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hispassion 08-25-2007 10:40 PM

300E - Warm air from vents and 3k rpm.
This will likely stump the best. So, get in, buckle up and hang on...

Re: 1990 300E W124 A/C Climate Control Problem

It's summer in Phoenix (as it is most of the year) and it's HOT! That said, on the way home from work Thrusday afternoon, the car started blowing warm (err, hot) air out of the center vents. Thought maybe it was the fuse gremlin thing and rotated each of my fuses 360 but there was no change for the ride in to work Friday morning.

This bothered me and taking some time at work, I read thread after thread with post after post looking for some help. From that research, I thought I'd check the following at the shop before going home:

Heater valve (some call in a Mono valve) - I took it apart and found that it had coolant in the coil area, likely caused by a poor seal. I cleaned up the plunger, determined that I need a new one and put it back together. The voltage to the valve was good with the KOEO and vent selected on the CCU. I supplied battery voltage to the valve and it clunked (before and after disassemble/reassemble). Also, hose was (after warm up) too hot to touch on the inlet side and cool enough to hold on to on the outlet side. Thus, I believe the valve to be working properly, at least for now. I'll replace it soon though.

Vacuum switching solenoid (behind the glovebox) - After removing the accumulated glove box junk and pulling out the box itself, I was able to access this solenoid. When I pulled off the green hose, it hissed, leaving me to believe the vacuum canister was not leaking. Though I wasn't able to test this compenent, I surmised that it was ok since the vents and ducts to seem to switch when called to and operate fine.

Climate Control Unit (CCU) - I pulled this out, removed the cover from the bottom and briefly inspected the circuit board. It looked ok, so I put it back together and reinstalled it.

While still in the shop, I restarted the engine and let it idle. The A/C system was working good and I had 60F coming from the center vents. Was this an anomoly, I thought. Maybe. So I closed the shop and began my drive home. It was 98F ambient and the AC system worked good until I took off from the first light. Almost instantly, the temperature in the center vents climbed to 90F. Obviously I didn't fix the problem. So was it the heater valve, the vacuum system, the control? What?

When I got home, I sat in the drive and thought. While looking at the vacuum diagram again, I thought maybe the check valve (behind the brake booster) was at fault, since earlier tests indicated that the storage canister was ok.

After I disconnected the vacuum (gray line) to the check valve, nothing changed. Check valve good. Next I disconnected the red line to the canister, it held vacuum. Canister good. Then I disconnected the green line to the switching solenoid. The center vents immediately stopped blowing any air, cold or hot. An inconclusive test, but I'm sure that wasn't the problem.

When I checked the voltage again at the heater valve, it was down to something like 11.9 volts. Yep, 11.9 VDC. So, I connected the voltmeter directly to the battery. It was reading somewhere around 12.3 to 12.4 VDC. I thought, this can't be good.

Now some history. You know I'm in Phoenix (110F on a good day) and I don't like to see my temp gauge into the upper limits. This is why I installed an 1100 Ohm resistor accross the terminal of the temp sensor for the aux fans. This tricks the fans into coming on sooner and does keep the engine cooler, typically below the 100C mark on the gauge. Also, I recently installed euro headlights from So, I believe my constant current draw to be up significantly.

Back to the low voltage issue. While idling and the a/c on, the aux fans on, the cabin fan and other minute accessories on, the voltage would not rise over 12.5 Volts. I increased the engine rpm to around 2500-3000 and instantly, the center vents began their hot air tyrant. At idle the cold air returned.

So, to deduce this whole thing and bring it to some sort of conclusion, I think I have a low voltage problem. But it prompts a few questions along the way.

1.) What is the lowest voltage the CCU operates at?
2.) Why would the system switch (or fall out) at a higher rpm?
3.) Has anyone else ever experienced this problem?
4.) Could the alternator be at fault or just too small?
5.) I think a larger alternator would fix this, would any of you recommend this?

To note: The heater valve never did get hot on the outlet side during the tests and even when the rpm's were increased to 3k in park/neutral. Also, the battery light doesn't come on at any time except during the KO bulb test.

Last night I didn't drive it and left it on a 2a trickle charge. The battery is now fully charged and I took it out for a test drive this afternoon.

