A/C switches from cold to warm sometimes as vehicle decelerates & re-accelerates.


Asked by Jul 18, 2013 at 08:12 AM about the 2001 Ford F-150 SVT Lightning 2 Dr Supercharged Standard Cab Stepside SB

Question type: Maintenance & Repair

My 2001 SVT Lightning is an all original with 96,000 miles that I bought new. I recently added an A/C recharge kit from Auto Zone to the a/c system. This was the first time any freon has been added. The problem mentioned above was happening before the freon was added.
The a/c now gets cold like it used to do. Sometimes as I decelerate to turn or stop and then normally accelerate the air coming out of the registers get warm for just a few seconds and then get cold again. It doesn't do this every time the vehicle starts & stops.
Do you have any suggestions are to what may be causing this problem? I was thinking that  maybe somewhere in the high vacuum to low boost changeover period may be causing the heater temperature control valve to internally change from cool to warm.
I haven't taken it to the dealer yet as I normally try to repair as many problems as I can.
Thanks, Robert

5 Answers


Your on the right track, sounds like there is a vacuum leak in the heat control system, it could be a little tough to find, I would locate the temp control valve and work your way back until you locate the leak, a mechanics stethoscope with the sound probe removed works really well to hear the leak.

2 out of 2 people think this is helpful.

Thanks. I'll give that a shot. Maybe I'll get lucky.....

1 out of 1 people think this is helpful.
Tracy Hooks

A vacuum leak usually effects the register control not the temp. If you're romping on the gas (accelerating hard) the PCM will shut off the AC clutch to provide less parasitic loss and make a little power more available. This will happen whenever the PCM sees a 20% change in position toward acceleration. I could be wrong about the vacuum controls but not usually. There is a vacuum "T" from the intake manifold that has a check valve, a vacuum reservoir and a connection through the firewall to the heater control. Check your vacuum lines and check valve.

Best Answer

I will check those out. I was thinking there was probably a check valve somewhere in there to hold the vacuum for the heater and any other vacuum operated accessories during those jack rabbit starts when the engine is under boost. I am familiar with the old style a/c cutout switch that went back to the Mustangs in the early 80's which was a normally closed mechanical switch that would turn the compressor off during WOT. I assume the PCM that you mentioned is the engine computer module that will send a signal to do the same function as the old switch would do. Thanks for the detailed troubleshooting and additional info. Robert

1 out of 1 people think this is helpful.
Tracy Hooks

You're quite welcome!

1 out of 1 people think this is helpful.

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