help!...road speed sensor?
Discussion
can anyone help? when i start my car (94 Chim 400 Serp) it revs up to 3000rpm ish then slowly drops to 2000 rpm and then again to what i would call a regular tick over. The tick over is even and does not fluctuate.
This happens every time i start the car - hot or cold. It also happens each time i dip the clutch at a junction - the revs repeat the same process as described above. When i drive along and lift off the throttle the car drives itself, as if the throttle is stuck open slightly or how it used to feel in an old car when you had accidentally left the choke on...
Before it is mentioned - the stepper motor is new and has been checked but i dont think it is this as once the idle speed is reached it is solid.
I have been thro' the bible and it mentions a road speed sensor - I am thinking it may be something to do with this sensor as, if the car is left standing and the idle speed is reached, the throttle can be blipped and the car behaves as you would expect (ruling out sticking throttles and advance retard problems - yes?).
Where is the road speed sensor? The bible says that to test it the cars rear wheels need to be rotated suggesting that it is near the rear axle? Does it read some marks on the rear axle perhaps? If so, could i have confused the sensor by spraying waxoyl on it or the marks on the axle?
I am getting desperate now...to the point that i may have to put the car into the dealer!! Would rather sort out myself so that i know its fixed (would also save a few quid no doubt!).
Thanks in advance.
This happens every time i start the car - hot or cold. It also happens each time i dip the clutch at a junction - the revs repeat the same process as described above. When i drive along and lift off the throttle the car drives itself, as if the throttle is stuck open slightly or how it used to feel in an old car when you had accidentally left the choke on...
Before it is mentioned - the stepper motor is new and has been checked but i dont think it is this as once the idle speed is reached it is solid.
I have been thro' the bible and it mentions a road speed sensor - I am thinking it may be something to do with this sensor as, if the car is left standing and the idle speed is reached, the throttle can be blipped and the car behaves as you would expect (ruling out sticking throttles and advance retard problems - yes?).
Where is the road speed sensor? The bible says that to test it the cars rear wheels need to be rotated suggesting that it is near the rear axle? Does it read some marks on the rear axle perhaps? If so, could i have confused the sensor by spraying waxoyl on it or the marks on the axle?
I am getting desperate now...to the point that i may have to put the car into the dealer!! Would rather sort out myself so that i know its fixed (would also save a few quid no doubt!).
Thanks in advance.
seems that way,funny ive said this because my speedo as just recently been lazy to wake up then 5 minutes into journey it fires up speedo works normally,oh umm here we go.sensor is connected to ecu to indicate road speeds,if you roll up to junction in neutral you will find idle stays fast until standstil,then drops back to regular idle speed.thats how i sussed there connected not all cars have this arrangment chims do,have you checked throttle position swith TPS.you may have to check wiring from speedo sense to ECU may come direct from speedo head,that may be faulty.
Edited by that daddy on Tuesday 29th August 12:23
[quote=that daddy]seems that way,funny ive said this because my speedo as just recently been lazy to wake up then 5 minutes into journey it fires up speedo works normally,oh umm here we go.sensor is connected to ecu to indicate road speeds,if you roll up to junction in neutral you will find idle stays fast until standstil,then drops back to regular idle speed.thats how i sussed there connected not all cars have this arrangment chims do,have you checked throttle position swith TPS.you may have to check wiring from speedo sense to ECU may come direct from speedo head,that may be faulty.
No - havent checked any sensors yet, was trying to narrow down which one first if possible. Wouldnt the throttle position switch not work all the time though regardless of whether the car was stationary or moving?
The maddening thing is the car behaves and sounds great at a stand still, rev the engine and the revs drop back to tickover no problem, Drive off or switch off and restart and its a different story...
Just had a look at your car - nice, smart wheels
No - havent checked any sensors yet, was trying to narrow down which one first if possible. Wouldnt the throttle position switch not work all the time though regardless of whether the car was stationary or moving?
The maddening thing is the car behaves and sounds great at a stand still, rev the engine and the revs drop back to tickover no problem, Drive off or switch off and restart and its a different story...
Just had a look at your car - nice, smart wheels
thanks petea,see what you mean now sounds speed sensor related alright,have another look round speed sensor wiring near driveshaft,may have seperate signal for ECU although cant be sure about this,anyone else be able to help,im bit like doctor with no patient,very hard to clarify from behind keyboard.keep us updated.
The RSS is mounted on or near the diff, and is triggered by the notches machined into the round plate the driveshaft bolts to (mine is on o/side). The sensor can get plastered in road dirt, or the gap between it and the notched wheel may me too large. Have seen various gaps cited but it needs to be close (it's threaded to adjust this)... gotta go play with mine as I occasionally get similar hiccups.
[quote=that daddy]thanks pupp,dident realise its adjustable,just thought it was road dirt sticking to all the waxoyl & dinatrol i sprayed on,ill have to clean the stator & p/up thanks again.any news petea.....?
Sorry guys - had to go to a meeting, just got back...
Will only be able to check this at the weekend at the earliest but its got a good chance to be something like this as i waxoyled the car when i carried out a service recently. Regardless of whether this is the cause it would seem like a good idea to clean it up!
Thank you both for your help - will let you know how i get on.
Sorry guys - had to go to a meeting, just got back...
