heat overpowers air conditioning at idle

[sleepstar]

what could this be?
when driving everything is fine and the A/c blows ice cold. but if i'm stopped in traffic suddenly the heat will overpower the air conditioning? is this monovalve relatated? i've already replaced it three times, although the build quality is a joke...do they really fail after like three weeks? or could it be something else? this is with the temp wheel clicked on MAX

[Jeremy5848]

I had a similar problem with my '85 300D-T years ago. The a/c would be cold only with the temp wheel dialed all the way to the cold side and clicked on max. I ended up rebuilding the electronic controller. It was damaged (burned PC traces) due to a previous failure of the aux pump (it's not fused in 123 models). The problem was that the controller was improperly telling the monovalve to open and deliver hot coolant to the heater core. In the short term you can clamp the coolant line at the monovalve and cut off the hot coolant. You won't have any heat, of course.

[sleepstar]

i was thinking of doing that just cause i wont need the heat for a long time. but even with it clicked on max, i still get heat. but only at idle. at like 20mph+ theres no heat at all.

[Jeremy5848]

Low refrigerant in a/c?

[sleepstar]

Originally Posted by Jeremy5848

Low refrigerant in a/c?

hmm... dont think so. the system was vacc'ed down and recharged with r12 like six months ago.. its ice cold when the heat isnt on...

[JimFreeh]

Might be lack of airflow across the condenser when stopped and at idle.
The electric fan should be triggered by high head pressure or temp.

If the electric fan is not coming on, and the clutch fan is bad, you will not get much airflow across the condenser.

This would be in addition to the monovalve being open, which must be the case, since heat is still working well, even at the max cool position.

Jim

[sleepstar]

the fan is manually wired to a switch inside the cabin and i always have it on in traffic. the A/c compressor comes on and off normally so i dont think its a low charge. it would have had to leaked an awful lot in just six months. like i said its ice cold when in motion. i can smell and feel the heat itself when idling... like the heater is on full throttle at the same time as the cold air is blowing...

[Lucas]

Flush your system? The Toyota 22R does this. Chunks get in the valve and either plug it or not allow it to close.

[Jeremy5848]

If all of the bits that we've discussed are indeed working, the only thing left is a problem in the controller. I suggest checking the system voltage at idle to make sure it's staying well above 12 Volts (prefereably above 13 but probably not with the tiny alternator that 123s were given).

If the electrical system isn't causing the controller to act up then I would try pinching off the coolant hose at the monovalve. That should stop the heat regardless of the engine speed and should be a quick test—you can use a Vice-Grip® pliers and cardboard or light wood to protect the hose from the jaws. If the heat stops but the air is only "cool," it suggests that the refrigeration isn't working well at idle; it also tells you the monovalve isn't staying closed.

There could be a problem in which the climate controller thinks that the car is much colder than it really is. One problem I had with my 123 was the temperature sensor in the center of the dash had a broken wire. This meant the car didn't know how warm the inside of the car was and it defaulted to one extreme (don't remember which, hot or cold). You can see the sensor by pulling the glove box liner and looking in and to the left.

[sleepstar]

good idea, i'll check the voltage. which hose should be pinched? the one below the monovalve or the one on the side?

[ROLLGUY]

Originally Posted by sleepstar

good idea, i'll check the voltage. which hose should be pinched? the one below the monovalve or the one on the side?

I would just pinch the hose coming off the head (easiest and most room to do so). Pinching off the hose going back into the block after the aux pump works also.

[Lucas]

I'm just curious why you have gone through so many valves... That's what happened when I had buildup in the cooling system.

[sleepstar]

i went through them because the build quality of the ones i got was so drastically poor i figured they were failing.

anyway i think theres something wrong with the controller. i was just out driving around and i mashed the CCU buttons and clicked and unclicked the wheel and i got it to stop supplying heat when clicked to MAX. it held 40 degrees just sitting at idle after that.