Tick over / battery ?

[jrgibb]

I've got a 525 td, 1993 I've been running it for about a year and the battery always seems like it's on it's last legs.

Since I've had it there's been a problem with the tick over, at startup it runs ok at around 900 rpm, then as the engine warms up it goes up to around 1200 rpm. When I put on the heated rear window and / or the fan (to No 4), the revs drop to around the 900 - 950 mark. Turn them off and up the revs go again.

My theory is that the engine management unit is sensing the poor battery condition and increasing the revs to charge it up?? I only base this on what my Lotus does which actually repeatedly blips the throttle which is pretty cool (it does stop as soon as it senses the throttle move!).

As I've been running it for a year I don't want to change the battery unless I have to.

One other point I usually need to give the glow plugs 2 or 3 goes before turning over for the engine to catch.

Thanks,

J

[pingu]

The battery charging is controlled by the regulator (part of the alternator). As far as I know, the ECU doesn't sense the battery voltage (unless the battery voltage is so low as to cause the ECU to start malfunctioning). So the fluctuation in idle speed is not the ECU trying to compensate for a duff battery.

When was the battery last changed? (They typically last about 5 years.) If the battery is shot then, given that winter is approaching, why not just get a new battery and be done with it?

It also sounds like the ECU is unable to properly control the idle speed. I've got a petrol so can't help you here!

[scoop2004]

with petrol cars AFAIK the increasing rev speed when u turn stuff on is to compensate for the extra load put upon the engine. When u put ur heaters on full the engine has to try harder to maintain idle,so theres a valve which opens to work the engine harder (this is the ISCV) altho normally the revs stay the same

i can only assume its the same for diesels

so maybe u have an overactive ISCV thats increasing the engine intake air a bit too much and increasing the revs