AC weakens after "burping" air out of cooling system

[DeltaFox]

I think the title has a pretty good summary of what happened to me. I am in the middle of an acid flush and here is what I observed.

After draining the coolant and putting in the acid to flush I had a hard time getting the air out of the system. During that time the AC was nice and strong. I know that the heater hose had no water in it because it wouldn't get hot like the radiator hoses, and inside the car with full heat set and the defroster on gave me ice cold air. When the heater core finally filled up with coolant I got tons of hot air in the cabin. So I turned the temp switch back to minimum temp and the AC was noticeably hotter. I would say about 10 to 15 degrees F warmer.

I suspected a monovalve and did some searching on it but I am not even sure if that is the right area to look into. The air inside or the fans rather change speed with the temp selector adjusted. Anyone have a clue as to what might be causing this or is that just a standard thing to have happen?

Is there a way to test the monovalve without opening the thing up? I.E - appling voltage to the controlling unit and checking inside temp.

[tangofox007]

Originally Posted by DeltaFox

I suspected a monovalve and did some searching on it but I am not even sure if that is the right area to look into.

I would start by making sure the electrical connector on the top of the monovalve is making good contact. The valve will open to the full heat position if the connection is bad.

[Jeremy5848]

With the engine NOT running, you should also be able to remove the plug from the top of the monovalve and apply 12 Volts directly from the car's battery or a separate battery. A few seconds won't hurt (many minutes might overheat the coil) and you should be able to hear a "click" or "thunk" as the electromagnet pushes the valve closed. No noise, bad monovalve.

I just tried with the monovalve in my '87. Got a nice soft 'thunk.' You will also get a little bit of a spark as the test mire makes and breaks connection. That's good -- no spark, no continuity.

It can still be bad if you get the thunk, due to a rip in the rubber valve. But that involves opening up the valve body. (Covered in Lesson 2)

Jeremy

[DeltaFox]

Thanks guys, I will test it out tonight. How long does it take to open up the monovalve to see if there is a tear on the rubber, 10 minutes, half an hour?

[winmutt]

Closer to 10.

[DeltaFox]

Ok after testing the electrical components everything looks good. The wiring into the unit is sending ~12V and when testing the unit I hear a soft thunk. Definately a piston or rod moving up and down. What didnt make sense to me is that when I disconnected the wiring into the monovalve I didn't get any heat from the vents inside. I did have the ACC set to vents only on max cool. I still expected that since 12V was the max cool signal that 0V would be the max heat and therefore when disconnecting that I would blow lots of heat.

I also tested the AC pressure to make sure I was not leaking. Holding right at the top of the full level. About 45 psi if I remember correctly.

If the mono valve is not bad is this just the case of converting to 134a? Would there be anything you could do to help it out. Bigger/more powerful fan up front? I have heard that a better condenser up front can make a world of difference but I would rather not open the system up if I don't need to.

Thanks again

[tangofox007]

Originally Posted by DeltaFox

I still expected that since 12V was the max cool signal that 0V would be the max heat and therefore when disconnecting that I would blow lots of heat.

The monovalve receives 12 volts any time the key is on. It is modulated through the ground circuit via the electronic temperature control unit.

With the monovalve disconnected (electrically), you should have more heat than you can stand.

Originally Posted by DeltaFox

If the mono valve is not bad is this just the case of converting to 134a?

When did the conversion take place?

[DeltaFox]

The conversion took place right before I purchased the car, about a month ago. The AC was colder at that point than it is now. I remember looking at the low pressure side (cold line into the evaporator) and seeing ice on the hard line right off the compressor. I don't see that anymore, but the line is still pretty cold. Its mostly been after I drained the radiator fluid and started the acid flush that I have noticed the temps seem weaker.

I do have a light hissing sound coming from my radiator cap now after the engine has been off for about 15 minutes or so. I'm thinking that means it needs to be replaced as its not holding pressure like it should. I don't believe that would affect the AC but just wanted to verify that the cap is bad.

[tangofox007]

Originally Posted by DeltaFox

The AC was colder at that point than it is now.

Other things being equal, R-134a isn't going to cool as well as R-12. Consider yourself lucky if it cools "well enough."

[DeltaFox]

Just for clarification I was never in the car when it had R12. The first time I was in the car the AC was cooler than it is now, and that was running with R134a. Currently, and I'm guessing the vents put out air at about 70 to 80 degrees. Its just slightly cool air, as compared to when I first bought the car and it put out cold air probably closer to 50 to 60 degrees.

[tangofox007]

I think I would try plugging a heater hose and see what affect that has. That should tell you whether the heater is the culprit or not.