I'm not really sure what the cause is, but here's a couple of comments:

1) I really don't think the low voltage is a problem at all. Due to the wiring harness resistance, I'm not at all surprised that you're only seeing 11.9v downstream. That should not be enough of a drop to cause the system to fail. If it was under 10v or something, yes, that's bad... but around a nominal 12v, I wouldn't bother spending any more time there.

2) The clutch is probably not slipping... the KLIMA would kill the compressor. It has a tach at the rear of the compressor, and compares the speed to engine speed. All you need to do is make sure the compressor (clutch) is not disengaging above a certain RPM (looks like 3k is the magic number here).

3) The fact that you can physically pinch off the heater hoses and prevent hot coolant from entering the heater core, I think we can rule out anything in there.

4) To check the compressor engagement, you could rig up a test light (small 12v bulb) in the passenger compartment, that turns on when the clutch is engaged. Go for a drive, see if the bulb (compressor) turns off just before the vent temps skyrocket. That would indicate a control problem.

5) Assuming the compressor is staying engaged, the problem appears to be refrigerant related... time to check pressures. If your lines have unmolested Schrader valves, it's likely still an R-12 system (and if so, DO NOT convert it!). If it has adapters screwed on that are "quick connect" type, it's been converted to R-134a... but there should be a decal under the hood indicating this. You can buy gauges (make sure you get the right type, R-12 or R-134a) and do this yourself if you want.

6) I believe there is another electrical safety circuit, separate from the high/low pressure switch, that can turn off the compressor. On the older models it's tied to a temperature switch. On later models it's tied to a temperature sensor. Might be worth investigating this, the sensor could be bad...?

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Dave
Boise, ID
Check out my website photos, documents, and movies!

I am by no means a seasoned MB mechanic, just a DIY keeping a '91 300E on the road and have learned a few things along the way.
It sounds like all of the a/c questions have been asked and answered but it seems that the [I]air delivery[I] has not been addresseed.
A'90 124 chassis will be needing attention to the vacuum pods within the dash. Sounds like your vac pods are sucking air and doing what they want to instead od what they should. If I remember correctly after doing mine last year, I had 6 out of seven bad pods. Sounds like you have a pod that is bad, probably the one on top of the evaporator case. I think these can be diagnosed off of the "christmas tree" behind the glove box. I would almost guarantee that if the pods are original in the car then thay are kaput!
Just an opinion! Good Luck!Thanks.

It is not refrigerant related. Sounds like the same issue I had with the 190E. I had a bad vacuum check valve, therefore when vacuum dropped due to acceleration the check valve did not hold the vacuum in the CC system and reservoir. Thus causing the blend air flaps to go to the hot position and thus dumping warm outside air in the cabin. Check the check valve that is on the firewall and enters the cabin to feed the reservoir and the CC system. Should be a plastic one. Also check for cracked rubber connectors on the vacuum lines throughout the engine bay. If you can't find anything there then look at them all in the car, you would need to most likely remove the dash to see them all but I doubt they are the problem. New check valves range from a few cents to a few bucks. Mine was actually no longer closing when the vacuum dropped on the engine side. I had another not letting enough vacuum to pass into the CC system therefore never enough to hold the flaps all closed properly. Currently if I use just my vents I get air from every other one due to a bad pod on the floor/windshield pod. So by just using the dash vents I get a vacuum leak when the system tries to close the upper and lower vent ways. So it could also be a bad pod. I can hear mine sucking when I try to close those vent conduits.

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~Jamie
_________________
2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

Simple test to eliminate the outside air entry theory: Tape off the air intake grilles, at the bottom/center of the windshield, below the wiper arm. Use the low-adhesive "blue" masking tape for easy removal. If the problem goes away entirely, then you can rule out a refrigerant issue, and start looking into the vac pods.

For the record, I don't believe this will turn out to be a vac pod issue... for the most part, the vac pods direct where the air goes, but have a relatively minor effect on air temperature. Even if the main air flap were full open, the evaporator should still be 40°F, and there should not be 100°F air coming through it. However I do agree that one or more of the vac pods are probably bad... I've had to replace at least one on almost every W124 that I've worked on (currently about 9 different cars).

A vac pod replacement tutorial is located here:
http://***************/forum/showthread.php?t=528

But this would not cause 100' air to come out the center vents. It would cause no air out the center vents.

