W123 Aux fan runs without key - PeachParts Mercedes-Benz Forum

Flyin4now · #1 04-08-2015, 10:48 PM

OK, I'm looking for some help here. Here's what I've been troubleshooting:

My 1985 300D has an electrical issue with the aux fan and blower motor. The PO wired a 12V 30A Bosch relay that supplies power to both of these. The aux fan has also been wired to come on whenever the AC is on.

Driving the other day, I thought I noticed a slight burning smell. Sure enough, within 20 minutes the AC and blower motor quit. I pull over and pop the fuse cover and notice that the 25A fuse for the blower motor/AC compressor was not blown, but the plastic of the fusebox that holds the metal tabs that the fuse rests in, was melted and now the tab had slid over and was resting against the adjacent tab. I did my best to bend the tabs back to their original position. Replaced the fuse and nothing worked. I went and bought a new relay and the minute I connected it, the aux fan started up with no key in the ignition. I even tried another new relay, thinking the replacement was defective. I loosened the fuse box and there didn't appear to have any damage on the underside.

Here's some of the things I tried with no luck:
I tried a different aux fan. Still came on with no key.
I swapped the CCU with 2 others (used).
I swapped the temperature regulator and fan regulator (both used) behind the glovebox.

If I jump the wires where the 12V relay plugs in, I can get the aux fan to run and the blower motor to run. I have replaced the brushes in the blower motor a while back. If I leave the new relay in and start the engine, the AC compressor does turn on. However, no blower motor!

I don't know what else to check. Thanks, in advance, for any help or suggestions!

vstech · #2 04-08-2015, 11:50 PM

Melting fuseblock is a sure sign of worn bearings in the blower motor...

84-300dee · #3 04-09-2015, 07:46 AM

Subscribed. I want to do what PO of your car did (I swapped the Auto CC for Manual) and want manual control of the relays for A/C switch and fan.

Attached is the wiring diagram for A/C. Hope that helps

Flyin4now · #4 04-10-2015, 10:13 PM

More info:

If I remove one of the wires on the AC pressure switch, the aux fan stops.

If the blower bearing is bad and drawing to many amps, could this have made the relay get too hot and fail in the "on" position, then in turn, it somehow ruined the pressure switch?

Thanks again.

Flyin4now · #5 04-19-2015, 09:50 PM

Update: (and request for help!)

I replaced the AC pressure switch. Problem still exists.

So, when this all started, I attempted to change the blower speed, which popped/broke something. That is when everything stopped. This is when the fuse hold was melted. I'm not sure if the blue #8 fuse actually blew or not, but what else could have been taken out in the process?

Like I said earlier, if I jump the terminals for the aux and blower motors where the relay plugs in, both motors DO work. The compressor WILL engage.

I also tried to "revert" the wiring back to factory, and nothing worked. No aux fan, no blower motor, and no compressor engage.

I'm scratching my head here so hard it hurts!

Thanks!

dkr · #6 04-20-2015, 10:16 AM

Quote:

Like I said earlier, if I jump the terminals for the aux and blower motors where the relay plugs in, both motors DO work. The compressor WILL engage.

What vstech said...

The blower motor will work, but it will just keep taking amperage until it blows a fuse or starts an electrical fire. There's another thread about someone with the same problem whose PO replaced the fuse with a larger one to keep it from popping and it later caught on fire and he had to pull the dash and replace a bunch of electrical wire.

Buy a new blower motor or rebuild the one you have.

Dkr.

funola · #7 04-20-2015, 11:27 AM

Quote:

Originally Posted by Flyin4now

Update: (and request for help!)

I replaced the AC pressure switch. Problem still exists.

So, when this all started, I attempted to change the blower speed, which popped/broke something. That is when everything stopped. This is when the fuse hold was melted. I'm not sure if the blue #8 fuse actually blew or not, but what else could have been taken out in the process?

Like I said earlier, if I jump the terminals for the aux and blower motors where the relay plugs in, both motors DO work. The compressor WILL engage.

I also tried to "revert" the wiring back to factory, and nothing worked. No aux fan, no blower motor, and no compressor engage.

I'm scratching my head here so hard it hurts!

Thanks!

Aux fan and blower motor are on separate fuses. Only the aux fan is switched by a relay (inside black plastic cover driver's fender). The blower is switched by the push button unit directly (no relay) I believe. If the wiring has been messed with, what's described above may not true anymore.

You can easily measure the current the motor is drawing by removing the fuse and clipping a 20 A ammeter in it's place.

BillGrissom · #8 04-20-2015, 03:09 PM

Search for my post on fixing the melted wires under the #8 fuse (cabin blower). I found the voltage drop was across the crimped cylindrical collar, so soldered across it. Others have suspected corrosion from water dripping on the fuse box (not in my case). I re-wired to power the "blower relay box" thru a new relay off the battery.

My guess is that your radiator fan runs all the time because the relay contacts are melted closed. Try swapping w/ another relay. This could be the factory relay. Perhaps he wired the new relay (actuated by AC clutch wire) to independently provide current around the factory relay. If so, its contacts could be fused shut. RollGuy has a post showing one way to wire it. I recall his new relay just works on the low-current "logic side" of the factory relay (controls its coil).

Flyin4now · #9 04-25-2015, 01:50 AM

I think the problem has been solved! Thanks to BillGrissom's post, I searched his and RollGuy's posts. I found the post about the blower motor regulator that is behind the glovebox. I don't know if the '85 300D is the only model that has this. I saw the post about jumping the two pins to run the blower on HIGH. I ended up swapping the regulator with another used and it fixed the problem with the aux fan running constantly. Blower motor functions properly and the relay that has been wired off the battery doesn't get very hot like previously.

Thanks for all your help and suggestions!