ACC heat /cool mode backards?

[nomorehurry]

There may be something about this on the forum somewhere, but I can't find it.

Car is 82 300DT, 210,000+ miles.

Air Conditioning systems runs fine for a while, then goes to high heat and then to max heat. Auxillary fan does not seem to come on while in cooling mode, but will engage while in heating mode.

Monovalve and wie are good, ACC unit is new (rebuilt) temp wheel has been cleaned and ohm resistance changes as dial is turned, all fan vents and speeds seem to work fine except I get continuous flow through defroster vents either in heat or cool mode.

Questios: what controls change from heating mode to cooling mode and vice-versa? Is there something that could be cross wired that would cause the fan to be reversed? (fan turns in correct direction).

Note: previous owner claims to be a mechanic, but I have already re-fixed much of his work. I am not a a mechanic....

Help is appreciated as both I and my local air conditioning expert are lost. Thanks all.

[moon161]

Unrelated don't wast your time if your serious.

My dad told me a story, old lady, her pipes froze. Called a weldor to come thaw them out.

Weldor womes, clamps electrode and ground on the pipe, has his partner start the genny and flip the welder on.

Pipes are all thawed, old lady doesn't want to pay. 'Ok, fine' he says, opens the door and yells 'FRANK, TURN THE SWITCH BACK TO FREEZE'.

Lady paid up.

[nomorehurry]

I appreciate your response, but I don't see how it relates to a confusing air conditioning problem. However, I will agree that it is an amusing story.

[moon161]

I was thinking backwards AC and backwards heating was the connection. No idea about your prob, just puckish.

[nomorehurry]

I know that as a kid I had to sit on a dictionary to eat dinner, ergo I became a smart-ass.

It's nice to see someone else with a warped sense of humor.

Have a nice day.

[vstech]

there are some threads on here about testing the ACC... search on brian carlton posts...
I'd likely be as lost as your a/c tech. Vacuum controls the pods that move the doors to route air. power from the acc turn the monovalve for heat on and off...

[nomorehurry]

VSTech, Thanks, I will check Brian's posts. I have been looking through the ACC posts but have not seen anything as wierd as the aux fan comming on with heat and not with air conditioning.

[tangofox007]

Originally Posted by nomorehurry

Auxillary fan does not seem to come on while in cooling mode, but will engage while in heating mode.

The ACC doesn't "decide" on its own whether to run the a/c compressor or not. Unless "off" or "economy" is selected, the compressor is engaged, regardless of the temp selected. The aux fan is energized on the basis of refrigerant temp, so it cannot come on until the refrigerant reaches the specified trigger temp, which will involve some delay following initial compressor engagement.

What you are experiencing is likely an uncommanded opening of the monovalve. Considering that the valve is closed electrically, I would investigate the possibility of a poor connection at the monovalve.

[compu_85]

I'd guess there's something wrong with the interior temp sensor or another temp sensor.

-J

[nomorehurry]

FWIW. After fiddling with it for a while, I got the aux fan to start while the AC was on.

I also found the defroster vent was staying partially open, which allowed cooled air to go through the defrost vents, which put cooled air on top of the dash, which allowed cooled air to be pulled into the in-car temperature sensor, which made the car think it was too cold, which caused the electronics to move into heating mode.

After using towels to plug the defroster vents, I put the AC on and it stayed cool for 20 minutes or longer while I drove to the junkyard for a defroster vent control. Even when the temp dial was moved off of max cool. Everything was good until on the way home the car decided to go to full heat and refused to change until it had been parked for a while. The the AC worked again with me doing nothing except turn the car off, wait, turn the car on. I will probably put the good defroster vent control on in the morning, if I can get to it.

The uncontrolled opening of the monovalve is a distinct possibility, but it has to be caused by something.

The air jet pulls cabin air across the temperature sensor (tested with a piece of tissue) and it was fine and good for a while. I did have a loose wire on the monovalve but re-soldered it. I have another plug with wires that I can splice on if the connection is being lost there.

[nomorehurry]

It seems to be working correctly not that I have replaced the defroster vent control valve. At least it worked correctly yesterday.

Here's what I have learned about these machines.
1 - machines are stupid!
2 - if there is a failure in the system, expect lots of heat.
3 - to fix them, try to think like a plant. (see above post about defroster vent being partially open and the consequences)
4 - when (after?) all else fails and you are beating your head against the garage wall, see item 3, then go back and check everything again. Do not say "I know that is right so I am not going to check it again." because that is probably the part that is broken.
5 - if at first you don't succeed, cry and try again, and again, and again, and...

I have said it before, but many thanks to all who post on the forum. Your knowledge and assistance has been of imense value in my rescue of "The Benz".

Miller Time!!

[tangofox007]

Originally Posted by nomorehurry

It seems to be working correctly not that I have replaced the defroster vent control valve. At least it worked correctly yesterday.

That's just a coincidence. No cause and effect relationship involved.

[JimmyL]

Next time it goes full heat, try and honk your horn. Sounds weird, but I had a cracked fuse. My car would go "full heat" randomly. Finally one day I honked the horn, which finally blew the fuse, which caused the car to go immediate heat. Replaced the fuse. Monovalve stayed closed, horn worked, AC worked.
So, check fuse for crack or just poor conductivity. Don't remember the fuse......
And of course, you might wan't to have a good look at your trunk badging. I don't really think you will find a "T" back there.....

[nomorehurry]

Originally Posted by tangofox007

That's just a coincidence. No cause and effect relationship involved.

Tangofox, there is a spelling error in my previous post. The "not" should be a "now". I did replace the defrost control valve and that did make a difference. My system is mostly working correctly, but the last time I drove it the heat still jumped to full. The defrost vent stayed closed.

It seems to take longer for the air conditioning to switch to full heat so I guess I am making progress. Something I noticed yesterday was that the top of the monovalve was getting warm-hot while the hoses to the monovalve stayed cool. To me, a bad coil might be a cause if the electromagnet coil in the monovalve is heating to the point it shuts off, that would cause the valve to go to open and allow hot coolant through the heater core. I think the coil was submerged in coolant at some time in the past and that might have damaged the coil. I would assume it is supposed to stay cool enough to touch without burning of the fingers, but my knowledge of electromagnets is very limited.

I searched the threads for a couple of ours but haven't found anything to indicate if the top of the monovalve (both plastic and silver cap) are suposed to stay cool or if they are supposed to heat up. Anybody know the answer? For info: the valve in in a 1982 300DT.

[tangofox007]

Originally Posted by nomorehurry

Tangofox, there is a spelling error in my previous post. The "not" should be a "now". I did replace the defrost control valve and that did make a difference.

"Now" is how I read it. The difference would be in how the air is distributed, not in the temperature of that air. I don't think the uncommanded heat problem is in any way related to the defrost air distribution.

Originally Posted by nomorehurry

To me, a bad coil might be a cause if the electromagnet coil in the monovalve is heating to the point it shuts off, that would cause the valve to go to open and allow hot coolant through the heater core.

I have seen cases where engine compartment heat causes the monovalve plug to develop a poor electrical connection, which causes the valve to open to the full hot position. If you want a "summer" solution to your problem, just plug one of the heater hoses.