Climate Control - PeachParts Mercedes-Benz Forum

YuCrew · #1 10-29-2007, 09:17 PM

Calling all experts. '84 380SL, 85k miles with ACC-3 (horizontal pushbuttons), rotary temp control, blower push buttons.
System completely inoperative. Replace fuse and everything works great. That was too easy. System seemingly working perfectly. High speed blower in defrost mode and I can hear AC compressor clutch engaging. Dampers opening and closing as expected in various modes. Move to economy mode, compressor shuts off. All fan speeds work as expected. Life is good. Now onto the ML to replace the driveshaft support bearing.

2 days later (today) drive approximately 10 miles...heater working fine. Good thing 'cause it's cold in CT. Then suddenly, without any noise/sounds/warning, the climate control system completely stops. No blower in any mode, including the defrost mode. No dampers moving. No AC clutch engaging. No circulation pump. All fuses in the fuse box are ok. Tested them. Swapped them. But the system acts like there is NO power. Not a single element of the system seems to be operating.

Any ideas or suggestions for a starting point or culprit? My first inclination is to start tracing 12V from the fuse block but the circuit diagram I have from the maintenance CD is difficult to read. Perhaps checking if 12V is at the pushbutton control and tracing that through the defrost circuit to the blower would be a start. I'm worried that 12V is not even there, I won't be able to troubleshoot the other 2 electronic boxes. Thanks.

Walrus · #2 10-29-2007, 09:32 PM

I have read anecdotal reports of similar climate control problems like you describe, caused by faulty ignition switch. Seems some previous owners may "overload" the switch by hanging a "Bunch" of keys on multiple rings, thus causing problems... Its a stretch... but has been reported. Try wiggling the keys in the switch.

I hate to imagine further...

meltedpanda · #3 10-30-2007, 10:15 AM

I would bet on the ignition switch, defrost not working is unusual as MB built in a failsafe with these to always have the defrost work, even when everything else fails

YuCrew · #4 10-30-2007, 04:19 PM

I will try the key. Hopefully that will be it but the key to this car is on a plastic key ring, with NO other keys. Thanks for the inputs. If this is the culprit, I will report back here.

ctaylor738 · #5 10-30-2007, 04:38 PM

You need power at Fuse 7 with the ignition on. The feed from there to the push-button unit is through a Black/Violet wire that goes to Pin 12 on Connector 2 of the PB unit.

I would start there.

JFawcett · #6 10-30-2007, 04:41 PM

Could be the pushbutton control unit.
My 87 560SL did that....it would be great and then, all of the sudden, part of it would quit. Meaning the blower would stop (but the AC would continue). The CCUs are really 3 components soldered together and with time and usage, those solder connections break and gremlins get into the workings.
You can get a rebuilt one for about $150 from an outfit in Georgia.

YuCrew · #7 10-31-2007, 07:48 AM

Thanks to all for your inputs, especially Chuck.

Chuck: I have voltage at fuse 7 and will now explore at the control unit this weekend. When fuse 7 failed last week, I did pull the pushbutton control and saw NO questionable solder joints but decided to re-heat ALL the joints. I don't know if I had just a fuse problem then or also a questionable joint since I stupidly did both before re-testing. My bad.

Dee8go · #8 10-31-2007, 08:25 AM

Quote:

Originally Posted by meltedpanda

I would bet on the ignition switch, defrost not working is unusual as MB built in a failsafe with these to always have the defrost work, even when everything else fails

That's what I was going to say. Having once owed a Buick Skylark whose heater died as I was driving through the Smokey Mountains in the snow in January. We were all wrapped in blankets and everyting was okay until ice began forming on the inside of the windshield!

I have a particular appreciation for the way MB's avoid this problem, until now, of course.

ctaylor738 · #9 10-31-2007, 08:43 AM

Are you sure that the fan didn't die? That can make it seem like everything else is dead.

If you are serious about troubleshooting this, an Alldatadiy.com subscription for the car for $25 is a pretty good deal.

YuCrew · #10 10-31-2007, 08:48 AM

Good question but fan motor was brand new, OEM (motor only) installed last year. I'm fairly confident it is not the blower but haven't checked it yet nor the resistor bank (I presume no resistance is engaged at full speed).

Will trace hot lead from blower to push button control unit and fuse to pb control this weekend. Do you know the pin # on the pb control that is the feed to the blower motor?

Dee8go · #11 10-31-2007, 08:48 AM

If it was just the fan, wouldn't you eventually feel some heat coming up through the upper vents, Chuck?

ctaylor738 · #12 10-31-2007, 10:11 AM

Maybe you would feel some heat, but it would be faint. Actually, I am not sure what the monovalve will do if there is no power at all to the system. The ACC sends a constant positive to the valve, and then sends ground on and off depending on the temp setting to close it. So you would think that with no power, the valve would be open and you would get some heat.

But WRT the pin number. The feed to the blower is through the push buttons and the blower switch, then to the relay, then to the pre-resistors, then to the blower motor. Simple things in the system get complicated very quickly because there are three black boxes that all sort of work together to give air direction, blower speed, and hot water flow.

I am thinking if you don't have Defrost, which is the most basic of the settings, that some really basic shared part of the push-button unit went TU and the unit can't get off the OFF position.

YuCrew · #13 10-31-2007, 11:21 PM

Thanks to all for your inputs. Here's a status report.

It's not the ignition switch, but given the sypmtoms, could have easily been the case.

Thanks to Chuck, I've traced voltage from fuse #7 to the ACC terminal 12, left plug. You may recall earlier post of this problem indicated a blown #7 fuse first. Replaced fuse and unit ran successfully for something less than 10 miles before NOTHING operational.

So, I opened the ACC and started to trace the circuit from pin 12. Continuity from pin to "edge" solder joint good. So onto the next component solder "pad" in the circuit. Hmmmm, no continuity. Oh, there it is....an open trace in the circuit. Not blackened or seemingly catastrophic...just small piece of path missing. I've repaired the board but have not installed it to test.

I'm wondering if the blown fuse and then a blown circuit board trace indicate a load condition in some fan mode that creates too much load or appoaches a short when in automatic mode versus full on or low. Perhaps a faulty resistor bank, which should be readily evident and only impacts the circuit when running on a speed other than high or low? I think I'll dig into the blower compartment next, starting with disconnecting it and allowing the balance of the system to operate (monovalve, aux pump, etc. which should put some heat into the interior). Any other next step, troubleshooting thoughts?

Thanks to all for your suggestions. I appreciate your inputs.

ctaylor738 · #14 11-01-2007, 08:30 AM

Nice wrench!

I think that you're on the right track. But doesn't maximum resistance occur on the low fan speed setting to reduce voltage?

Richard Wooldridge · #15 11-01-2007, 10:18 AM

I seem to remember reading sometime back on one of these posts that the aux water pump can short out internally and cause blown traces. I've been thinking that it might be a good idea to isolate some of the loads on the climate control by adding a relay and separate fuse to the fan circuit and the aux water pump circuit. This would ease the load on the climate control unit and also make it easier to troubleshoot, as a blown fuse in one of the relay circuits would isolate the defective part.