And BTW hispassion, what's coming out the side vents while all this is going on?

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2012 E350
2006 Callaway SC560

Wow, thanks guys for all the input. This along with gnashing my teeth has helped eliminate a few things. These are the things I know it's not.

1) It's not a voltage issue.
2) It's not the compressor clutch or Klima (MAS) relay.
3) It's not (at least for now) the vacuum pods.
4) It's not a refrigeration issue (suction line cold during heat up)

This morning on the way to work, I had a rather warm ride in. The ambient was 89°F and without many rpm's at all, I had temps up over 100°F and at one point (during heavy acceleration) it got to 110°F. Since it was only 89°F outside, 110°F at the vents had to be heated air.

When I got to my favorite (no shade) parking space at work, I opened the hood and felt the suyction line. It was cold and sweating. The heater hose (on the same side of the car [outlet?]) was hot as was the aluminum futher in towards the heater core.

So, at lunch I grabbed my pinch-off pliers, asked the fleet tech here if I could borrow another pair and I pinched off both the inlet and the outlet to the heater core. Then I went for about an 8 mile drive. The ambient was 102°F. The vent temps were 60°F regardless of rpm, fan function, vehicle speed or acceleration.

Therefore, it has to be hot water (coolant) getting into the heater core. Now, before I told you all that I pinched off the inlet to the heater core and this is true. I also pinched off the outlet to the heater core with no change in symptoms. But when I closed off both inlet and outlet, there is plenty of cool air.

And I wonder. Is it possible for the engine water pump to force water backwards through the heater core and past the heater valve - and if so, how? The one thing I didn't check through all of this was the Aux Pump. Do you suppose if it isn't pumping towards the valve that it can allow water to circulate backwards?

For now the symptom has a band-aid (well actually two) on it. So, a final thought and question for this post is; I saw somewhere on this board that someone had come up with a vacuum operated heater valve modification. I'm sure I could find it by searching, but have any of you read that post/thread and would you recommend that mod? Or should I just spend the money on a new pump and heater valve?

Thanks again guys for all your help, thoughts, suggestions and diagnostics... I think we're almost there!

Regards,
Michael B.

Oh, Brewtoo, almost forgot - the side vents I determined a while ago also blew warm/hot air. I just didn't update ya.

__________________
1992 300CE - 157kMi (my white beauty)
1992 300CE - 220kMi (sold)
1990 300E - 189kMi (sold)
1990 300SL - 102kMi (salvaged parts available)
1986 300E - 230kMi (salvaged parts available)

Expert RV Generator Repair
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When no one else can, I do!

We judge ourselves by our intent.
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Yes, now that you have gnashed your teeth, if you will wring your hands a while I think you will have it!

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2012 E350
2006 Callaway SC560

Sure sounds like the monovalve to me. It's a relatively rare failure (unlike the older styles). I think I'd try replacing that first. BTW, your vent temps are pretty high... I'd expect <40°F while cruising at speeds above 40mph. My vent temps only approach 60°F when poking along in stop-and-go traffic on a hot day.

I'd still check all the vac pods while you're at it, this is relatively easy after you access the 7-port manifold behind the glovebox. Each line should hold vacuum... if not, the pod on the other end is dead.

Brewtoo, I took your advise and have been wringing my hands all afternoon, Stopping only long enough to hit the F5 (refresh) key on my keyboard.

Gsxr, I think you're right. In my first post I noted there was coolant in the coil area, but dismissed it as a minor issue since it was clunking and hot water was only on one side of the valve. In the thread (linked below), Richard Wooldridge explains to someone else "Check your auxilliary water pump - the monovalve is just like the water valve in your dishwasher or washing machine, it requires water pressure to operate it. It has a primary valve, operated by the solenoid, that controls a bleed to allow the water pressure to operate the main diaphram, and if there isn't enough water pressure the monovalve will allow water to flow, even when it has the proper voltage." He goes on to say that he replaced the whole thing with a vacuum operated valve.

I know he's talking about a monovalve and I haven't seen one on an MB. But I have repaired/replaced many a dishwasher/washing machine inlet water valves. I've had my heater valve apart 2x and it doesn't resemble an inlet valve for a major appliance.

I guess the question that remains is; would it be a good idea to move to a vacuum valve rather than an electric OEM valve? Also, if the Aux Pump isn't working, could the coolant reverse direction on high rpms? I will search threads for when the aux pump is supposed to be on - on a W124.

