86 S class climate PBU in 87 300TD - PeachParts Mercedes-Benz Forum

mbehensky · #1 12-07-2014, 04:03 PM

Hi, All-

I had a problem with the climate control push button unit on my 1987 W124 300TD wagon. The blower motor and amplifier were fine, but the climate control never ran the blower. I knew it was probably the PBU, since if you put some voltage on the pin at the PBU the blower motor turned on fine.

I took a trip to Pic N Pull yesterday, and, unfortunately, the only W124 they had with a PBU was a 1995 gasser. I remember reading something about the control for the auxiliary water pump changing sometime in the 1990s, so I didn't think I wanted that one.

I grabbed the PBU out of a 1986 S class gasser they had there (don't remember the exact model). I thought that it would slip right in, but after I got back to my car I realized that the plastic housing is different. I did test it, plugging it into the connectors on my 300TD, and the blower came on fine. While I was driving home from PNP, it seemed like it pretty much worked.

After I got home, I decided to try and switch the boards out of the SD housing into the W124 housing. Here's my notes on the process:

1. The first step is to remove the bottom plate on both units. Take off the 4 small screws holding it on, and use a small flat bladed screw driver to pry the bottom plate loose. There are 3 places at the back where there is a little plastic tab holding it in place.

2. Next, you have to remove the buttons to get the boards out. These are really hard to get off. I destroyed one switch on the good unit by pulling too hard. You can use a small screwdriver to pry between the button and the white plastic shaft that goes into the switch. Do your broken unit first so you can get the hang of it.

3. You have to disconnect the temperature control potentiometer from the control wheel before you can get the board out. Before you start, turn the control wheels on both units to Min (or Max, you just want them to both be the same). There is a plastic pin that connects the wheel to the pot. It looks like a flat bladed screw, but it is just a pin. You can use the small flat bladed screwdriver to pry the pin out. It is really long, and you have to pull it out a long way to free up the board. Try not to rotate the wheels when you remove the pins. The point of all this is that the wheel can be put back in the wrong orientation (there are two possible positions).

3. You should be able to remove the bottom board now. It is connected to the upper board (the one with the microprocessor on it) via a wide flat cable, to the fan control button board with a flat cable, and to a board with the lights on it with a flat cable. You first have to remove the processor board. The board has a tang that fits into a slot at the back of the plastic case. On the donor unit (the SD), you can clip out the plastic to make it easier to get the board out. On the W124 unit, I stuck an Xacto knife blade in the slot between the board and the plastic to hold the plastic out of the way, and pried it out. You could try and get it loose at the front end, but I didn't want to crack the clear plastic light diffuser.

4. You can just pull the fan control board out easily, but the light board looked like it would be hard to get out. There are only two wires on the flat cable, so I unsoldered them on the bottom board and re-soldered the SD board to these wires. I don't think polarity matters, since they are incandescent lights, but I made sure I didn't swap them anyway.

5. Now put the SD boards in the W124 unit. I got the processor board in by putting it in under the tangs on the light diffuser and snapping it in at the back. The fan control board just gets stuck in place (it gets held by the buttons. The bottom board gets held in by the bottom plate, but don't put the plate back on yet.

6. Now stick the buttons back in. The buttons can get put in backwards; each button has a white section that should face the light diffuser so they will light up properly. You have to push pretty hard to get them to snap into place.

7. Put the plastic pin back that connects the temperature wheel to the potentiometer. Make sure you have the wheel at Min when you put the pin back in. You can get it put in 180 degrees off, so be careful. If you get it wrong, you wont be able to rotate the wheel all the way without breaking the pot or pin, and the temperature will be way off. Make sure the wheel can rotate all the way from Min to Max by turning it gently. If it won't spin easily, you may have the wheel 180 degrees off (this happened to me, because I didn't leave the pot at a known location when I took the pins out). Try spinning the wheel a half turn and putting the pin back in.

8. After making sure that all the buttons work properly, snap the bottom board on the W124 unit back in, and replace the screws.

Enjoy your brain transplanted PBU. Mine seems to work OK, although I haven't tried the air conditioning functions since all my freon leaked out long ago.

Max Behensky