Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

A/C is cold until the fan is on high, then it's warm

When the a/c fan is on any setting other than the highest setting the air comes out cold. When the fan is set to the highest setting, it only comes out warm. This happens regardless of a/c or max a/c setting, regardless of fresh or recirculated air, and regardless of car speed. (2005 Mustang GT)

First, determine if it’s the A/C clutch kicking out when the fan is switched to high.

It could be the blending door. Somehow on high setting it’s blocking off AC.

Gotta be low system charge. U could check pressures, but I say evac and recharge with proper fill. Bet it works fine than. U are probably used to <great performance from a/c. Riding in a car with good a/c might surprise u.

I checked the compressor and it is cycling on and off every few seconds in all scenarios. If it needed a recharge wouldn’t that mean it should only be blowing warm air? The fact that the air is still cold (at low fan speeds) but the compressor cycles on and off rapidly confuses me. Are those typical characteristics of a low charge or something else?

Temp at driver vent, max ac, should be around 40-45. Do you have this at low fan speeds?

Air can feel cold even with a low charge causing short cycles.

I got out my thermometer and took some measurements at the driver’s side vent. (It’s 75F outside) On all settings other than high I get 66F at the vent, that’s with normal & max a/c and fresh & recirculated air. Turn the fan on high and it goes up to ambient temperature (also regardless of norm & max, fresh & recirc).
Obviously not as cold as it should be, I just thought I wouldn’t get any cold air at all if it needed charging.

When the compressor cycles on and off every few seconds, that is a symptom that the system needs to be recharged. When the blower is on high speed, the system can’t cool the higher volume of air.

The poor cooling performance and rapid cycling of the compressor clutch is vary likely due to a very low charge level. The system won’t be completely disabled until there is no pressure detected in the system (less than 25 PSI), in the mean time the compressor can be damaged by operating undercharged due to poor lubricant circulation.

An AC system has a max flow rate that it can fully cool the air. At a certain air flow rate the AC can’t keep up, and the air that does come out then will be warmer than at lower flow rates. My guess is your AC system isn’t working at the capacity it did when new, and as posted above by the experts here, the charge level is probably the first place to look.