AC Intermittent

[Rick Iverson]

[FONT=Verdana]Fellas;[/FONT]

[FONT=Verdana]Denise and I met our oldest in St Cloud last weekend to drop off my 1995 GMC 3500 w/20' Pace Shadow GT trailer. On the way down, the AC was super cold with an OAT of 85°F and an empty truck and trailer. [/FONT]

[FONT=Verdana]Dan drove up from the Cities in his prepared (barely streetable if not illegal as hell) BMW to pick up the rig, as he and his brother along with the Boys are going to RA for a track day next Monday. We had dinner, the Girls went shopping and we loaded his car.[/FONT]

[FONT=Verdana]On the way back to the Cities, he called to announce the AC was tits-up, and that OAT was perhaps cooler than what the AC was putting out. His car weighs 2300lbs, whereas my FC with tools, platen, scales, support equipment, sundries and personal items easily run 2300lbs.[/FONT]

[FONT=Verdana]At his home, he tested the AC Freon level (whatever that means), and it indicated OK. He disconnected the trailer and went for a drive, and the AC put out super cold temps again. What gives?[/FONT]

[FONT=Verdana]VR[/FONT]

[FONT=Verdana]Iverson[/FONT]

[john f]

Evaporator froze up?

john f

[Rick Kirchner]

Could be that under load the A/C system cuts out to preserve crank horsepower.

Whenever I jump on it in our CRV the A/C cuts for probably 15 secs, long enough to blow hot air if its not set to recirculate.

[Dick R.]

I don't know how your system works but my 88 Dodge Van system uses vacuum to regulate the "blend air" in bi-level mode. Under moderate load vacuum goes away and just the heat is directed to the cockpit. Apparently the default no vacuum position for a flapper is "heat".

This assumes no actual problem in the system or possible "as designed" cutting off of the A/C in order to cool the engine under heavy load.

Any chance he was driving faster than you normally do?

Dick
CM85

[Rick Iverson]

Fellas;

Thanks for the thoughtful input.

Dan is on his way to Road America at this writing. It's amazing what a little PM on tire pressure (both trailer and truck) as well as repositioning the car aft to ease on the tongue weight. He just reported the AC is ice cold.

Having said that, there must be an algorithm to determine the tongue weight vs position of the car with respect to the axles. Any ideas?

Iverson

[Rick Kirchner]

http://continuouswave.com/whaler/ref...culations.html

[R. Pare]

Very common for the evaporator to freeze up in very humid weather, especially if you are drawing the air from the outside instead of recirculating the interior air.

[DaveW]

Very common for the evaporator to freeze up in very humid weather, especially if you are drawing the air from the outside instead of recirculating the interior air.

In my experience, when the evaporator freezes up, the air flow out of the vents also gets really slow, so it's fairly easy to diagnose.

[Rick Iverson]

Dan called last night to report after a short stop at Wausau for gas and dinner, the AC failed again when he continued. I'll have him check the air flow out of the vents on the way back.

So if the evaporator freezes up, what's the fix?

Iverson

[Larry]

...go back to your flying, and figure a CG for the trailer. Then you can figure the CG loaded with the car at various places. With some experimentation, you'll be able to figure the best CG.

For CG calculation, establish a datum reference line. This is strictly arbitrary, but I would suggest something like 2' in front of the tongue. Weigh the tongue and wheels, and you can determine the CG using the basic formula of weight x arm = moment. Therefore, moment/weight = arm.

Hauling on my little, open trailer, I've found that I must load the car on backwards to get enough tongue weight.

Larry Oliver
International Racing Products

[R. Pare]

So if the evaporator freezes up, what's the fix?

Iverson

Wouldn't call this a "fix", but when it happens, turn it off, let the ice melt, and switch to interior recirculation when you turn it back on - once the humidity is removed from the interior air, there is no moisture to freeze onto the core.

There probably is a "fix" that the AC experts know about, but I ain't one of them!

[Frank C]

The AC could be about to fail, of course. I drove from Oklahoma to Chicago in August 2008 in my 2002 Saab 9-3, and about Rolla, the AC started cutting out intermittently. It would turn back on after switching on and off. In Chicago, it failed totally. I had it looked at and the Saab dealer in Downers Grove (great guys - they had worked on the car before during a Christmas visit water pump problem) told me that it would not lock up and I could drive back to OK safely, where repairs would be cheaper, particularly if I did not use the factory compressor. I have had the AC fail on an S10 towing a race car in Memphis. It made it back to Tulsa, 75 miles from home, where it locked up and became undrivable with the regular belt arrangement. Be prepared just in case! I think it is common for the vehicle to command the AC to cut-out under full throttle. This was particularly apparent in my somewhat underpowered 4.6L F150.
- Frank C

[Rick Kirchner]

If, on top of all this, the gas milage goes to hell then the compressor is locking up. I've had it happen twice.

[DaveW]

Dan called last night to report after a short stop at Wausau for gas and dinner, the AC failed again when he continued. I'll have him check the air flow out of the vents on the way back.

So if the evaporator freezes up, what's the fix?

Iverson

On my tow vehicle, a '74 Chevy Van, which has an AC system installed in 1984 by a guy running an AC business out of his garage, the evaporator freezes up all the time. What I did was to put an on-off switch in the line to the compressor clutch so I can run the AC fan (which is separate from the heater fan) w/o the compressor running. That thaws it out in short order.

[stephen wilson]

AC systems usually freeze for one of two reasons, low refigerant pressure, or low air flow. ( dirty filter, bad blower, etc. )

[DaveW]

AC systems usually freeze for one of two reasons, low refigerant pressure, or low air flow. ( dirty filter, bad blower, etc. )

Of course, you are correct, but that assumes it was properly designed in the first place...

My compressor is probably too large for the evaporator and air flow rate.

[Rick Iverson]

You guys are sooooo smart. I'm about two light years out of my league.

Thanks, Fellas.

Iverson

Post Scriptum - I had to lay the law down on these guys early this AM: opposite sides of the track at ALL times. I can easily see this track day escalating into full fledge competition, given the chance. You know brothers!

[BobC]

FWIW
I'm not an expert on A/C
I did have an aftermarket (dealer installed) A/C on a 320i BMW that froze up.
Problem turned out to be a sensor that should cut out the A/C compressor when certian conditions are met but if the compressor was allowed to continue running, the system would freexe up.
That was a great problem to have in west Texas in August with a black car.
Bob Cancellieri

[pitcrew89]

2 common causes: a/c refridgerant low & low pressure cutout switch faulty [most common]