Aux water pump, monovalve 86 SDL - PeachParts Mercedes-Benz Forum

probear · #1 05-22-2008, 06:18 PM

I've just got this 86 SDL back on the road and having a slight problem finding one of the problems.
I have replaced the CCU and also the klima, the A/C system is fully charged and working, however, it appears that the monovalve is full open. Now, when we installed this engine, we did the acid wash and thoroughly flushed the system, then rebuilt the monovalve, and everything seemed fine for a short while.
Then came the heat. At idle, (with the A/C not working), the heat would be like a blast furnace. When moving down the road, it was not quite as hot, but still hot. When the A/C was hooked back up, the heat would equal or slightly best the cooling temperature. The blower motor seems to work at various speeds, and the aux heater pump feels like it is working.
A clamp on the heater hose lets the A/C blow really cold air.

Suggestions where to start?

Jeremy5848 · #2 05-22-2008, 09:26 PM

With the CCU set to maximum cooling, check the voltage at the monovalve. It should be close to 12 volts, which closes the valve so no hot coolant gets into the heater core. If voltage is present but you still get hot air, then the monovalve solenoid is hung up or the rubber diaphragm is torn.

If you don't have 12 volts across the monovalve then something is wrong with the CCU or one of its sensors.

Jeremy

LUVMBDiesels · #3 05-22-2008, 09:37 PM

Replace the fuse.The fuse drives the monovalve and the transmission kickdown. I had to do mine as the kickdown shorted out and blew the fuse.

#4 05-22-2008, 10:36 PM

Quote:

Originally Posted by LUVMBDiesels

Replace the fuse.The fuse drives the monovalve and the transmission kickdown. I had to do mine as the kickdown shorted out and blew the fuse.

That happened to my 300D also, but it was obvious because the same fuse powers the tach.

probear · #5 05-23-2008, 11:21 AM

Quote:

Originally Posted by Jeremy5848

With the CCU set to maximum cooling, check the voltage at the monovalve. It should be close to 12 volts, which closes the valve so no hot coolant gets into the heater core. If voltage is present but you still get hot air, then the monovalve solenoid is hung up or the rubber diaphragm is torn.

If you don't have 12 volts across the monovalve then something is wrong with the CCU or one of its sensors.

Jeremy

OK, pulled the monovalve plug and checked the voltage. With the control on high heat, I got 3 or less, and it varied, after turning the control to max a/c, the voltage went to consistent over 12....
Looks like the monovalve is going to be opened up.

THX !!

Jeremy5848 · #6 05-23-2008, 11:56 AM

Quote:

Originally Posted by probear

OK, pulled the monovalve plug and checked the voltage. With the control on high heat, I got 3 or less, and it varied, after turning the control to max a/c, the voltage went to consistent over 12....
Looks like the monovalve is going to be opened up.

THX !!

Those readings look correct but you should check under load, i.e., with the monovalve connected to the plug. Can you pull the plug partially off of the monovalve pins and get your voltmeter probes in there? or use a couple of wires?

Nonetheless, if you are getting heat with the CCU set to max cooling and 12 volts on the monovalve then it is definitely the monovalve.

Jeremy

probear · #7 05-23-2008, 04:08 PM

Disassembled the monovalve, cleaned it, flushed it, checked the diaphrams and reassembled. Tested it and it was operational. Also check the resistance across the terminals and that also checked out OK.

Started car again, and noticed that once again, hot air from the driver's side vent, BUT cold air from the passenger side vent. Also seemed line the center vents were not to normal flow, sooooo, now checking out the vent layout and vacuum hoses...

Any more suggestions?

Jeremy5848 · #8 05-23-2008, 05:46 PM

Mercedes climate control systems have built-in schizophrenia: they are capable of "blowing hot and cold" at the same time. The intent is to run the a/c as a dehumidifier and then warm the air (if needed). To that end, the CCU and the flaps have a complex set of operating variables. I would guess that some of the flaps are not working correctly.

Your '86 SDL is in some ways similar to my '87 D; there should be a manifold behind the center-right of the dash with a lot of hoses running to the various vacuum pods that control the flaps. You can remove one line at a time from the manifold and test the pod with a Mityvac. Any pod that does not hold a vacuum is probably bad. Replacing pods is another problem, often requiring dash removal.

If the pods all check out good, the other possibility is a bad CCU. This is easier to get at than the pods but more expensive to replace unless you can find one in your local junkyard.

Jeremy

probear · #9 05-23-2008, 06:27 PM

Thanks, but the CCU is a new reman. The last one that I got from them (in my other SDL, works flawlessly), but I will still keep in mind that even new can fail.

The vents seem to be a logical place to look but the confusing part is that at max cool, their is still heat, but the monovalve is suppose to be closed, thus not allowing hot coolant into the heater core, yet, I have heat.

Jeremy5848 · #10 05-23-2008, 07:08 PM

Quote:

Originally Posted by probear

. . . The vents seem to be a logical place to look but the confusing part is that at max cool, their is still heat, but the monovalve is suppose to be closed, thus not allowing hot coolant into the heater core, yet, I have heat.

As an experiment, run the car and operate the climate control system in a/c mode but do whatever you do to get hot air coming out of one of the vents, as you have described. Now open the hood and carefully pinch the coolant hose that the monovalve is in. Use a clamp or a pair of pliers or whatever it takes. With the hose clamped, does the hot air gradually become cold or at least no longer hot? If so, the monovalve is somehow being opened even though it is supposed to be closed.

Remember that the monovalve is fail-safe: with the power off, it opens and allows hot coolant through. This is so you will always be able to clear a fogged windshield. Anything that interrupts the 12 volts to the monovalve can result in unwanted heat. Check for dirty or misaligned connectors, broken wires, etc.

Another experiment: remove the plug and connect 12 volts and ground directly from the battery to the two pins of the monovalve. I don't think that polarity matters but check and see. With a constant 12 volts on the monovalve, it should be closed and the heat should go away. Does it? Now remove the 12 volts. The heat should come back. Does it?

Be careful not to leave the 12 volts directly on the monovalve for a long time (more than 15 minutes). I'm not sure it was designed to dissipate that much power (even though it has hot coolant going through it). The CCU produces a variable duty-cycle pulsating DC, which dissipates less energy. This is probably overkill but I don't want to suggest something that would damage your car.

The only other thing I can think of is to ask whether your SDL can have different temps set for driver and passenger. My wife's '96 E300D can do that and it has a "duovalve" instead of a "monovalve." But it's a much later car.

Jeremy