Does the ECU take a signal from the speed sensor?

[ATG]

97 Chim ... as per title, does the ECU take a feed from the speed sensor? I ask because I've seen it said in several threads that it does, but I was told by a Midlands independent that it did not. A couple of months ago, my speedometer died. I noticed at the same time that my Chimaera stalled a few times on approach to traffic lights ... i.e. if changing down a bit slowly so the engine was idling while the car was still rolling along at a fair pace, the revs fell off and the car died. When I took the car in to have the speed sensor replaced (looming MOT, otherwise would have tried gapping it first) I asked if this idling problem could be a result of the speed sensor having died, but was told the ECU didn't take a feed from the sensor, thus there could be no connection. I assumed the stalling was just coincidence, even though it seemed to clear up as soon as the sensor was replaced. I must say I can't immediately see why the ECU needs to know the car is moving, especially to control the idle. But I just noticed a post on the Griff forum that suggested the ECU is fed from the speed sensor, so I thought I'd see if anyone can give a definitive answer.

[spend]

Depends on the box, only LT77 boxes have separate transducer solely for ecu (speedo from diff). Yours will certainly have the black magic box which splits the speedo transducer signal for the speedo & ecu.

Dave

[dlkbs1]

ATG,

I am as confused as you, according to the Bible the ECU does take a feed, according to two TVR garages it does not (although one did come back to me and say they think it might).

I have a 93 4.3 which has a SD1 gearbox and the speed sensor is supposedly on the gearbox as opposed to the Diff on yours later models. I have very similar symptoms, once warm it stalled virtually everytime clutch was dipped for more than a second. One check I read was to cruise along at 30 or 40mph and put it into neutral, if the revs drop off below 500rpm or it stalls then the likely problem is a the speed sensor has gone or is on it's way out, or the gap needs setting.

Now I have done the obvious - stepper motor cleaning etc - and had no success. The one thing I did do that worked for a while was re-adjust the tickover air bleed(allen screw on the top of the plenum by the throttle) - Factory setting is 2 and half turns open. Checked mine and it was 4 turns so reset it and rebooted the ECU. It was perfect for 3 weeks tickover rock steady at 950ish rpm. Never stalled once. However driving on Sunday tickover went completely, even stationary would not tickover. Rebooted the ECU and hey presto tickover fine again.

My guess at the moment is the ECU is getting an error code (although I don't have a reader to confirm or know anyone who does), possibly/probably the road speed sensor and this is causing the mixed up settings for tickover.

Let me know what you find out!!

[steve-V8s]

Not a definitive answer because to a certain extent I am guessing with the Chim, however it is normal practice for the ECU on most cars to use a road speed signal to raise the tick over slightly until the car has stopped or has been stopped for a few seconds. On both the V8s and the Griff this is noticeable by the tick over revs being quite high when coming to a halt, they then drop down to normal in two distinct steps by means of the ECU controlling the stepper motor. On My 98 Griff the sensor is mounted alongside the Diff, without it the engine does indeed stall very easily.

[spend]

dlkbs1 said:

I have a 93 4.3 which has a SD1 gearbox and the speed sensor is supposedly on the gearbox as opposed to the Diff on yours later models.

I'm pretty sure that should read 'in addition' rather than 'opposed'. Then again "standard" and TVR is a very risky assumption

I can only recall that very early Griffs had no speed sensor on the diff (and they dont have speed input to the ecu - pre-cats). The SD1 box is the LT77 Vitesse spec box, the speed sensor was only used for the ECU. The early GKN Diffs had the speedo speed sensor on the front of the diff, the later BTR diffs had it on the side. When the T5 box was adopted the ecu had to be fed from the same sensor as the speedo, so the magical black box to split the signal was introduced.

Dave

[AntonyJ]

spend said:

dlkbs1 said:

I have a 93 4.3 which has a SD1 gearbox and the speed sensor is supposedly on the gearbox as opposed to the Diff on yours later models.

I'm pretty sure that should read 'in addition' rather than 'opposed'. Then again "standard" and TVR is a very risky assumption

I can only recall that very early Griffs had no speed sensor on the diff (and they dont have speed input to the ecu - pre-cats). The SD1 box is the LT77 Vitesse spec box, the speed sensor was only used for the ECU. The early GKN Diffs had the speedo speed sensor on the front of the diff, the later BTR diffs had it on the side. When the T5 box was adopted the ecu had to be fed from the same sensor as the speedo, so the magical black box to split the signal was introduced.

Dave

Dave, my precat (april '92) definately has the signal to the ECU ,definate drop in revs after maybe 1-2 seconds,when the car comes to a halt.

[spend]

Is that normal Ant? So do you have some kind of splitter to supply the ecu & speedo? I'm sure someone said the precats only used the speed transducer on the box (the Stewart Warner dials as I recall) ~ but they do confuse me a bit. Dont think the Chims had quite so many variations as the Griffs.

Dave

[AntonyJ]

spend said:

Is that normal Ant? So do you have some kind of splitter to supply the ecu & speedo? I'm sure someone said the precats only used the speed transducer on the box (the Stewart Warner dials as I recall) ~ but they do confuse me a bit. Dont think the Chims had quite so many variations as the Griffs.

Dave

As far as I know the signal is just split in the wiring, never found anything like a splitter box whilst doing my under dash delvings.

[Bill_H]

Hi, Ive got exactly the same problem on my 4.0ltr Chim.
Tried sticking a scope on the ECU pin that supposedly is the speed sensor - but got no reading.
Now it seems from the other posts that its unlikely to be an under dashboard problem - so a question.
Exactly where is the speed sensor on a 1996 Chim with the T5 box?
I spent an hour or so under the car looking around the back end of the gearbox and could only find something incredibly inaccessible that may be the reverse switch!
Can find nothing that looks like the speed sensor picture I have in workshop notes.
Like ATG I have the MOT due so cant carry on driving on the rev counter!

[spend]

The speed sensor is on the diff with the T5 box. GKN diff its on the front by the propshaft, BTR diff is on the nearside driveshaft (like an ABS sensor). They are split in the dash to feed speedo & ECU from the same signal.

There is a speedo drive on the T5, but I've never heard of it being used in TVRland?

Dave

[CaptiV8ted]

From the bible...

"Road Speed Sensor.

This sensor has 2 jobs: it drives the speedo and it tells the ECU whether the car is moving or not and this info is used to control and adjust the idle. If the sensor fails, the speedo will also fail. Don't rely on this though, as it could be a fault with the wiring or the signal is incorrect or noisy - ENOUGH TO UPSET THE ECU BUT NOT THE SPEEDO ITSELF....

...Test with ECU plug disconnected, ignition on, pins 6 and ground. Result should be fluctuating between 0 and 12v as the lugs pass the sensor (6 per wheel revolution)"

Copyright Steve Heath.

May need a sillyscope to see signal as a multineter will average out the readings.

Thankyou and goodnight.

[David Beer]

Test. i seem only to be able to post few words.

Edited by David Beer on Tuesday 13th November 19:37

[RWDKurt]

Resurrecting an old thread here. My 93 Chim has both sensors - one on the LT77 (SD1) gearbox, and another on top of the GKN diff. Discovered this tonight in an effort to fix my dead speedo / odo. I don't seem to have any problems with idle and stalling which backs up the view that each one does a separate job.

Shame it isn't covered in the Bible - perhaps only a handful of cars have this setup. Can anyone confirm that Dave is correct about which one does which job?

Cheers

Kurt

Edited by RWDKurt on Tuesday 2nd September 21:56