A/C not working. 1984 300SD - PeachParts Mercedes-Benz Forum

na5kar · #1 05-13-2014, 02:41 PM

Hello All. I'll get right to it. We bought a 300K + mileage 300SD. Converted to run on Veggie. The A/C compressor belt was not on the pulley. Previous owner says that the compressor seized and threw the belt. I took off the compressor and will get a new one through Advance Auto, along with the dryer and expansion valve. Before spending on those things, I decided to make sure that the blower and switches work. The fuses look ok. There is power to the fuse panel. With the key on, the buttons for the A/C generate a 'thump' sound under the dash on the passenger side. No blower though. I took off the carpeted panel on the passenger side under the dash. There is a black plastic box with maybe 10 pins to the left of the blower. I removed the connector. One of the male pins (on the box) was very loose and the receiver slot for that pin on the connector is corroded. My thought is that this might be an A/C controller??? Next, I disconnected the 2 pin plug for the blower itself. I connected a hot and ground lead to the two male pins of the blower plug. The blower works. I also removed the switch panel behind the wood faceplate. There is power to some of the wires, but there are way too many wires for me to decipher what is what. So here we are. Did my diagnostic efforts tell anyone where my problem might be? Thanks a million in advance for guidance.

uberwasser · #2 05-13-2014, 02:49 PM

I just went through fixing the exact same symptoms on my '85 300D. Same system, with components located in slightly different locations.

Here is a great page with lots of good info on testing the components of your climate control.

http://userweb.windwireless.net/~jimc/mamerepairs.html#acctest

That black box you mentioned is commonly referred to as the blower control unit/relay. It's basically a box full of switches. When told to by other components in the climate control, it switches current between the various coils in the blower resistor (on the W123 chassis the resistor is on the inside of the front passenger fender not far behind the headlight under the hood). Normally if this relay is bad the blower will be stuck on a certain speed. If the resistor has a broken coil, however, it could potentially lead to no power to the blower.

So there are two other possibilities here - the push button panel, or the temperature control unit, which is the silver box next to the black blower relay.

The page I linked will tell you how to test that power is getting to and from the correct pins on the push button unit. If you can confirm all is well there, you can move on to the temperature control. I don't think this page has much info on testing that, but if you can verify that the push button is good, and the resistor passes visual inspection, I'd lean toward that unit. It is basically the brains of the operation, combining your desired inputs from the push button, the current state of the system, the current conditions inside the cabin, etc. and then adjusting the system to bring it all in alignment. If this unit is bad, lack of power to the blower can be on symptom.

On my car, this ended up being the problem. Swapping the temp control unit with a junkyard spare ended up bringing my blower back to life.

na5kar · #3 05-13-2014, 02:56 PM

Thank you uberwasser. The W123 page looks very interesting, but mine is a W126 (I think). Do you know of a site for this model, or does the W123 page work for mine as well? Thanks

uberwasser · #4 05-13-2014, 03:00 PM

Pictures to help with parts identification:

Temp controller:

Blower relay:

Blower resistor:

na5kar · #5 05-13-2014, 03:04 PM

Awesome! The black box I was referring to is the Blower Relay. Could a damaged pin (or the whole relay) be the culprit of why the blower does not engage?

uberwasser · #6 05-13-2014, 03:06 PM

The earlier W126 had the same system as the "second-gen" W123. I'm not sure at what point they transitioned to a different climate control setup on the W126.

Assuming you have the same system as mine, then you'll have all the same components but just located in different areas. For example, the blower control and temp control units are both behind the glove box on my car but are down next to the blower on your car, as you mentioned.

However, even if you do have the later system that page does have pin layouts and diagnostic steps if you scroll up a bit.

To help identify which you have, if you have a panel that looks like this, with the thin white AUTO button, your system is the same as mine:

uberwasser · #7 05-13-2014, 03:09 PM

Quote:

Originally Posted by na5kar

Awesome! The black box I was referring to is the Blower Relay. Could a damaged pin (or the whole relay) be the culprit of why the blower does not engage?

I suppose it's possible, but the failures I tend to read about relate more to the unit sticking on one speed. So the blower, in that case, would work but you couldn't slow it down or speed it up.

