Air conditioning Evaporator Freezing Up
 

I'm having an issue with my 96 vert. The a/c starts off fine with the air blowing out of the vents at full force and cold. After driving for an hour or more the force of the air coming out of the vents is diminished to what feels like 1/4 of the force. I noticed the line from the accumulator to the evaporator is frozen with hard white ice. I've changed the cycling switch since it appears the compressor never cycles. I've checked the controller for codes and it's only 00. I've put gauges on it and it's about 23-25 on the low side and about 180-190 on the high side at about 68% humidity and 80 degrees. Is the system low on freon? Is there something else I'm missing?

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Here is a chart I use to convert to 134. It looks like you are low on Freon by 1/2 12 oz can. I am not an expert but I have the gauges and my pressures were correct for 90 degrees.
If you are freezing up there would have to be moisture in the system, has it been open for an extended period of time? Also I was happy to see on first test my fans were coming on when ac is on if yours are not after warm up could present a problem.

Thanks for the reply. The system hasn't been open to my knowledge. It seemed ok earlier in the year. The car has 40k miles. It's a 96 so it came with 134a. The fsm called for 28 psi on the low side. Do you think 23 -25 is low enough to cause the freezing and non cycling of the compressor?

I noticed my ac delco conversion does not really cycle all that much when it is on. I'm thinking you are mabey low 1 12 oz can causing Freon to over cool and not work properly. I would add to the correct psi and see how it works, wearing protection of course.
I wanted to edit the psi can only be determined by outside temp.

The cycling switch should open at 22.5 psi and close at 46 psi. It will cycle at the same pressures always. Higher ambient and engine temps will cause it to cycle less as it will be harder to get down to the 22.5 with higher Freon temps.

Now if the system is low on Freon it will cycle more (short cycle) and the evaporator can freeze up. If it short cycles that’s an indication it is low on Freon.

If the low side goes much lower than 22.5 like 18 or 20 it can freeze up if it does not cycle.

When you first start the car, AC to max, fan to 10, how often does the compressor cycle?

The "freezing up" he's seeing is on the outside of the tubes! If he had moisture in the system, the a/c would quit working because of the moisture freezing on the inside of the system. Moisture in the system is not the problem.

:cool:

The compressor is not cycling. I've already changed the cycling switch but the compressor continues to run all the time. I tried pulling the wiring plug from the cycling switch and that disengaged the clutch so it seems the wiring is good. The only other thing I can think of is the new cycling switch is defective.

Why wouldn't you go directly to the FSM diagnostics for "COMPRESSOR ALWAYS ENGAGED" - That seems to be a relatively sure way to deal with your particular issue.

8A-67-5 in the FSM

Sometimes those diagnostic trees seem foolish and many times maybe are but in your case I'd think a very good direction to head towards.

I assumed this is a C68 car - if the car is a C60 then it's very different!

Wvzr-1. I've looked at the fsm. I'm unsure what wire to disconnect when it says to remove blower wire. I'll take another look this morning. Thanks.

In the "ALWAYS ON" I don't see any reference to "blower wire".

Is it actually "ALWAYS ON"? C68 or C60?

It's a C68

WVZR-1. I was looking in the a/c section of the fsm. I'll get the other book and take a look shortly. Thanks for the help. I'll post an update once I go through the page you recommended. Thanks!

I should clarify the compressor clutch is always engaged when the a/c is on but not when the a/c is off. I'm going through the fsm for the c68 and so far everything checks out but I now need to pull the control head. My question is when the service manual say clutch always engaged does that mean only when a/c is on or that the clutch is engaged even with a/c off?

Thanks for the info. The pressures seem close to what the FSM is calling for so that's probably not it. I guess I'll pull the dash apart to get the head unit out and try disconnecting it. As I posted earlier the only thing I'm not sure of is what the fsm means when it says clutch always on. Is that on constantly when the a/c is on or is it stuck on even when ac is off?

Thanks for the reply. If my thinking is correct, if I set the a/c on auto and turn the temp up to 90 the compressor should shut off? I'm going to try that now before I pull the head unit out.

ok, car a/c on auto a/c temp set to 90, outside temp is 77 and compressor runs continuously. I assume I need to pull the head and see if it disconnects when I disconnect the head. Any input would be greatly appreciated.

These numbers are normal for an engine that is just sitting and idling.
"I've put gauges on it and it's about 23-25 on the low side and about 180-190 on the high side at about 68% humidity and 80 degrees."

Now connect your gauges again and with AC to MAX, REV the engine to about 1500 or so and hold it there for about 30 seconds and watch the low pressure side. It should come down to 22.5 psi and then cycle (cycle switch opens) and the compressor should stop. The pressure should then rise by itself to 46 psi and re-engage the compressor. This should repeat for an normal system operation.

See what your results are.
If you go much past below 22 psi, possible bad cycle switch.
If you can't get down past 22.5, possible overcharged system.

Other things that will affect system operation is airflow over the condenser. Check for excess leaves and debris on the condenser. Both fans should be operating at low speed with the AC on for a 96vette (check it).

I didn't see the diagnostics that way but I ain't read or reviewed them previously. I interpreted the chart as you disconnect the cycling switch first and then based upon the results you either go to the PCM to check commands after a disconnect or you to the AC control and disconnect.

Actually with an OBD-2 car I might be tempted to see how it responds to Tech1 commands or with the proper Tech2 module you could do the same.

For a '96 car I don't believe I'd just throw parts at it without doing the Tech1 or Tech2 command checks.

Ok, tried the heat and when it's set to the floor vent it turns off the clutch. I put the gauges back on and ran it for a 5-7 minutes between 1500 and 2000 rpm. The lowest the pressure dropped to was 21 but the clutch never disengaged. Maybe the Napa (China) cycling switch I put in was defective? Not sure what else to think except either the cycling switch or a/c head? I know disconnecting the cycling switch disengages the compressor.

Here's a C68 compressor command diagram from the FSM

Attachment 48113807

Attachment 48113808

Circuit operation

Attachment 48113811