Odd A/C issue '02 S/T

[ptf18]

Fellows. The '02 S/T we have has "developed" an odd Air Conditioning "issue". Its started a couple weeks ago and is a "come and goes" type of problem.

This is what happens. We'll turn the A/C on in either the "normal" or "max" setting and the the output air will be cold as heck. On short "trips" 10-20 miles all's well.

BUT...... sometimes..... after traveling 20 miles plus.... the cold output air will turn warm. One time I checked the compressor only to find that the clutch was NOT engaged (as I suspected). I tapped on the Hi/Lo pressure switch but that did NOT cause the clutch to engage.

If we turn the A/C system off and leave it that way for awhile ( couple...3 minutes) and turn the system on again....Out comes very cold air.

Thoughts? I'm thinking that its the Hi/Lo pressure switch. I've looked at the visible wiring and don't see damage to it.

ptf18

 

[koda2000]

There are 2 separate switches. A High and a Low pressure switch. I'd have your refrigerant level/pressures checked with a set of A/C gauges.

I had a '68 280SL Mercedes roadster that had aftermarket A/C installed in it. It would behave this way because the evaporator would freeze up after about a 1/2 hour (I don't believe it had hi/low pressure switches). Once it froze up you had to turn the A/C off and let the evap core warm up. Then it would work again for a while. IDK if this was a refrigerant pressure issue or a design issue with the system. Maybe if it had pressure switches it wouldn't have froze up... ?

 

[429CJ-3X2]

I think I read recently that's the reason our a/c cycles instead of running continuously.

 

[boominXplorer]

I'd say low charge and it's freezing up. Have someone recover what freon you have and see if it's close to 30oz. Then change the orfice tube and the receiver dryer while it's decharged (both can be had for around $50 in parts). Recharge and add the correct amount of oil and retry. You popa hole in the old receiver dryer and measure what oil is drained so you know how about much to add back in.

 

[ptf18]

Fellows: thank you all for your input. Wife's been driving the S/T back and forth to work (40ish miles r/t) and a bit locally running errands the past 4 days....

A/C work great she says......

Odd......

 

[Big Z]

I'm with Boomin. It kinda sounds like your coil is freezing over. This is most likely to happen on lower fan speeds and with max a/c (recirculate mode), and also on somewhat cooler days. If it happens again, try turning it to panel air only. If it starts getting colder after a few minutes without a/c, you've pretty much confirmed it. The ice will melt and allow air to pass through, instead of around, the coil again; it'll be cold for a minute and then warm up again. When your low side pressure gets too low, the temperature drop at the very entrance to the evaporator core can be below freezing causing ice to form. It then spreads from there.
This is why you may need to top-off the refrigerant with a half a can or so if you haven't had the a/c serviced in a long while. I probably wouldn't jump right to system evacuation and more major maintenance just yet.. It's fairly normal to lose a very small amount of refrigerant every year so this problem can show up if a top-off hasn't been done in years. Other, free 'fixes' could include keeping your fan speed higher and using normal a/c mode instead of max (unless you need the boost in cooling or otherwise have the fan on a higher position). Add some heat rather than dropping to low fan speed if it is getting too cold while doing this. Keeping good airflow over the coil is the best thing you can do to prevent freezing aside from a proper refrigerant charge.

Another possibility is your a/c relay is getting weak, causing intermittent compressor cycling. Or the same for the high/low switches. You can always ohm across the switch prongs when it's not working. Or stick a jumper wire in the harness-side plug. It's mostly likely the low pressure switch that's interrupting the operation. Because it seems likely you actually do have low pressure.. and it's probably taking forever for the switch to sense the pressure has climbed high enough for it to return to the 'closed' position.

 

[ptf18]

Thanks Fellows for your input. Alls been well..... till this afternoon. Its 106 here in the DFW area and we were returning home. had driven about 20 miles with the a/c on "normal" (which I believe uses outside air to go across the evaporator verses "max" which recirculates the cabin air). The both of us noticed that the cold output air was getting "warmer". The wife turned off the a/c system and told me to leave it off for a couple minutes (see who wears the pants in this family dont you). I did as instructed and after a bit turned the a/c back on.......

Alls nice and COLD...

Thank you all again for your input.... I'll use your suggestions and see what I find.

Miserable working on cars when its 100 plus out..... maybe when I was 16 it was ok....not so much at 60 plus.....