A few intermittent problems

[andre372]

The car has a few intermittent problems for which I haven't found the cause.

1) Engine drops revs a little when braking to a stop
2) Bulb warning lamp lights up while all bulbs are actually ok

The braking issue happens maybe once a week and not necessarily throughout the day. With normal braking power when coming to a stop, revs drop a bit and cause this slight stutter. Once the car has stopped and with brakes still applied, revs rise to normal. It's a matter of maybe 50-100 rpm.
How is braking related to engine? The vacuum pump is operating all the time and depending on the condition of vacuum pipes and servo there simply is vacuum for power assistance in the servo or these is not. Am I missing something?

The bulb warning issue has haunted me before, and was the known issue with Tourer back lights. When the warning lamp came up, the LH rear side lamp wasn't working and the brake lamp was on half power. A little knock turned things to normal. This happened maybe once a month or two.
Now for the past two weeks, the warning lamp comes on but the bulbs are all okay (yes, even both of the license plate bulbs). The warning lamp normally lights up when the car is started but not necessairily every time. While driving it goes off after a while but lights up again sometimes.
Could it be due to corrosion in the rear ligt cluster and turn on the warning even when everything looks okay?

Any ideas please?

[andre372]

Problem no 2 was caused by a bad connection of RH brake bulb. Simplest of simple.

[Mike Noc]

The jury is still out on whether problem number one is a problem.

Personally I think it's in the software to drop the revs on the CDT until the car comes to a stop and then they return to, for want of a better phrase, we can call normal idle.

If you lift the brake pedal and then brake again once the revs have stabilised it doesn't happen.

There have been a few threads on the subject but no conclusions.






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[andre372]

Originally Posted by Mike Noc

The jury is still out on whether problem number one is a problem.

Personally I think it's in the software to drop the revs on the CDT until the car comes to a stop and then they return to, for want of a better phrase, we can call normal idle.

If you lift the brake pedal and then brake again once the revs have stabilised it doesn't happen.

There have been a few threads on the subject but no conclusions.

Thanks. I'll give that a try next time it occurs. Although I have tried it (lift pedal, wait, brake again) but don't quite remember the results.

I wonder, though, which sensors apart from the brake pedal switch and wheel speed (ABS) sensor indicate to the ECM about braking/slowing down. This "problem" has never happened when free wheeling or lightly braking, but only when braking normally or rather heavily.
I reckon the vacuum pump doesn't put varying load on the engine or does it?

[Mike Noc]

No I don't think it has anything to do with the vacuum pump. The CDT is drive by wire, and there is no input to the ECM for the amount of available vacuum, so it is completely separate. The ECM will control the revs despite varying demands from the power steering, air con or any other normal loading.

Once the revs have found normal idle they will stay there even if you re-apply the brake, and every time the engine drops to 'anti stall idle' it will also regain normal idle once you have stopped, and it will do this repeatedly with the same time lag.

It doesn't appear to have anything to do with the clutch pedal sensor either, as has been suggested.

I think that, depending on how you brake, this is what is programmed to happen.

[andre372]

Originally Posted by Mike Noc

No I don't think it has anything to do with the vacuum pump. The CDT is drive by wire, and there is no input to the ECM for the amount of available vacuum, so it is completely separate.

Once the revs have found normal idle they will stay there even if you re-apply the brake, and every time the engine drops to 'anti stall idle' it will also regain normal idle once you have stopped, and it will do this repeatedly with the same time lag.

It doesn't appear to have anything to do with the clutch pedal sensor either, as has been suggested.

I think that, depending on how you brake, this is what is programmed to happen.

I brought the vacuum pump thought up because it's the first and only thing that comes to mind at the moment that could, related to braking, put load on the engine.
I suppose it could as well be programmed to do so and as you suggest - depending on how to brake - the ECM has to decide from e.g the wheel speed sensor readings from how big of a deceleration to start "stalling".

It took me 5 minutes to write the last sentence, does it even make sense?

[clf]

Could the drop in revs be caused by the ABS sensing, adding load to the alternator?

[andre372]

Originally Posted by clf

Could the drop in revs be caused by the ABS sensing, adding load to the alternator?

I don't know much how these things work but it got me thinking about something. A/C compressor is engaged/disengaged with a "clutch", so it whether adds a certain amount of load on the engine or not. Power steering and water pumps add constant load I suppose. But the alternator. Some people are concerned about how turning on different electrical devices in the car affects mpg (isn't that the point in fitting DRLs as well). So the alternator must change it's load depending on power consumption and thus have a clutch of some sort. Or isn't this how it works. I'd been thinking about it before but never really looked for an answer.
Isn't the ABS pump operating purely on electricity...? But why should the ABS pump have anything to do with normal braking and consume that much power to affect engine revs...

[clf]

Originally Posted by andre372

I don't know much how these things work but it got me thinking about something. A/C compressor is engaged/disengaged with a "clutch", so it whether adds a certain amount of load on the engine or not. Power steering and water pumps add constant load I suppose. But the alternator. Some people are concerned about how turning on different electrical devices in the car affects mpg (isn't that the point in fitting DRLs as well). So the alternator must change it's load depending on power consumption and thus have a clutch of some sort. Or isn't this how it works. I'd been thinking about it before but never really looked for an answer.
Isn't the ABS pump operating purely on electricity...? But why should the ABS pump have anything to do with normal braking and consume that much power to affect engine revs...

I dont know myself, how they work. I think they work electrically and hydraulically. But the sensors themselves are electrically operated. So my thinking was,as you brake, the sensors draw current as they detect if the ABS is required or not when it senses the speed etc. Although I wouldnt have thought they would have drawn that much current, perhaps along with the brake lights too, they maybe draw enough to dip the revs.

[Mike Noc]

No it isn't a loading issue, as, if you let the revs stabilise at idle then brake again they stay exactly where they are.

Also, if the revs drop to the anti stall idle they will come back up to normal idle just after the car has stopped even with your foot still on the brake.

And you can load the power steering up to the limit, switch the air con on and off and switch everything electrical on when idling and the engine revs still stay where they are.

That's why it looks more like being part of the engine map to me, with the ECM letting the engine fall to anti stall revs in certain conditions, and then once you have stopped restoring the engine speed to normal idle.




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