connector behind the diff.
Previously it's been said that most speedo faults are due to the speed sensor on the diff, and as a recap on that point, it's worth checking the sensor is clean and correctly gapped. The correct gap is pretty much no gap, so wind it in until it touches and then back off enough that it doesn't touch when you rotate the rear wheel.
Clean the connector lots, use contact cleaner, maybe even remove and solder it and you stand a chance of fixing the problem.
To check it's working as expected, unplug the connector and wedge a couple of lengths of wire in either side of the connector. Handily they should be red and black from the sensor so it's easy to work out which length of wire goes where in a multimeter. With both rear wheels in the air, start the car and drive it. With a multimeter on AC volts, look for a steady rise of voltage the more you rev, which in 2nd gear without revving too much will be between about 1.8 to 3.3v, rising towards 5v if you go a bit more nuts with your speed.
If you get no voltage or tiny amounts, replace the sensor.
Having established the sensor is good, and the connection is good, test the speedo by removing the dash cowl, and unplugging the round-pinned 7 pin plug hanging from the back of the speedo. Check then clean the connection pins and plug it back in. The yellow wire from the rear of the speedo is the one that needs the signal from the sensor by the diff. The other side of the plug the yellow wire goes to is grey, and this fat grey wire runs the length of the tunnel to behind the helmet holder. It's easy to spot because it's on its own at the bottom of the plug.
There's nothing between the speed sensor signal and the speedo (It doesn't go through any boxes or the ECU). Take a length of wire from the red side of the speed sensor to the yellow or grey side of the plug behind the speedo, and a length of wire from the black side to an earth somewhere (or the black wire in the plug behind the speedo. Run the car in the air as before. You must have the speedo 7 pin plug plugged in to its counterpart for this test to work because the speedo takes a 12v live signal via the green wire that goes on to the back of the speedo itself as well, and it won't work without that feed. Lighting is also from that connector, but a separate 12v feed.
If the speedo is fine, it'll work perfectly with this temporary direct feed from the sensor, meaning you know there's an issue in the loom somewhere.
The connector you're interested in is J30/J33 and it's the massive 36 pin connector behind the helmet holder, and you want Pin 26. It should be a grey wire from the speedo plug to J30/33 along the tunnel. Check for continuity of this section.
Rearward of the connector, it from J30/33 it splices to go to the door control ECU pin B9 as a one way connection to deactivate the door buttons (the biggest plug, 2nd row, last pin), and then of course to the speed sensor. Check for continuity from J30/J33 to these places to track down where the problem is.
Hopefully this gets you to track down the problem.
If it is the speedo, the lead time for a new one from Caerbont can be less than a fix, and not much more expensive. I tried JDO instruments who were very quick but unable to properly test my 220mph speedometer, which it turned out was perfectly functional thankfully.
Hopefully this helps someone, and saves a load of time searching Google for odd posts here and there or following wiring diagrams to work out what goes where.
Edited by NuddyRap on Tuesday 4th July 14:44
Useful info, thanks.
I have a minor issue whereby my speedo jumps around while stationary. It works fine on the move and only exhibits this problem after driving for a while. Maybe the plug going into the speedo needs some WD, or I have a bad earth somewhere.
What was your issue in the end, I don’t think you said..?
I have a minor issue whereby my speedo jumps around while stationary. It works fine on the move and only exhibits this problem after driving for a while. Maybe the plug going into the speedo needs some WD, or I have a bad earth somewhere.
What was your issue in the end, I don’t think you said..?
Edited by FarmyardPants on Tuesday 4th July 19:11
FarmyardPants said:
Useful info, thanks.
I have a minor issue whereby my speedo jumps around while stationary. It works fine on the move and only exhibits this problem after driving for a while. Maybe the plug going into the speedo needs some WD, or I have a bad earth somewhere.
What was your issue in the end, I don’t think you said..?
My issue was that the speedo just didn't work at all - it stopped mid trip and then sporadically sprung back to life the next time I took the car out. I have a minor issue whereby my speedo jumps around while stationary. It works fine on the move and only exhibits this problem after driving for a while. Maybe the plug going into the speedo needs some WD, or I have a bad earth somewhere.
What was your issue in the end, I don’t think you said..?
Edited by FarmyardPants on Tuesday 4th July 19:11
As it turns out, the speedo and the speed sender are completely fine, so the issue in my case was a loom break.
On internal connections, there's nothing wrong with a bit of WD40 but I'd avoid it on any external connections and use a proper evaporating contact cleaner as it'll hold dirt, attract water and possibly then corrode the pins or interfere with the connection the next time it's made or jiggled about.
In your case I'd suspect a wire problem first, and a speedo fault next. The speedo connections are pretty simple - speed signal of >~ 5v down the yellow wire that attaches to the back of the speedo. Black is earth and green is 12v live. Jumping could be caused by a problem somewhere sending a small voltage down the yellow wire (which is grey from the speedo connector to the junctions behind the helmet holder as above). This could happen internally to the speedo, on the loom, or at the back of the speedo if a wire is loose.
A quick check would be with the ignition on by the car not running to unplug the speedo, put a multimeter probe in the grey wire connector, one in the earth of the same plug and see if you get any voltage. If you do, the problem is voltage making its way in to the speed signal wire and you just need to find out where.
If you don't, it would be useful to make direct contact with the yellow wire on the back of the speedo and an earth to see if you get a voltage reading. Assuming the wires aren't visibly touching each other to transfer the voltage, you'll find the problem is your speedo has an internal connection problem doing that for you.
When you're on the move if it's reading fine there's probably enough voltage being made to more than cover any loss of voltage via whatever the small short is.
Edited by NuddyRap on Thursday 13th July 16:00
Edited by NuddyRap on Thursday 13th July 16:01
Great write up, I'm sure that will be very useful to many people
If anyone does need a speedo sensor, you will find an advert here
https://www.pistonheads.com/gassing/topic.asp?h=0&...
If anyone does need a speedo sensor, you will find an advert here
https://www.pistonheads.com/gassing/topic.asp?h=0&...
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