Headlights and idle speed

There is indeed an ECU. It's an Bosch Motronic M1.3. I've googled and I can't find much about feedback loops except that they relate to O2 sensors. This car doesn't have O2 sensors or a cat.

I don't think this is a feature, even though a lot of people say "They all do that". My prehistoric MG Midget didn't do that.

Assuming that the ECU _should_ compensate for the increased electrical load, what should happen then? Is it supposed to monitor the electrical system and increase the fuel supply when the foot is off the throttle and the RPMs go down to idle?

There's no feedback loop that's why. On a 205 I should imagine that you haven't got a very sophisticated ECU if you've got one at all. So you increase the load on the engine and the speed drops. That's why you've got to put your foot down to get up a hill.

The message from Antony Gelberg contains these words:

More modern engines may well be able to, but older systems didn't have such clever electronics to sense the drop and make up for it.

: > I can get around this by setting the idle when everything is switched on : > but this seems like a bodge as obviously the idle will be a little high : > with everything switched off. : >

: > So what causes this? My pea-brain suggests more current needed, hence : > more load on the alternator. Then my head starts to spin. Why can't the : > engine maintain idle speed regardless of alternator load? : : There's no feedback loop that's why.

That behaviour appears even with a feedback loop. It's the type of feedback which is important - for this sort of thing you need an "integral component", which does something about an error which would otherwise be constant over time.

Example: toilet cistern. Water level falls -> ball valve opens -> water level rises. Allow a small leak and the water level only rises to the point where the leak is matched by the influx. To get the water level right up you need something which says "Hang on. I'm putting the same amount of water in as I normally do at this water level and yet the level isn't rising. Better add a bit more."

Ian

The message from Antony Gelberg contains these words:

It'd also have to increase the air supply.

On most modern cars, the ECU monitors the idle speed and controls a small air valve that bypasses the throttle in order to keep the idle speed where it wants it. When you turn on a big elecrical load, the idle speed starts to drop, the ECU sees this and opens the valve a little to keep it constant. On my car, when the engine cooling fan starts up, you can just see a movement in the boost gauge. You can also feel it working if you load up the engine slightly with the clutch.

My old 1990 Bluebird had a much cruder system called "idle-up". There was a solenoid valve in the carb which just increased the idle by around 100RPM. Selected heavy current users like the headlights and the heater fan were wired so as to turn on the valve whenever they were on. The effect of the load was more-or-less cancelled out by the valve, so turning on some things would cause the idle speed to go up, whilst others caused it to go down.

Not sure which setup your 205 has if any. On the MG, the charging system may not have been able to generate enough current at idle to run the headlights, so they would mostly run from the battery rather than drawing power from the engine - hence the engine wouldn't notice. You can tell when that's happening, because the headlights will get noticibly brighter when you rev the engine.

Cheers,

Colin

No it just monitors the idle speed & slightly advances the ignition in an attempt to compensate. The MG didn't do it because it's alternator couldn't cope at idle. I suppose the obvious question is why worry about it? If you want to see how it works then the old Haynes Fuel Injection manuals turn up on ebay.

Hunting is usually a sign of too rich a mixture, isn't it?

Previous message got lost. Anyway.

If the engine had no means of altering the throttle, by means of a valve, or anything else, then there is another possibility. If the engine is at a higher RPM, it will suck more air through the closed throttle. You want the engine to have some available power margin at closed throttle, to stave off stalling when moving off. If there is less reserve power, due to the electrical system wanting a goodly fraction of a horsepower, then the only way to maintain this reserve power is to increase the idle speed.

This I think could be a simple matter of programming - to mostly overcome the lack of anything to simply feed the engine more air to remain at the same RPM. And of course, if you can move hardware stuff to software, then the engine just got cheaper, and possibly even more reliable.

The book says idle speed 850-950rpm. So they came out of the factory idling at 900 (to take the average), then stalled when the driver put the headlights, fan, and heated rear window on?

For me, this lot together lose about 300rpm at idle. Even a well-tuned example is surely going to be borderline stall at 600rpm. Hence I assume that the 300rpm drop is too high.

Hmm. I really should check the timing. :)

Because it's annoying that when it's set up to idle okay with all the electrics switched on, that the idle when they are switched off is 1100 rpm.

Just bid on one. Pretty cheap and interesting. Don't all rush to outbid me.

Because it doesn't as it is a fairly basic injection system.

If you search the archives, it looks like a lot of them *do* do that. Someone was advertising his ruined souped-up example on ebay boasting about his 1500rpm idle! It's funny, I can drive around all day with the tailgate with Paint From Hell, but when I have a problem like this, I'm not happy until it's sorted.

I forgot to say when I posted before - I noticed tonight that when I put the fan on full, the ignition light comes on for a split second and I could swear it stays on faintly until it's turned off. Who knows if it's related but I'd better check the belt next weekend.

I love the variable speed fan control. Touch of class, that.

Oh right, they don't all do that, that's high, check the mixture & the timing.

Yep, but it's an indication somethings up,

Nope. See bottom of post.

Shouldn't do. My carburretored Capri doesn't.

OK, one thing I can think of is that the battery is only partially charged so it is having to do that as well. This is especially true if you are doing short journeys of less than 15-20 minutes a day.

Take the battery off the car and fully charge it.

Is it still hunting? As someone else remarked, check the mixture.