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Old 08-25-2007, 10:40 PM
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hispassion hispassion is offline
I Want A 1935 500K!
Join Date: Nov 2006
Location: Peoria, Arizona
Posts: 112
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.
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)

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