Gas W126 climate control in diesel? - PeachParts Mercedes-Benz Forum

bustedbenz · #1 08-07-2009, 01:08 PM

In chasing my climate control problems (A/C is cold when it works, but cuts out fairly early in a trip and can never be persuaded to re-engage) I've decided to try a different dashboard "computer" -- replace the control box and see if it's sending bad signals to the system. Doesn't cost me anything to try, IF I can use the parts I've got.

I went to pull-a-part two or three weeks ago and got a climate control unit that looks like it's in MUCH better shape than mine. The buttons press more firmly, it wasn't covered in dust, the temperature wheel has a firmer rotation and clicks into the detents more strongly; it appears to just be generally less worn out and possibly okay. There's a tag on it that says it was inspected and found okay in 2001, which means it was somebody's rebuild/replacement. Point is, I'm optimistic enough to assume it works at least as well as mine does until I learn otherwise.

Here's the catch. It's out of a gasser w126, not sure which one or what year but I think it was late 80s. I know for a fact that it's the same "generation" of w126 as mine, rather than the older one. I haven't checked each plug to see if they match but IIRC from last time I had mine out a year ago, they seem to. It has the exact same complement of rocker switches up at the top for defrost, hazards, etc; in the same order with the same markings, so I'm assuming they're identical switches if the pins all line up. (I just took the thing out with the whole wood trim piece and all the switches, since the wood looks better than mine too. Going to change it in as a unit.)

Is there any reason that shouldn't work? The only visual difference I see between my button box and the potential gasser replacement is that mine has a very thin "O" marking on the off button, whereas this one seems to have been printed in "bold". Everything else seems identical as near as I can make out. Does somebody know that they in fact do completely different things under the dash or run on a different voltage or something? If I were an engineer, I'd put the same climate mechanisms in any w126 no matter how it was powered, but I can't assume that without somebody saying "Sure, try it, you won't shred anything."

compu_85 · #2 08-07-2009, 01:16 PM

It should work just fine, all the Gen 2 W126s have the W124 climate control system, gas or diesel.

-J

tbomachines · #3 08-07-2009, 01:18 PM

It should plug right in. What other troubleshooting have you done so far? Have you tackled the KLIMA?

bustedbenz · #4 08-07-2009, 01:29 PM

Quote:

Originally Posted by tbomachines

It should plug right in. What other troubleshooting have you done so far? Have you tackled the KLIMA?

No, I haven't. I can't even physically locate the KLIMA at the moment. I haven't really had time to troubleshoot it, and what time I have spent on it has been in trying to further my guesses about whether I'm looking at head gasket or head problems. (Still waiting on an oil report.)

I've just checked every fuse I can find to prove that they're good, and I've put gauges to it and verified that my freon levels are adequate to be cooling at least somewhat.

The trouble I've got is that the compressor works but doesn't seem to stay engaged; it just randomly (no button pushing from me) clutches out and never comes back the rest of the day. Engine restarts don't help, pushing every button on the box doesn't help... it just dies. So I figure it's got to be electrical somewhere. Figured the computer was as good a place to start as any, since it is already showing signs of not working right. Specifically -- it works fine all summer, but any time the ambient temperature is below 40F or so and I start the car, it takes 20 minutes before the system responds to me at all. It doesn't matter which button I push; if it's cold outside (when I desperately need heat on the glass to make it safe to drive) it simply doesn't do anything. And when it does engage on those days, it does so with a HUGE flurry of relays clicking in and out rapidly, the blower motor starting, stopping, starting, stopping, starting, etc... and just generally a whole demonstration of electrical foolishness. And then when it does finally settle down frequently it's stuck in "heat" mode for 15 minutes until I either pass out from the temperature or it finally realizes i asked it to cool down half an hour ago.

So all this suggests that my button box needs replacing even if I still have other problems. If the compressor cut-out doesn't stop doing that when I put the new computer in, I'll keep troubleshooting.

Brian Carlton · #5 08-07-2009, 01:54 PM

It plugs right in alright...........but, I believe the logic is different on the gasser. The diesel provides a ground signal to the Klima to allow it to close. The gasser provides +12V for the same task.

I do not believe that they can be exchanged. Check the part number............probably different than the diesel.

pawoSD · #6 08-07-2009, 01:57 PM

Quote:

Originally Posted by Brian Carlton

It plugs right in alright...........but, I believe the logic is different on the gasser. The diesel provides a ground signal to the Klima to allow it to close. The gasser provides +12V for the same task.

I do not believe that they can be exchanged. Check the part number............probably different than the diesel.

Why would they make that backwards between the two?

You could always use a relay to reverse that instruction.

micalk · #7 08-07-2009, 08:47 PM

It might not hurt anything, but the part numbers between a 88 420 SEL and 86 SDL are different. Check the wiring schematics in the FSM, the troubleshooting section should have good schematics. Or take the covers off and look inside. If they are different, it shouldn't take too much inspection to make that determination.

I'd verify that the tests in the FSM pass or fail before adding an unknown (the gasser ACC) into the mix. You should be able to test the climate control system with a multimeter if you know what you're doing. Might have issues with the monovalve - when mine was sticking open it would give me heat with the temp wheel turned all the way cold, and the hotter the interior got, the faster the fan would blow making it even hotter. Well, the control unit didn't know that the monovalve wasn't working, it just knew that it was hot, and that it needed more air to cool it off. My point is that you need to check all your inputs to the control unit before you decide it's bad. And by the same token, you need to check that when the control unit sends a signal to a relay that the relay actuates and performs its intended function.

I haven't looked at the schematics in this particular case, but usually 12V is hard wired to a relay and a ground is used to actuate it. So now I've just given a SWAG instead of fact which I think puts me in the category of Senior Member. Woo-hoooo!