87 300sdl AC Issues
 

My symptoms are "cool" but Not Cold air on Normal setting all blower settings work properly, flow comes out the right vents. Temp wheel seems to go from cool to hot and back properly. But, going down the road, it can fluctuate from cool to a mixture to roasting hot and back to cool.

A few years ago, the monovalve had the repair kit installed and think it was OEM instead of MTC. I have another OEM on order. The cabin temp aspirator is Not sucking air. I found it by removing the glove compartment insert but can't figure out how to actually get to it behind a vertical air duct and a horizontal plenum.

I'm thinking about going to an decent shop to check AC pressures. It has been converted to R134. The aux fan appears to come on when called. If someone could help me with the best way to remove test and/or replace the aspirator and the ohms for the sensor to see if it's good I would appreciate it.

Tomorrow, I'm going to try to clamp the heater hoses to see if I get all cold

If the aspirator isn't working, it won't be pulling air past the cabin temp sensor and could cause some unexpected behavior.

Cool but not cold could be refrigerant charge, or more likely the monovalve leaking by. Go lay eyes on the fat refrigerant line in front of the brake booster. If the metal part is sweaty and cold to the touch when the A/C is running, it's likely to be the monovalve or even could be the pushbutton unit. If the monovalve loses voltage, it will open and pass hot water through the heater core. There is no blend door, so it will be heater vs. A/C. Heater will win.

I would describe the refrigerant line as cool to the touch, not cold and sweaty. I don't think I'm getting outside air but it's been in the mid 90's here lately. Econ will give me what feels like heated air.

Klima is brand new. OVP had a blown fuse now good which gave me back AC compressor, ABS and tach. Car had been sitting with very little use in the last couple of years.

I realize a dead aspirator and untested sensor will probably explain the problems but I can't find the value to test the sensor against and the best way to get the aspirator out right now. OEM mono valve kit is on the way as well.

The aspirator motor just unclips from the bracket it is sitting in. You should be able to work one side loose and roll it out of the bracket. Then you can unclip the hose and the plug on the rear. Try spinning the little fan in there, the motor may be seized up. Set the temp wheel to max cold (dot on the blue band) and see how it does. That bypasses the cabin sensor and defaults to max fan and max cold.

If you have a very high load on the A/C and you're in a low humidity area, you may not get a sweaty return line. It should be relatively cold though. If it isn't, you may be slightly low on charge. Checking with a set of gauges would be prudent before attempting to tweak the charge.

EC should just give you ambient air from the outside intake if you're set to max cold on the temp wheel. If it feels heated, it probably is if the monovalve is leaking by.

My aspirator is on the firewall behind the glove box and to the right and embedded in foam. In between is a vertical duct going to the vent above and below it is a wide horizontal duct, I can hardly even get a finger on it.

Is it a better choice to remove the carpeted knee bolster under the glove box and get at it that way?

Did they move the aspirator in '87? In my '86 it's immediately to the right of the glovebox. There is a duct in the way to the right hand vent, but you can work around it. It just unclips and rolls up and out.

Focus on the A/C system health first. By putting the temp wheel on the blue dot, you're effectively bypassing the temp sensor. The system should try to run as cold as it possibly can. If you start noticing the compressor cycling, you could have a problem with the evaporator temperature sensor, but you'll never know until you observe the system in operation.

Grasping at straws until you confirm the refrigerant circuit is good first. There is a steady small leak rate, so not unexpected to discharge over the years.

This car did have a new mono valve insert installed by Mark at Stu Ritter Mercedes in Denver about 4 years ago but I didn't know for sure but suspected that he installed a genuine MB repair kit.

After receiving a new one and installing it, no joy. It would blow cold air until the coolant got hot then the blast furnace started.

After a lot of reading and testing I finally cracked the code. My mono valve and the coil which is the top portion of the removable part of the repair kit is good.
Putting a multimeter on the two pins gave me 4.80v on max cool. It should be 12v. Putting the positive probe on brown and negative proble on battery negative gave me 12v. That means the CCU is not switching ground.

To prove it I removed the two pin connector on the mono valve and used two jumper wires. One from battery positive to brown position on the valve and battery negative to the ground position on the valve and I get a thunk, valve closed no more hot coolant into the core and wonderful cold air.

The good news is the mono valve is good. The bad news is probably the CCU ground switching function is toast. All other functions work like auto where turning the temp wheel slightly cooler or warmer work with the exception that I won't get hot since the valve is now held closed.

Most owners with a mono valve problem is no heat. Usually it means it's stuck closed or you don't have 12v to the brown pin. Other like me have a ground problem in the CCU which leaves the valve always open. Or, more likely leaking past the rubber parts of the valve.

Is there a temperature regulator in a 87sdl that handles ground? Is it under the blower motor?

Ground to the monovalve is handled on the CCU board itself in a darlington transistor package. If you're handy, pull the CCU and look for crappy/broken solder joints and burnt traces, I'd be willing to bet money you'll find more than one.

Thank you Diseasel, I was hoping for someone to confirm the CCU controls ground switching. BTW, I have been following your posts with great interest. Very well done

I successfully repaired my CCU which was a process itself. Cutting to the chase, pop the CCU apart and inspect the circuit card. Look particularly closely at the traces on the back of the card, the main connection pins, and the pushbutton area. If you see cracks or rings around the joints, flow in some fresh solder, that's likely all that's wrong. If you see burnt traces or charring, things can get more involved and you may be better off with a rebuilt or 2nd hand CCU.

Taking my own observation of no ground to a known good mono valve and using Programma's test procedures, I ordered a remaned CCU. Hopefully, this will fix it. I'm going to keep the original for a fixable spare.

After receiving my remaned CCU from Programma, my hvac is working correctly. Next is pulling my fuel sending unit. I found out the hard way that it's stuck at just over half full.

They never seem to work well in my W126s, even with new sending units. I just go by mileage on the trip meter.