Here's the thread...
http://www.peachparts.com/shopforum/diesel-discussion/188981-mono-valve-voltages-post1524566.html

Lastly, I believe the system to have been converted to R134, though I don't know how thorough a job it was since it was done before I bought it. Heck it was 105°F ambient with 60°F vent temp for the ride home which gives me a 45° temperature differential. Should I still expect 40°F from the vents with R134 when it's 105° out? If so, I'll have the system checked for proper charge sometime this week.

I did notice (before this problem) when driving around at night when "reflective" temps are a non-issue, my vent temp is down considerably. Phoenix is desert and we have reflective tempertures off the pavement of 135°F on a good day.

I'll check the pods too as you described.

Thanks again and kind regards,
Michael B.

__________________
1992 300CE - 157kMi (my white beauty)
1992 300CE - 220kMi (sold)
1990 300E - 189kMi (sold)
1990 300SL - 102kMi (salvaged parts available)
1986 300E - 230kMi (salvaged parts available)

Expert RV Generator Repair
Go To: MiGen Repair
When no one else can, I do!

We judge ourselves by our intent.
We judge others by their actions.

Check your heater core and evaporator temperature sensors as well -- a bad heater core sensor can turn the heat on unexpetedly.

Replace the monovalve, too, it's defective if it had coolant in it.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Peter,

These temperature sensors might be located where? I suspected the monovalve, but when the outlet of the valve was cool to the touch, it put me in a diagnostic tail spin.

It wasn't until I realized the outlet side of the heater core was hot that lead me to believe it might be flowing in a reverse direction. Maybe it does flow from the engine block through the heater, through the valve and so on, but I thought it flowed from the water pump through the aux pump, up to the monovalve, through the heater core and back to the block. Can anyone confirm the path of coolant flow for the heater?

Thanks!
Michael B.

__________________
1992 300CE - 157kMi (my white beauty)
1992 300CE - 220kMi (sold)
1990 300E - 189kMi (sold)
1990 300SL - 102kMi (salvaged parts available)
1986 300E - 230kMi (salvaged parts available)

Expert RV Generator Repair
Go To: MiGen Repair
When no one else can, I do!

We judge ourselves by our intent.
We judge others by their actions.

Assuming proper charge and all, it must have been converted as my '83 300SD gives similar vent temps at high noon (58F to 63F) on the highway and it was converted to R-134a just over three weeks ago before I moved here to Phoenix.

I don't know how colder mine would be with R-12, but a good A/C should give you about 40F at the vents even with the ambient at 110F. I've driven a car around here that can even hold 41F in city traffic in the afternoon!

I had never thought about it before, but yes, the heating circuit does flow from the driver's side to the passenger's side. The feed pipe comes from the driver's side of the engine, from the cylinder head, which would be the hottest part of the cooling system - makes perfect sense. The reason you felt a cool hose on the other end was because your AC evap was cooling the liquid as it went through the heater core - d'oh! The ACC manual seems to corroborate this theory, based on the diagram in 83-600, if you note the direction of the arrows by the monovalve.

My 1994 model with factory R-134a cools remarkably well... I've seen vent temps around 42-44°F, even in 100°F ambient temps, at low humidity (15-20%) here in Boise. That's when maintaining a steady speed though, not stop & go traffic. It's not quite as good as my R-12 systems, but it's quite acceptable. On a long freeway trip, my R-12 system can get in the 34-38°F range. BTW, these numbers are on lower fan speeds... the temps will be higher when the fan speed is higher, since the air has less time to cool as it passes through the evaporator.

I'm not so sure if the conversions are as efficient. Usually the conversions are done poorly, meaning they do NOT flush out the entire system to remove the old mineral oil. This often results in system failure (Google "R-134a Black Death" for some info) several years later. My '87 that was wrecked had been converted and I was never impressed with the performance, and I could hear the compressor operate... made me wonder how much longer it would have still functioned. The early factory systems are tolerable, the later ones I believe are better.

I have not read the entire thread. That said, I don't think that your vacuum pod test was valid. Somehow find a vacuum source, remove the glove box again and check the pods individually. If memory serves me correctly, the second line from the top of the switchover valve is the recirculation pod. Pay particular attention to that one.

If the compressor is turning, the clutch is not slipping, at least not significantly. I think you're chasing a red herron with the voltage thing.

My $0.02,