After removing the (fan tricking) resistor and making sure the battery was fully charged, I took the ol' girl out for a spin. Esentially to get a pair of locking clamp pliers to pinch off the heater hose. I wanted to do this just in case there was an outside chance coolant was slipping by the heater valve. Also, I connected a voltmeter to the valve, ran my meter lead to the inside and set the meter on the dashboard. It read a constant 11.9 volts. Note however that the battery measured 13.2 V just before this and I suspect it still does. When I turn the dial to full heat, the voltage dropped to "0". So, this seems to be working fine.

Ironically, the engine didn't run any hotter than it did with the "fan tricker resistor" installed. That said, I was getting about 50F out of the vents in city traffic. However, and this is where it seems to rear its ugly head, when I rev the engine above 3000 rpm (sitting still or through delaying an upshift) the temperature out of the vents climbs to over 100F! When I bring the engine back to 2000 rpm or less, everything returns to normal. And yes, I have the heater hose clamped off as an extra precaution.

Mechanically and electrically, I'm very good at what I do. Referigeration is where I'm a little weak and this has me stumped.

Your thoughts and responses are most welcome.

Michael B.

saumil 08-25-2007 11:35 PM

What are the low and high side pressures ?

Is the belt tight ? Though with a loose belt I would have expected the opposite.

Does your car have the Klima relay ? There are tests you can do to determine what causes the compressor cut-off. If you are interested, I can send you the steps.

When you get hot air, I am assuming that your compressor stopped working ?

If it cuts off at high RPM and comes back on low RPM, it is possible that your high side pressure may be rising too much, and this could happen either because you have non-refrigerant in the system or too much refrigerant or the aux fan is not working.

When the hot air comes thru the vent, do you have to stop the car and restart for the a/c to work ?

hispassion 08-25-2007 11:38 PM


Seems we're posting quite effectively on two different forums. These are all good questions. I'll answer them to the best of my ability.

I don't know what the pressures are as I don't own a set of gauges. Seems I wasn't focused on the system basics and was think more along the lines of a control issue.

The belt is tight. Checked that.

I beleive this does have a Klima Relay, however I don't have to shut off / restart the engine to experience a recovery.

I don't believe the compressor is cutting out, however I didn't rev the engine with my eyes on the compressor clutch. And I didn't hear the compressor click off/on if that's any indication.

The aux fans are working (low and high speed). It may have been converted to R134 and I don't know if the system is overcharged.

Maybe it's time to visit the local shop who has a set of guages and can tell me what's going on inside or maybe I'll just buy a set of guages and test it myself.

What about the expansion valve? Could a failure of this cause these symptoms?

Michael B.

saumil 08-25-2007 11:55 PM

I dont know how an expansion valve failure would cause a RPM dependant failure. It controls the amount of rerigerant entering the evaporator and if everything as far as temperature is concerned is the same, RPMs should not affect its operation directly and because you are getting good cooling at low RPMs, it should atleast be opening normally to a certain level. RPM will change the operation of the compressor for sure and could result in higher high side pressures causing the compressor to shut down but if you say the compressor does not shut down then again there is a puzzle. Maybe the clutch clearance on the compressor is too much and is not engaging properly, search for clutch clearance and you may find something useful related to your problem.

brewtoo 08-26-2007 09:38 AM

The Klima relay on this car is built into the MAS relay.

How would high pressure shut down the compressor? There are only two switches at the drier and that's not what they do.

When it warms up, do perceive that ambient air is entering the cabin, or heated air?

saumil 08-26-2007 11:51 AM

The pressure switch is a 2 function switch, will close when pressure is above a lower limit and below a upper limit.

I just looked at the wiring diagram for a 1990 300E and there is not much involved in compressor cut-off, much less than my 87 260E. There is a pressure switch, a compressor speed signal and a engine speed signal.

Need to know for sure whether the compressor is cutting off at higher RPMs or not !!

brewtoo 08-26-2007 02:25 PM

My schematics show S31 to be a simple switch that opens below two bar and closes above three bar. It prevents the compressor from running with a low charge. I still don't see a high pressure switch, but there is usually a relief valve.

