Climate Control Problem - PeachParts Mercedes-Benz Forum

braverichard · #1 10-17-2004, 10:02 AM

This is about my '84 Euro 500SEL. When I purchased it some months ago, the A/C wasn't blowing at all. Not that it wasn't cold, but no air was coming out of the vents. Same goes for the heater. So, naturally, I thought the blower motor was the problem. Before I even started diagnosing the issue, I had the opportunity to purchase a used blower motor for a very good price of $20. So I did. A few days ago, I removed the old blower motor. I tested it by connecting it to my car battery and the thing blew strong! So it was working. I connected a voltmeter to the power connection to the blower motor and there was no voltage flowing to it. At that point I started suspecting the push button control unit (PBCU). Is there another step I should take in the diagnosis before I decide to replace the PBCU? Some additional info: (1) the current PBCU was actually a used one that the previous owner used to replace the original one in 2002 (just 15,000 miles ago) when the original one stopped working (2) I already replaced the fuse (3) at night, all the buttons on the car light up except the buttons on the PBCU.

Any help would be appreciated. I did several searches but most results deal with units that work on and off. Mine has NEVER blown air at all, yet the blower motor works.

braverichard · #2 10-17-2004, 11:37 AM

Additionally my mechanic informed me over the phone that the problem could be with the resistor.

stevebfl · #3 10-17-2004, 01:37 PM

Presuming you have an automatic system as installed in US cars the blower is controlled by a separate controller called a blower controller, under the passengers side of the dash.

braverichard · #4 10-17-2004, 03:17 PM

Yes indeed I do have the automatic system as installed in US-Spec 500SELs. Blower controller? Never heard of that. Are the instructions for its removal in the repair manual?

sixto · #5 10-18-2004, 01:41 PM

Would an 84 have a solid state blower controller or a resistor pack?

Sixto
95 S420
87 300SDL

jbaj007 · #6 10-18-2004, 03:24 PM

Can't speak for his Euro, but my '84 126 has the resistor pack. Looks like a little toaster between the firewalls by the brake booster.

braverichard · #7 10-18-2004, 03:41 PM

The Euro should be the same as the US-Spec. Could the problem be from the resistor? How can I test it? Oh by the way, can you explain its location better... or better yet you could just post a pic of the location. Not quite sure of where the brake booster is. Thanks!

stevebfl · #8 10-18-2004, 04:05 PM

Never seen a resistor pack fail and it not likely it would take out all speeds. The blower controller is also pretty bullet-proof, but they do go bad.

From the drawing it also is plain that the PBC could be not telling the blower controller what to do. Fuses?

braverichard · #9 10-18-2004, 09:04 PM

All fuses in the fuse box are new. I've been suspecting the PBCU. It doesn't light up at night like the rest of the dash buttons, so it may not be working but I don't want to buy one as replacement when the one I currently have still works and the problem is something else. I already did that before... with the blower motor. Can you give me a better description of the location of the resistor pack?

Mjornir · #10 10-19-2004, 01:01 PM

When you press the buttons on the PBCU, can you tell if the vacuum elements that move the air duct flaps are working? I had a similar problem with my '84 300TD - the ACC was completely dead. No fan, no vacuum actuators, no AC compressor. After a lot of disassembly and testing, it turned out that the PBCU wasn't getting any power at all because the connectors that plug into the sides of the PBCU were corroded and the ignition switch was worn out. I cleaned the connectors and replaced the electrical part of the ignition switch and now the ACC works perfectly.

braverichard · #11 10-20-2004, 11:03 AM

No when you press the buttons you can't tell that ANYTHING is happening. Everything just stays the same, as quiet as they were prior to your pressing the buttons. Is that how it was with your 300TD?

Mjornir · #12 10-20-2004, 05:38 PM

That's exactly how it was with the 300TD. I'd checked the PBCU, temp controller, fan controller, resistor pack and installed all new fuses - and still no response from anything when I pressed buttons. I decided to pull the PBCU out and check each pin on the connectors to see if it was getting power at all. I had left the ignition switch in the "on" position and as I was pulling the connectors off of the sides of the PBCU, the loose/corroded pins made contact and the vacuum switchover valves clicked and the blower kicked on. I used a small piece of rolled-up sand paper to clean the contacts in the two connectors and pliers to squeeze a few of the loose contacts back together. That fixed that problem, but sometimes the ACC would shut off when I hit a bump while driving. I could get it to come back on by shaking the key a bit, so that was the worn out ignition switch. I just finished replacing the switch last week and so far everything seems to be working normally.

cheesman · #13 10-20-2004, 05:48 PM

I went with every fix I could think of turned out to be a burnt contact in the ignition plug. $42CDN. Plus 140 CDN for parts that I did not need. But does it ever blow now.

braverichard · #14 10-20-2004, 09:01 PM

Thanks for the info. That car has a host of other issues. I'll be sending it off to the mechanic on Friday or so, but I won't have him look into the climate control issue. I'll diagnose it using your tips and then if I fail, I'll send it off to him.

braverichard · #15 12-19-2004, 11:46 AM

Finally got the chance to troubleshoot this. I pulled the PBCU and saw that it was really messed up, some wires seemed burnt. So automatically I felt that it must have been defective. I purchased a used one and now it works well. The only problem now is that the PBCU can only keep the air coming in full blast. Once I push the automatic button the blower motor goes off. But at least right now I can get the system to work under full blast. It will just have to stay that way until I can purchase another one.

Thank you all for your help.