Discussion
I have recently had a loss of speedo/odometer reading at cold start-up but after a few miles the gauge springs into life. At this point (with the engine warmed up) I notice a misfire or slight cutting out.
I have connected Rovergauge which shows a fault code for the Road speed sensor and, with the engine hot and at tickover, the system cannot settle to a steady idle with the lambda readings unstable.
I am assuming therefore that the ECU is struggling with a lack of imput and then a sudden return of signal from the speed sensor?
Having checked all the ignition components (which have been replaced during the last few months including a new stepper motor) I can find nothing obvious which could cause the misfiring and the fuel pump primes and runs ok.
The only thing which I have just noticed not working is the reversing light and am wondering if the speed sensor shares the 12v feed and maybe the sensor is ok but the wiring is partly broken somewhere. I have checked the fuse and it is fine and the wiring that I can see looks ok also.
So, my question is, should I be able to measure 12v at the speed sensor connection, which I found under the centre console, with the car stationary and ignition on?
Apologies for the long-winded post!!
I have connected Rovergauge which shows a fault code for the Road speed sensor and, with the engine hot and at tickover, the system cannot settle to a steady idle with the lambda readings unstable.
I am assuming therefore that the ECU is struggling with a lack of imput and then a sudden return of signal from the speed sensor?
Having checked all the ignition components (which have been replaced during the last few months including a new stepper motor) I can find nothing obvious which could cause the misfiring and the fuel pump primes and runs ok.
The only thing which I have just noticed not working is the reversing light and am wondering if the speed sensor shares the 12v feed and maybe the sensor is ok but the wiring is partly broken somewhere. I have checked the fuse and it is fine and the wiring that I can see looks ok also.
So, my question is, should I be able to measure 12v at the speed sensor connection, which I found under the centre console, with the car stationary and ignition on?
Apologies for the long-winded post!!
Sorry to bump this topic but would appreciate any advise from the experts.
From the bible I read that to test the speed sensor you jack up the rear so that you can turn the wheels and put a meter across the connection, but it doesn't say what the reading should be if the sensor is ok?
I am assuming the 12v signal is switch on and off every time the castellated wheel passes the sensor (ie. 12-0-12-0 etc.) but am not sure it's that simple!!?
I'm hoping that the problem I have encountered is simply the sensor breaking down and in need of replacement and not the 'black box' behind the dash that converts the signal for the ECU as I believe these are no longer available!!
Anyone have any experience with this?
From the bible I read that to test the speed sensor you jack up the rear so that you can turn the wheels and put a meter across the connection, but it doesn't say what the reading should be if the sensor is ok?
I am assuming the 12v signal is switch on and off every time the castellated wheel passes the sensor (ie. 12-0-12-0 etc.) but am not sure it's that simple!!?
I'm hoping that the problem I have encountered is simply the sensor breaking down and in need of replacement and not the 'black box' behind the dash that converts the signal for the ECU as I believe these are no longer available!!
Anyone have any experience with this?
There are four possible culprits - the speed sensor on the diff, the cables/connectors, the speedo control box in the dash (cars with the T5 gear box have this, the earlier LT77 cars don't need it), and the speedo itself.
In my experience, if you lose both the speedometer and the odometer (milometer) together, then your issue could well be speed sensor (or its connections) - the speedo control box in the dash is not getting a proper input. I doubt the sensor puts out 12 volts - fractions of a volt I would have thought. You are looking for a consistent on-off pattern. I had this issue last year and mine was a failing sensor, but it is worth checking first that it is correctly spaced from the wheel on the diff that you have already identified - just adjust the gap until it gives the best signal.
I seem to remember that the idle on the T5 cars, especially coming up to junctions, does rely on speed sensor input, so you could be correct about your running issues.
The one other thing that could be causing you a problem is the box under the dash, they have been known to fail.
Have patience, a proper expert will be along soon. If you can't wait on this lovely sunny Sunday morning, get adjusting and testing the sensor by the diff first.
Good luck
In my experience, if you lose both the speedometer and the odometer (milometer) together, then your issue could well be speed sensor (or its connections) - the speedo control box in the dash is not getting a proper input. I doubt the sensor puts out 12 volts - fractions of a volt I would have thought. You are looking for a consistent on-off pattern. I had this issue last year and mine was a failing sensor, but it is worth checking first that it is correctly spaced from the wheel on the diff that you have already identified - just adjust the gap until it gives the best signal.
I seem to remember that the idle on the T5 cars, especially coming up to junctions, does rely on speed sensor input, so you could be correct about your running issues.
The one other thing that could be causing you a problem is the box under the dash, they have been known to fail.
Have patience, a proper expert will be along soon. If you can't wait on this lovely sunny Sunday morning, get adjusting and testing the sensor by the diff first.
Good luck
Check the sensor connections on the rear axle. They are exposed to a lot salty road dirt. Clean and brighten with emery. The plastic plug very near and above the exhaust is a pig to undo. Need to push pull with a thin electric screw driver to remove. A new sensor solved the problem on my 1999. Early cars I believe had the sensor on the gear box.
Thanks for your suggestion EGB but my sensor doesn't have any connections exposed to the elements but instead has the connector block inside the cabin under the rear of the centre console, which I've checked and is fine.
On searching the forum I came across this post from May.
http://www.pistonheads.com/gassing/topic.asp?t=128...
Did anything come of Mark's (blitz) attempt to fix the speedo calibration unit (black 'fag packet sized' box behind the dash) or Steve Heath's replacement part?
On searching the forum I came across this post from May.
http://www.pistonheads.com/gassing/topic.asp?t=128...
Did anything come of Mark's (blitz) attempt to fix the speedo calibration unit (black 'fag packet sized' box behind the dash) or Steve Heath's replacement part?
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