C208 Heater blower only works with engine off?

[Jebsy]

Hi guys.

New to the forum. I just bought a 1999 CLK 230 Kompressor (C208). It's a bit of an old duffer. I'm having a bit of a strange problem with it...

The heater blower wasn't working when I got it. After reading the forums here, I pulled the motor and rotated the brushes, which got it working again. But, it only seems to work with key in position 2.

It runs every time at position 2, with no problems. However, as soon as I turn the key further to start the car, the blower cuts out.

I've only had the car a few days. So far, after rotating the brushes, it's worked once when I started it, seemingly by chance. It also worked a second time, when it just came on after about 20 minutes of driving. But most of the time it just refuses to do anything with the engine running (but it always works on position 2, before starting).

When it's working, the EC and recirculate switches both seem to work (they light on and off with each press, the recirc leads to the sound of vents moving). When it's not working, both those switches fail to light or operate. Heated rear window works regardless.

I'm a bit confused as to why it always works just on position 2, but then fails (except occasionally) as soon as engine is running.

Any ideas?

It's the basic heater controls with the rotary switches, not the fancy lcd display one.

 

[Jebsy]

More info:

Cabin filter is clean and dry. No evidence of water ingress into motor/fan housing.

It shuts off ok with ignition off (not running all the time or flattening battery, like some I've heard about).

When it's working, I can use the fan controller to change speed or turn on and off. I can also set recirculation on and off (flap moves and airflow felt/heard to change). Everything seems to work ok when the fan spins.

The two times it started with engine on, the car was stationary. So it's not air-flow that's starting it. First time, upon starting in petrol station. Second time, after a 20min drive, and subsequently sitting with engine running for a few minutes, it just started up (fan control already switched on).

If anyone has wiring diagrams, manual pages, or suggestions/theories, I'd love to hear them. I've got a test-meter and I'm not bad with a screwdriver, but I'm a little perplexed about where to even start.