A/C works great at idle/ not when driving

Friendly format provided to inquire about automotive a/c systems.
Archived Forum

Moderators: bohica2xo, Tim, Dougflas, HECAT

Posts: 1
Joined: Wed May 30, 2018 4:08 pm

A/C works great at idle/ not when driving

Postby csmall2331 » Wed May 30, 2018 4:50 pm

Ok first off the vehicle in question is a 2001 jeep cherokee. New parts: condenser, evaporator, drier, compressor, a/c lines with orifice tube. I also replaced my clutch fan and ensured electric fan does turn on. The low and high pressure switches are also new. After replacing these parts I took the jeep to a certified a/c shop for a evacuation/recharge. Everything was working great or so I thought. After a few months I noticed while driving I would get intermittent cold air. But as soon as I stopped at a stop light I would get nice ice cold air again. I decided to buy an a/c manifold gauge set to see what my head pressures were. At 83 degrees I was getting a reading of 32 psi low side and about 250 high side. This was at idle in the driveway. Air was nice and cold coming out the vents. As soon as I revved the engine my low side psi would quickly dip down and eventually trip the low pressure switch? My high side psi stayed fairly stable. Of course when that happens the clutch stops engaging and both my high side and low side would equalize and eventually go back to normal after a minute or two. So what is going on guys? Is this a classic sign of a low charge? Restrictions? Kinda the opposite of everything I have read about ac systems. Usually the increased airflow causes the ac to run cooler when driving? Mine is great at idle and intermittent when driving.
User avatar
Posts: 517
Joined: Wed May 11, 2016 4:12 pm
Location: Las Vegas, NV

Re: A/C works great at idle/ not when driving

Postby bohica2xo » Wed May 30, 2018 7:33 pm

Well, your condenser is not doing a great job. At 83f ambient & idle I would expect less than 200 psi on the high side. Jeep never had the best airflow over the condenser, and this may be more or less "normal" for your vehicle.

But the condition you describe could be a charge level issue, a blend door issue, a cabin fan airflow issue...

Does the airflow from the vents seem the same as it ever was? If it is cycling on the low temp because of a partially blocked air side in the evaporator, you would see what you describe. A dying fan motor looks the same as a blocked evaporator.

Try a full load test.

Doors open. Cabin fan on highest speed. All vents open. Raise engine rpm to 2,000. Run for 90 seconds, and check pressures & vent temperature while the engine is still at 2,000 Record all of that & the ambient temperature - and post it here.

If it is cycling during a full load test, record the run / stop interval IE, 3 seconds on 5 seconds off.

If it is low on charge, running a full load test will not hurt anything. The compressor only runs while there is refrigerant returning oil to it.

Return to “Automotive Air Conditioning Forum”