Will only be able to check this at the weekend at the earliest but its got a good chance to be something like this as i waxoyled the car when i carried out a service recently. Regardless of whether this is the cause it would seem like a good idea to clean it up!
Thank you both for your help - will let you know how i get on.
UP DATE...
Got the car jacked up last night and couldn't find anything that looked remotely like a road speed sensor! Looked all round the drive shafts and the diff but no toothed wheel, etc - is it inside the diff? or does it have a cover or something?
There is a picture in the bible but i couldn't see anything like the item in the picture?
Only other thing i can think of is the throttle position sensor (the butterfly valve) but as i said earlier in the thread, i would have thought this would have affected the revs when the car was stationary as well...whatever is wrong only materialises when the car physically moves???
Hmmm....looks like its a trip to TVR @ Hexham H C...
Got the car jacked up last night and couldn't find anything that looked remotely like a road speed sensor! Looked all round the drive shafts and the diff but no toothed wheel, etc - is it inside the diff? or does it have a cover or something?
There is a picture in the bible but i couldn't see anything like the item in the picture?
Only other thing i can think of is the throttle position sensor (the butterfly valve) but as i said earlier in the thread, i would have thought this would have affected the revs when the car was stationary as well...whatever is wrong only materialises when the car physically moves???
Hmmm....looks like its a trip to TVR @ Hexham H C...
If you hold the car on a slope using the handbrake with the engine ticking over, and then ease the handbrake off to let it roll forward without touching the throttle, do the revs pick up? If so this would suggest that the behaviour is caused by the road speed sensor. It's supposed to pick the revs up when it's moving, so may not be the result of a mechanical fault. But for example you may be able to 'fix' it by disconnecting the road speed sensor input so that it just gives you a constant idle speed - or even disconnect the stepper motor completely.
GreenV8S said:
If you hold the car on a slope using the handbrake with the engine ticking over, and then ease the handbrake off to let it roll forward without touching the throttle, do the revs pick up? If so this would suggest that the behaviour is caused by the road speed sensor. It's supposed to pick the revs up when it's moving, so may not be the result of a mechanical fault. But for example you may be able to 'fix' it by disconnecting the road speed sensor input so that it just gives you a constant idle speed - or even disconnect the stepper motor completely.
Thanks for that...easy enough to try before admitting defeat!
Will give it a go next time i am in the car, which will be after the weekend as i am off to European Drag Racing finals at Santa Pod - V8 Nirvana!!!
Mines a 94 M 4L and the toothed disc is between front of diff and the prop shaft with the sensor mounted above the disc. Its a swine to get to, prob best from offside with wheel off. I tried last week to undo the sensor as I had an erratic speedo but couldn't budge it, could get no leverage. However took connections off and cleaned them and tightened them up and having initially put them back the wrong way round, I now have a rock steady working speedo.
Good luck
Good luck
myquest said:
Mines a 94 M 4L and the toothed disc is between front of diff and the prop shaft with the sensor mounted above the disc. Its a swine to get to, prob best from offside with wheel off. I tried last week to undo the sensor as I had an erratic speedo but couldn't budge it, could get no leverage. However took connections off and cleaned them and tightened them up and having initially put them back the wrong way round, I now have a rock steady working speedo.
Good luck
Good luck
Thanks for that - will check in that location before admitting defeat...
Was off work yesterday following return from Santa Pod (getting old now and need a day to recover)so called service @ HHC just on the off chance they could fit me in - they couldn't - they said what i expected, that it would be the stepper motor. Cant see how this can be the case when, when idle is reached, it is rock solid...they said that after fitting the new stepper motor the idle should have been reset?
Anyway, will check road speed sensor and see what happens...
UPDATE...
Did a few post winter / pre summer / pre MOT checks on the Chim, including locating and cleaning (as best i could) the road speed sensor (its located on top of the main drive shaft in front of the diff).
cleaned it with a solvent and blasted it with an air line a few times, drove the car and it felt exactly the same!...was a bit dissapointed..
Anyway, kept driving the car (as you do) and it got better and better and now its OK, spot on, better than ever...! and this is the only thing i did that would affect this.
Can only think the ECU has re-tuned itself and it is now working correctly - perhaps resetting the ECU after cleaning the road speed sensor would have been wise..?
Worth noting if anyone is experiencing similar issues (see earlier in the thread) - going to give the road speed sensor an annual clean as part of my servicing routine (although i think i may have accidentally caused the problem in the first place by spraying waxoyl too close..)
Did a few post winter / pre summer / pre MOT checks on the Chim, including locating and cleaning (as best i could) the road speed sensor (its located on top of the main drive shaft in front of the diff).
cleaned it with a solvent and blasted it with an air line a few times, drove the car and it felt exactly the same!...was a bit dissapointed..
Anyway, kept driving the car (as you do) and it got better and better and now its OK, spot on, better than ever...! and this is the only thing i did that would affect this.
Can only think the ECU has re-tuned itself and it is now working correctly - perhaps resetting the ECU after cleaning the road speed sensor would have been wise..?
Worth noting if anyone is experiencing similar issues (see earlier in the thread) - going to give the road speed sensor an annual clean as part of my servicing routine (although i think i may have accidentally caused the problem in the first place by spraying waxoyl too close..)
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