1993 W124 300D Climate Control Blower Issue, Blower Always ON - PeachParts Mercedes-Benz Forum

VwTravis · #1 06-06-2014, 04:25 PM

First off I would like to say that this site has helped me out immensely over the years. I have been I lurker for a long time, but finally decided to reach out for help on an issue that recently developed in my 1993 w124 300D. Hopefully along the way I can help a couple of people out.

The problem first started when I noticed that my car would go past the 100C mark with the ac on in traffic. In the past 4 years that I have owned the car it has never done this so I started to get nervous. In the back of my mind I knew that it was the fan clutch, but had a friend look at it anyway. Well Thats when my problem started.

We started by trouble-shooting the Aux Fans by applying power/ground to the fan connector near the a/c dryer. After doing this one fan came on (low speed fan?). After this we tried to test the other fan. We proceeded to the blue temp sensor on top part of the radiator hose. We pulled the connector and jumped the 2 wires, nothing happened. We then applied power/ground to the connector, nothing happened. This is where think I screwed up big time.

When I turned on the car I then noticed that the Climate control Blower Would not turn off. None of the buttons would respond when pushed.

I replaced my blower motor control unit with a spare, but that did not work. The blower motor itself is less than a year old.

Now I am lost and need guidance. I searched the forum but did not find anything pointing to my problem, only threads with people having the opposite problem.

Any insight would be much appreciated, thanks.

At this point this is where I am at....

sixto · #2 06-06-2014, 05:17 PM

IIRC the blower controller gets a signal through a yellow wire from the PBU. There's a junction block just outside the blower case beside the brake booster. Anyway, check the voltage at the small wire/s feeding the controller. Skip solid brown wires and the big wires that power the blower motor. If you see anything like a 6-7V signal from the PBU when the PBU is set to OFF, the PBU is fried.

At any rate, it's poor practice to troubleshoot an automotive electrical system by randomly applying +12V and ground. That's okay for basic consuming devices, but switches and sensors usually feed sensitive electronics. Hopefully a fried PBU is the worst of it.

Sixto
MB-less

VwTravis · #3 06-06-2014, 05:31 PM

Thanks for the help Sixto. I will Check the Push button Unit signal.

I know I'm an idiot for doing this, but at the time my friend convinced me it was a good idea.

Now where did I put my Multimeter....

dieselbenz1 · #4 06-06-2014, 06:25 PM

My bet the PBU has fried you coud place a 1k pot between the 2 leads where the temperature sensor is and if the fans don't kick in when you vary the resistance down to around 300 ohms it would be the PBU. Also check the recirculating pump for current draw if it is more than 2 amps your PBU was fried before you started.

VwTravis · #5 06-06-2014, 07:07 PM

Thanks for the reply Dieselbenz. Its getting late, so I will report on my findings tomorrow.

VwTravis · #6 06-07-2014, 01:59 PM

The yellow wire at the connector is getting 1.2v. The red one is getting battery voltage, and I think the black one is a ground. Here is a picture..

The recalculating pump is getting .58 amps. However it appears that it is is always on and the solenoid (thing my finger is pointing to?) is hot to the touch. Here is another picture...

VwTravis · #7 06-08-2014, 02:49 PM

Sorry for the late response guys. I posted my findings earlier, but it appears that I need moderator approval to post anything with pictures.So these posts might be out of order, sorry about that.

Another thing I just realized was that in addition to the blower always being on, the heat is also on.

So I decided to take the PBU out to see what will happen. With the PBU removed the Blower fan and recirculating pump are not on anymore. So for now I will drive her around like this until I can fix the problem.

Thanks again for all the support

sixto · #8 06-08-2014, 03:32 PM

That'll leave the mono valve wide open for full heat to the cabin.

Sixto
MB-less

VwTravis · #9 06-08-2014, 04:04 PM

I see. Will the mono valve sill be wide open (supplying full heat to the cabin) if I unplug it? Also even with the PBU still plugged in I was still getting full heat in the cabin.

Since I had the PBU out I decided to open it up and found some discoloration (like white chalk) on the inside cover. This discoloration was above a part of the circuit-board that looked to be burnt. I will try to post some pictures of this.

VwTravis · #10 06-13-2014, 05:49 PM

Sorry for the long delay folks, I got busy with work. After opening up the PBU and seeing damage it was pretty clear that I needed a new PBU. I didn't feel like spending the $$$ for a new one so I took a gamble and ordered a used one from e bay for $50.

The past week I have been using the Mini, and lets just say the NY roads were not kind to my backside. Well anyway, the unit arrived in the mail today and actually looked like new. I plugged it in turned the key and all seemed normal again. I will need to take on a drive, but all seems well. This is a huge relief, thanks to all that helped. I will post pics of the damaged PBU when I get a chance.

I hope that people will learn from my mistake and NEVER NEVER NEVER troubleshoot MB electrical problems by randomly applying power to connections.

dieselbenz1 · #11 06-13-2014, 09:12 PM

Keep an eye on the engine temp if there is a possibility you were given a gas model version they run 10 degrees warmer. There is a fix for that if it is a gas version PBU.

VwTravis · #12 06-14-2014, 11:43 AM

Crap I didn't know that gas and diesel W124's used different PBU's. Just to make sure I compared my old unit with the working one and they both have the some part numbers. #124 830 33 85 and 9 140 010 119 both are Bosch units made in France.