Re: Headlights and idle speed
- From: Colin Stamp <col.dustbin@xxxxxxxxxxxxxx>
- Date: Sun, 26 Mar 2006 22:31:33 +0100
On Sun, 26 Mar 2006 20:36:53 +0100, Antony Gelberg
<antony@xxxxxxxxxxxxxx> wrote:
Malc wrote:
"Antony Gelberg" <antony@xxxxxxxxxxxxxx> wrote in message
news:4426ebbb$0$21368$db0fefd9@xxxxxxxxxxxxxxxxx
My 205 GTi exhibits the oft-documented issue where the idle speed
decreases when the headlights are on. I'd say it loses about 100rpm
although it's hard to be exact as it's hunting. :)
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. 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.
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?
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
.
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