A/C periodically blows hot, and I do mean HOT - PeachParts Mercedes-Benz Forum

SirMatt · #1 05-15-2003, 10:10 AM

My 1984 300D is doing a really strange thing of late. The A/C will suddenly blow hot air. Not warm air or ambient outside air, but HOT air. Heater core on a 100 degree day air. Really unpleasant little shock when it's 95 outside. It will work just fine and then poof suddenly your a schnitzel in a big German convection oven. I can't figure it out. Sometimes it starts out that way when you start the car but other times it will just suddenly blast out hot air in midstream.

Any ideas? Most of the time if I turn the ACC system off for a period and then restart it the situation will resolve itself.

Thanks in advance for any help.

smartin · #2 05-15-2003, 12:04 PM

Greg (1985 300TDT) - What happens is that hose (to temp sensor on dash?) is completely gone? How should system work with regard to hot air? Steve

leathermang · #3 05-15-2003, 12:10 PM

If the air coming out of the vents is hotter than the outside air temp... then it has to be coming from contact with the heater.. meaning flap /control of the flaps/ sealing of the flaps... types of things... the AC does not have any way to heat air ( to my knowledge )....
Fortunately these problems usually cost less than dealing with the AC.... but may be harder to deal with due to location and number of components potentially not working... like diaphrams, vacuum lines, temp sensors... etc....

GregS · #4 05-15-2003, 12:40 PM

There is a tube made out of foam behind your glovebox. The foam deteriorates and looses its seal on the fittings it is attached to or gets a hole worn in it from rubbing against the glove box. Don't inspect, just replace it. I used small diameter foam pipe insulation from Home Depot. This is a very common problem.

GregS
'84 300D,
'90 300CE

lrg · #5 05-15-2003, 01:13 PM

While your foam duct (tube) for the temp. sensor may need replacing, I doubt that is what is causing your sudden heat problem. On hot, sunny days the dash gets hot and if anything a leaky duct will cause the sensor to read a hot temp and turn the A/C up, not the heat up. Also, if it were a simple vacuum pod problem I doubt you'd be getting anything more than ambient air temp from the outside. If you are getting full on hot you likely have an electrical problem that is opening the monovalve and heating the heater matrix. The next time this happens try spinning the temp setting to full cold and see if it makes any difference. Also, on a separate ocassion try pulling the plug on the monovalve (I'm, 99% sure that unplugged/unenergized the valve is closed) and see if when the system goes into unwanted full heat mode if it gets as hot. If you want to get fancy you could rig a bulb to the monovalve plug and observe when it gets turned on. If the sudden heat problem goes away with the monovalve unplugged then it is definitely an electrical/electronic problem.
If so you likely have either a bad temp sensor or bad climate contol unit. Both are relatively easy to replace. If it's the CCU look for a rebuilt unit, they're far cheaper than a new one. Good luck.

Fredmburgess · #6 05-15-2003, 01:51 PM

When it goes to full hot, is it also switching to air out the defrost vents? Defrost, max heat, is the default when the pushbutton c/c unit fails. That's what my car did when I got it. I replaced the whole pushbutton unit with a rebuilt - about $200, I think (ouch) but it fixed the problem. Not sure issues are related but make sure the auxilary water pump is working (key on, engine off, heat on, pump should run). When these fail I believe they can fry the ciruit board on the pushbutton control unit. Circuit board in mine was visibly fried. After reading about the aux water pump I put a 1.5a fuse in the line for the aux pump just in case.

Good luck! Hey, at least you know you've got heat...

SirMatt · #7 05-15-2003, 03:21 PM

It could definitely be electrical in nature. I've got some Gremlins running around in the system. Cruise is schizo and when plugged in will periodically give me the clickity clack crap from the actuator. High beams absolutely refuse to work. They work in "flash" mode (pulling on the stick) so it's not the lights themselves or the wires, but they will not turn on for driving. In fact I lose all of my headlights when I try to push the stick forward for high beams. Running and tailights stay on, headlights take a nap. Probably a combi-switch issue.

Anyway, when this happens the heat blows out the center registers too. I may be wrong but I think when the ACC is set to high heat the air only comes out the side registers and the floor. When this happens it blows out full bore from all the registers. And yes, it's serious heater core air, significantly and noticeably hotter than even the 95 degree ambient air outside. Setting the wheel to MIN makes no difference the only way to make it stop is to hit O and restart the system after several minutes.

Rick Miley · #8 05-15-2003, 03:26 PM

I had a 190E that did that on a trip through the mountains. Uphill over a certain altitude was full heat. Turn it off, crest the hill and turn it back on, normal operation. Sorry, but I never found out what caused it. We don't have any mountains in FL so it was never an issue again. Hope you find it.

#9 05-15-2003, 05:10 PM

In addition to checking the foam hose to the temp sensor, you need to check the monovalve.

With the engine cool, go to the valve on the firewall near the battery with four screws on top. Remove the four screws and lift up the top. If it is flooded in there, you need a repair kit. It is about $35 and takes five minutes to replace.

Good luck,

Palangi · #10 05-15-2003, 09:08 PM

I vote for the monovalve also. They usually only last 3 to 5 years. The symptoms fit. The foam hose which was mentioned really only affects automatic mode, causing it to react very slowly. Go for the monovalve repait kit.