1997 940 Idle Control Valve

[TonyS9]

Funnily enough my car has now developed an ABS warning light. It comes on after a few minutes of driving, before which the brake pedal vibrates on light braking sometimes.

Speedo works normally most of the time but it seems to cut out (drop partially or to 0) intemittently, usually on braking.

I've put a new sensor in a few years ago, it had gone OC, no speedo problems that I can remember. Hubs are all new, struts+brakes refurbed 5-10 years ago.

Havn't done any testing yet, will update soon.

[Laird Scooby]

Originally Posted by TonyS9

Funnily enough my car has now developed an ABS warning light. It comes on after a few minutes of driving, before which the brake pedal vibrates on light braking sometimes.

Speedo works normally most of the time but it seems to cut out (drop partially or to 0) intemittently, usually on braking.

I've put a new sensor in a few years ago, it had gone OC, no speedo problems that I can remember. Hubs are all new, struts+brakes refurbed 5-10 years ago.

Havn't done any testing yet, will update soon.

Those symptoms are consistent with a sensor failure Tony, given your speedo is intermittent, particularly on braking, i would first look at the cable from the sensor to the car, there is a plug in the left hand cubby hole in the boot/luggage area that runs (through a separate grommet in the spare wheel well) alongside the fuel tank pump/sender unit cable. I had to renew mine as it had broken in several places, discovered while changing my back axle. When i say "discovered", the breaks were there but still making contact before it was touched to move it out of the way to drop the old axle.

Usually the connector in the car is inside a blac box about the size and shape of an old Swan Vesta matchbox that is clipped together - these don't always go back together again after!

[TonyS9]

Originally Posted by Laird Scooby

Those symptoms are consistent with a sensor failure Tony, given your speedo is intermittent, particularly on braking, i would first look at the cable from the sensor to the car, there is a plug in the left hand cubby hole in the boot/luggage area that runs (through a separate grommet in the spare wheel well) alongside the fuel tank pump/sender unit cable. I had to renew mine as it had broken in several places, discovered while changing my back axle. When i say "discovered", the breaks were there but still making contact before it was touched to move it out of the way to drop the old axle.

Usually the connector in the car is inside a blac box about the size and shape of an old Swan Vesta matchbox that is clipped together - these don't always go back together again after!

The senor I was talking about replacing before was a front sensor, so I guess the speedo works off the back one (diff) only?

[Laird Scooby]

Originally Posted by TonyS9

The senor I was talking about replacing before was a front sensor, so I guess the speedo works off the back one (diff) only?

On the 7/9xx, the sensor for the speedo has always been in the diff Tony. When ABS was introduced, the reluctor ring was changed (intially) to a 96 tooth from a 12 tooth that the 7xx had used for the speedo. Hence on 1988 models onwards with ABS, a speedo converter was added, a small black box hung on the ABS ECU bracket above the accelerator pedal. This was a divide by 8 counter with a couple of op-amps hung after to recreate and shape the pulse into a sine (or half-sine) pulse for the speedo.

In late 91/early 92 with the advent of the (then) new Bosch ABS-2E system, the reluctor rings standardised at 48 teeth although some late 7xx and very early 9xx models changed to 48 teeth from late 1990 and with it an ABS ECU change. This ABS ECU can be used on the earlier 7/9xx with the 96 tooth rings but will show a duff sensor at about 4mph. Conversely an early ABS ECU won't show a duff sensor on a 48 tooth untl about 15mph - the normal figure is 8mph.

The purose of this waffle is to explain how the speed and speedo can help diagnose faults with the ABS system.

You see now why i said your fault pointed not only to the rear sensor but also to an intermittent cable fault. If the sensor simply didn't work, your ABS light would come on at about 8mph, not at some point down the road at a different speed. Because it senses an intermittent connection before indicating a malfunction, it is pulsing the pedal when you brake below a certain speed. However the malfunction is not a definite one for the ECU to narrow down to a certain thing.

If your car has the later diagnostic box, you should be able to pull a code that indicates which sensor it is Early (pre ABS-2E) systems had no on-board diagnostics.

I'm certain that the code will indicate something along the lines of "Faulty speed sensor, back axle, open or short circuit cable" which gies you a choice of three possible as the cause. Most likely is the cable so that's where i'd start looking.

Note that some very late cars with IRS had two rear sensors and therefore a different ABS ECU, however the speedo sensor is i believe still in the diff on those but i might be wrong on that.

[Laney760]

I've been thumping on the dash and the clocks for years to no avail. I guess I'm back to square one. To eliminate the missing speed signal I'll first have to have the clocks taken out and sent away for repair before I move onto the sensor and since the clocks were repaired twice when I first got the car and then failed again almost immediately afterwards then I'm not that confident of getting them fixed anyway. I'll just have to live with the high revving after high speed or overtaking for a while longer. Thanks everybody. It's a shame as the speedo / revving problem are the only thing wrong on this car, absolutely everything else works perfectly, I put the car in the start of Covid and got my garage to do every single thing that needed doing (in order to give them work and keep them afloat).

[TonyS9]

I'd investigate the missing speed signal first.

Mine had a chaffed wire, that was shorting out on the Panard Rod which is very close to the diff. Wasn't even broken, just needed cleaned up, some tape and pulled through the O-rings a little. I think its always been like that.

On board computer threw a code of 2-3-5 which is rear axle sensor.
Plug into A-3, ignition on, press button 1s.

Was seeing occassional speedo loss under gentle braking, and ABS light after a few minutes driving.

[Laird Scooby]

Originally Posted by TonyS9

I'd investigate the missing speed signal first.

Mine had a chaffed wire, that was shorting out on the Panard Rod which is very close to the diff. Wasn't even broken, just needed cleaned up, some tape and pulled through the O-rings a little. I think its always been like that.

On board computer threw a code of 2-3-5 which is rear axle sensor.
Plug into A-3, ignition on, press button 1s.

Was seeing occassional speedo loss under gentle braking, and ABS light after a few minutes driving.

Glad to hear you found what i expected Tony!

[Steve940estate]

Originally Posted by TonyS9

What exact point in the journey did you notice it was idling high? LH2.4 keeps the idle high after a cold start until it warms up and you stop after that. Again we don't know the reason for this, I feel it is a bug. I notice it using pulse and glide, but the designer didn't expect the car to be traveling in neutral alot. It is very odd that it stops when you stop.

The O2 sensor control mixture at idle not idle speed. You need more air to increase the idle speed, not more or less fuel.

The missing speed signal combined with WOT the idle goes to about 1800rpm in my experience

High idle noticed when coming to rest for the first time after start up. This probably is nothing to do with Laneys problem. Cold start and drive off before the initial high idle speed had dropped. Onto motorway drive for a few miles the come off and stop at traffic lights. Idle stays high until I switch car off and restart, it's fine then.
Thinking about speedos I did fix a problem on mine by tightening up some of the screws that attach the speedo unit to the circuit board which also form electrical contacts. I have probably posted about it on here but can't remember the details now.

[TonyS9]

Yeah it definitely should drop when you stop. Even yesterday I noticed this in mine, drove about 2 miles through town then stopped at traffic lights, idling close to 1000rpm, took about 5s to drop to 700 or so when I stopped.

That was with a bad ABS, but speed working most of the time.

Any codes stored in any of the systems?

[deeman940]

i have exactly the same problems with mine,,,like the OP i was going to send the clocks off.....
When i bought the car the owner said it works if you thump the dash but that stopped working and i don't want to be thumping my dsh regularly.
Does anyone have a link for the speed sensor?