I think the more common sources of complete lack of power to the blower are the push button panel (easy to eliminate as the problem by testing for power at the appropriate pins laid out in the diagnostics on that site I linked) or the temp control unit.

But run through those tests, try jumping the blower from the push button plug as noted there, see if power is making it through the chain. This will help narrow things down.

You will also be able to jump the plug with the steps noted there to see if the compressor is really seized.

na5kar · #8 05-13-2014, 03:45 PM

my push button system is the same as yours. It has the 'clear' Auto button. I read that link you sent me. I'm an ok diagnostician, but that was a little bit like Chinese to me. My issue ... not the guy who wrote it. I will give it a shot though.

uberwasser · #9 05-13-2014, 03:50 PM

The other method of diagnosis is process of elimination. If you have a junkyard near you with a few of these cars, you can head there and pull these parts. They usually aren't more than $15/ea. Then just start swapping to see if things change.

tram · #10 05-13-2014, 03:54 PM

Quote:

Originally Posted by na5kar

Hello All. I'll get right to it. We bought a 300K + mileage 300SD. Converted to run on Veggie. The A/C compressor belt was not on the pulley. Previous owner says that the compressor seized and threw the belt. I took off the compressor and will get a new one through Advance Auto, along with the dryer and expansion valve. Before spending on those things, I decided to make sure that the blower and switches work. The fuses look ok. There is power to the fuse panel. With the key on, the buttons for the A/C generate a 'thump' sound under the dash on the passenger side. No blower though. I took off the carpeted panel on the passenger side under the dash. There is a black plastic box with maybe 10 pins to the left of the blower. I removed the connector. One of the male pins (on the box) was very loose and the receiver slot for that pin on the connector is corroded. My thought is that this might be an A/C controller??? Next, I disconnected the 2 pin plug for the blower itself. I connected a hot and ground lead to the two male pins of the blower plug. The blower works. I also removed the switch panel behind the wood faceplate. There is power to some of the wires, but there are way too many wires for me to decipher what is what. So here we are. Did my diagnostic efforts tell anyone where my problem might be? Thanks a million in advance for guidance.

I've seen this problem several times on W123 and W126 Diesels and Gassers both... everything works but the blower motor, blower motor checks out OK. In all of these cases, the ignition switch was to blame. Sometimes, you can wiggle the key or move it back towards off slightly to activate the blower.

The dead giveaway is to look at the notches on your switch and surround. they *should* line up in "run" mode. Notches that do line up exactly don't always automatically mean a switch is "good", though.

Let us know what you find.

uberwasser · #11 05-13-2014, 04:28 PM

That's true, could also be the ignition switch. Wasn't in my case, but it's a possibility.

na5kar · #12 05-13-2014, 04:43 PM

tram, that's a good suggestion. In fact, the ignition switch was replaced before I bought the car. The only thing that makes me question that is the 'thump' I hear when I rotate the blue/red temp dial to the 'coldest' position. The key is in the 'run' position when I hear this. Now, I have to plead complete ignorance here. I can barely understand how the push buttons work. No manual and all.

na5kar · #13 05-13-2014, 04:44 PM

uberwasser, I will definitely try the junk yard idea

tram · #14 05-13-2014, 11:05 PM

Quote:

Originally Posted by na5kar

tram, that's a good suggestion. In fact, the ignition switch was replaced before I bought the car. The only thing that makes me question that is the 'thump' I hear when I rotate the blue/red temp dial to the 'coldest' position. The key is in the 'run' position when I hear this. Now, I have to plead complete ignorance here. I can barely understand how the push buttons work. No manual and all.

New parts can be bad... just ask gerrygv or whatever his username is... on the other hand, maybe not. LOL.

But seriously- in the cases I observed, including that of my old '84 380SE, the blower motor was the ONLY thing that didn't work.

uberwasser · #15 05-14-2014, 12:42 AM

Same with my temp controller. The A/C compressor would turn on, the push button would click and things behind the dash would clunk. The only thing that didn't happen, no matter what I did, was the blower working. I confirmed by testing for power at the blower and got none. 12V direct to blower showed it worked flawlessly.

So if you ignition switch is new, while it's possibly bad, I think the low hanging fruit is swapping the temp regulator with a good used part, at least after you check the resistor.