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

20

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

77,735

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.
20

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

1 out of 1 people think this is helpful.
21,725

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
20

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.
21,725

You're quite welcome!

1 out of 1 people think this is helpful.

Your Answer

Add photo

Related Questions

  • My 1993 ford lightning 16 answers

    My truck has two gas tanks and it has a switch that you can press to change from the front tank to the rear. Well the switch works when it is on the rear tank it shows the correct amount of gas but wh...

    1993 Ford F-150 SVT Lightning

    -

    Maintenance & Repair

  • What would cause engine to shut off once I put it in gear? 3 answers

    When truck is in park and idling, it runs fine. As soon as it is put in gear (reverse or drive) idle goes down and engine shuts off.

    1999 Ford F-150 SVT Lightning

    -

    Maintenance & Repair

  • 2000 Ford Taurus rough idle after warmed up, turned off then back on 13 answers

    If the car is cold and started it runs fine no matter how long it is used without turning it off. Once the warmed-up car is turned of and the restarted the idle is between 300-500 RPMs in all ranges....

    2000 Ford Taurus SE

    -

    Maintenance & Repair

  • i have a 77 f150 that has a problem running when it is cold 53 answers

    77 f150 351v8 . i have to start it multiple times until it stops stalling then it runs fine once warmed up

    1977 Ford F-150

    -

    Maintenance & Repair

  • Heat blows cold in 2004 Ford Explorer 3 answers

    Has 4.0L, V6 engine. When air comfort controls are set to heat it blows cool/cold air forcefully -- always when idling and sometimes when driving. When the engine is working hard while driving, it blo...

    2004 Ford Explorer XLS V6

    -

    Maintenance & Repair

F-150 SVT Lightning

Looking for a Used F-150 SVT Lightning in your area?

CarGurus has 132 nationwide F-150 SVT Lightning listings starting at $5,995.

ZIP:

Search Ford F-150 SVT Lightning Questions

Ford F-150 SVT Lightning Experts

#1 Tom Demyan
Tom Demyan
Reputation 120
#2 Guitarman1930
Guitarman1930
Reputation 50
#2 Logan Carter
Logan Carter
Reputation 50
View All

Related Models For Sale

Used Ford F-150
3,363 Great Deals out of 205,065 listings starting at $1,273
Used Ford Mustang
724 Great Deals out of 38,166 listings starting at $1,750
Used Ford Mustang SVT Cobra
7 Great Deals out of 274 listings starting at $5,499
Used Chevrolet Silverado 1500
2,546 Great Deals out of 147,463 listings starting at $2,000
Used Ford F-250 Super Duty
456 Great Deals out of 42,343 listings starting at $2,800
Used Dodge Ram 1500
570 Great Deals out of 12,811 listings starting at $1,295
Used Chevrolet Camaro
664 Great Deals out of 38,931 listings starting at $2,995
Used Chevrolet Corvette
358 Great Deals out of 20,118 listings starting at $3,688
Used GMC Sierra 1500
1,143 Great Deals out of 57,189 listings starting at $995
Used Jeep Grand Cherokee
1,087 Great Deals out of 69,712 listings starting at $495