The test in the factory manual for S31 does not provide for anything but a low-pressure test.

saumil 08-26-2007 03:23 PM

O.K., I see the misunderstanding, the high pressure switch that I referred to is the same switch you are referring to, I meant to say that the pressure switch will cause compressor shut off when pressure is higher than the upper limit.

hispassion 08-26-2007 05:16 PM

Thanks for the input guys. However, the compressor is NOT cutting off!

The MAS control module has no affect (as I understand it) on climate control except to cut off the compressor in the event of a failure.

In the Phoenix summer heat, there really isn't any way to tell if it's heated air or just ambient air. Heck, the cold water faucets here spew hot water from July through September.

Since the compressor is not cutting off and the vent temp comes up almost instantly, I'm now led to believe something is failing in the A/C system. Compressor maybe? With my limited experience in domestic refrigeration, compressors (I think) have reed valves and maybe there's a weak reed valve. Would this cause the high rpm heat up issue?

With this thought in mind, I'm surmising this scenario...

If it can't compress (above a certain RPM) and the freon is allowed to pressurize the evap, this could essentially heat soak the evap making it seem like the heater comes on or the heat ducts are open.

Something to note; when the heat does rush in, there are no plenum door noises, just a change in temperature.

I welcome your thoughts, ideas and diagnosis.

Michael B.

saumil 08-26-2007 05:48 PM

Compressor is possible but it could just be the clutch clearance that has increased for some reason, still dont know why it is RPM dependant. The clutch is the first thing to check and even replace. If expansion valve shuts, you will also loose cooling but dont have a clue why that should be RPM dependant.

Check the voltage on the battery also, unregulated output from the alternator does wierd things, and the alternator output is RPM dependant if unregulated. But again, a whole bunch of other things will stop, including the car if the alternator regulator has failed.

In short, running out of ideas.

hispassion 08-26-2007 09:01 PM

If the clutch clearance has increased for whatever reason, the clutch may slip, the comparator signals wouldn't match and Klima portion of the MAS Controller would cut off the compressor; correct?

So, as determined earlier today with my wife depressing the accelerator in neutral (risky) to 3000 rpm while watching the temp guage stuck in the center vent rise from 60F to over 90F and I visually locked onto the compressor watching the it spin, the clutch did not disengage.

Since I currently work for an Emergency Generator Service Company, I'm somewhat familiar with alternators, current, regulators and their operation. However, after a complete (triclke) charge and the voltage over 13.0v, it still happened. I have deduced the need for a larger alternator, but for now, this one is working. I suspect the reason voltage was down (mentioned in my first post) was because of the aux fans being on nearly constant. The tricking resistor has been removed for now.

Saumil I thank you very kindly for your input. If you're out of ideas, is there anyone else out there that can offer a suggestion or diagnostic. As someone else said in another thread, "I throw my self at the mercy of this forum and its collective knowledge".

Most respectfully,
Michael B.

brewtoo 08-26-2007 10:45 PM

The low side line, where the schrader valve is on the left fenderwell, does it stay cold when the temp at the vents goes up?

saumil 08-26-2007 11:06 PM

One more thing, is there enough charge ? Atleast get a gauge that will let you look at the low side, the pressure should be approx. between 35 and 45 on the low side. You may be at borderline

hispassion 08-27-2007 12:01 AM

Saumil - I will make an appointment tomorrow at the local shop to have the pressures checked on the system.

Brewtoo - I started the car, let it run for about 5 minutes, confirmed the vent temp at 60F, ran the rpm up to 3k for about 20 seconds while holding the suction line. It was cold and remained cold, but I wouldn't be able to make ice cubes with it. I returned the engine to idle and looked inside the car to see the vent temp guage close to 100F!

So, maybe it's not refrigerant related. What now? Why is this issue rpm dependant? I could understand it if it was a under load with vacuum near zero, but it's not. I can accelerate hard, but as soon as I get over 3k, the heat cometh.

This is a strange one for sure. Your help is appreciated.

Michael B.

saumil 08-27-2007 10:34 AM

If possible, take it to a shop that only does auto air, you will get more accurate feedback about your a/c's condition.

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