Fan running continuously with ingition off - flattened my battery

[jeremy156]

Hi,

My W210 developed an odd fault after a long journey.... it seems the electric fans in front of the radiator switched on after I parked and clearly didn't stop until my 100Ah battery was flat the point that the poor thing barely read 6V on my meter! By the way, I checked the residual current to keep the car alive at 130mA, sounded about right. Anyway, after I'd re-charged the battery overnight, I put it back into the car and sure enough, the fans started. When I turned the ignition they stopped and I had a short drive. As soon as I stopped, the fan came on again. The only way I could stop it was to disconnect the battery. I later realised I could disconnect just the fan when I discovered the connector hidden behind a panel behind the grille. Now I can leave the battery connected and rely on the engine's fan to keep it cool (and avoid using the a/c).

So... my challenge now is to figure out why the main fan feels the need to run whenever the ignition is off. Has anyone experienced this before?

 

[fnkhan68]

the electric fans in front of the rad are designed to run with the a/c only as the engine is under extra load when it is on hence the need for extra cooling. out of curiosity, have you checked if the electric fans are running with the a/c off?

 

[jeremy156]

Ignition on, engine off, fan stops.
Ignition on, engine on, a/c off, fan stops.
Ignition off, fun runs.

I haven't tested with a/c on - I'll connect the fan back up and try that when I get home.

 

[Apial]

The same happened to my W124. I removed the a/c relay and it stopped. I took the relay apart and found a welded together contact. I did a tempoeray repair by sanding the contacts surface smooth and replacing it before I purchased a new relay.

No more problems!

 

[jeremy156]

Perfectly reasonable then that I should go and hunt down the relay - but under what circumstances *should* the fans operate with ignition off in a W210?

The last journey before the fault was a very wet and fairly windy one, so perhaps some water ingress may have been the final straw for a relay somewhere.... the question now is how to identify the correct relay to test/replace. Any idea where to look? Is it one of those on the right under the bonnet with the fuses?

 

[Tigger]

There is something called an Auxilliary Fan Control Unit which sits behind the front bumper under the nearside headlight.

You're absolutely right that it is water ingress that causes it to fail and it does tend to happen after a long slog in the rain. If I remember correctly the part costs around £70-£100 and the headlamp and / or bumper has to be removed.

Its a bit of really bad design that it is placed in such a vulnerable spot.

I paid for it to be replaced by the stealers once and then 18 months later when it started going wrong again I tried a different ploy...

If you look at the fans from the front, at the top right there is a small plastic cover that you can remove which gives you access to the fan's electrical connector. After stopping the car if the fans are still running (unless you've been caning it), disconnect the connector and replace the plastic cover.

Once the car has had a chance to dry up, you will find that you connect the fan up and everything is fine until the next long drive in the rain.

If I was to replace it again, then I would ensure it was installed with a bag around it or heatshrink or something.

 

[jeremy156]

Oh, headlamp and bumper off - just what I like to hear...! I'll see if I can work out how to get access to this control unit tomorrow and see if it looks like it's leaked or anything.

Yep, the only way I can stop my battery going flat overnight at the moment is to disconnect the fan. Even after a good drying out period, it still starts the fan the moment I connect the power.... something is definately dead!

Thanks - I'll report back.