Random Revs

[AlRaven]

My revs used to sit rock steady at 950 at idle but now bounces regularly up and down from about 800 to 1800 and sometimes drop as low as 600, I've finally cleaned the stepper motor but no change - any ideas where to look next?

[MikeTVR]

Check earthing straps particularly the one under the offside wheel arch. Electrical noise on the road speed converter under the dash stops the converter from working properly. This causes the auto setting of the idle speed to fail.

Mike

[blitzracing]

After your prompting, the failed speedo calibration unit I had to bits was just bad solder joints on the main plug. The earth pin was open circuit so the thing was triggering most of the time, car moving or not. I had to use a plumbers flux to get the joint to reflow!

[taylormj4]

This is interesting. Can anyone advise how the speed sensor interacts with the ECU and how it can be checked, e.g. if speedo is working is that suggestive that the sensor is functional ?
Presume the electrical noise issue is that the signal-noise ratio becomes poor and the frequency of the square wave (presumably) from the sensor becomes distorted and unreadable ?

[blitzracing]

The input circuitry is poor, and allows the inputs to float above the point it triggers a speed signal even when the car is not moving (using a frequency to voltage converter). TVR mike has a mod that puts a small voltage on the relevant pins to remove this effect if its not just bad earthing. The output goes to pin 39 on the ECU as a fixed signal , but this is only part of the circuits function, as it still has to alter the pulse string to feed the speedo.

[taylormj4]

blitzracing said:

The input circuitry is poor, and allows the inputs to float above the point it triggers a speed signal even when the car is not moving (using a frequency to voltage converter). TVR mike has a mod that puts a small voltage on the relevant pins to remove this effect if its not just bad earthing. The output goes to pin 39 on the ECU as a fixed signal , but this is only part of the circuits function, as it still has to alter the pulse string to feed the speedo.

So, if the circuit isn't working correctly, what sort of problems will this create, i.e. what is the speed sensor used for other than running the speedometer ?