A/C not working

CANNON1349

MECHANIC
2008 FORD F-150
4.2L
V6
2WD
AUTOMATIC
153,000 MILES

Customer came with an A/C problem, but the pressures are good. Low side 55 PSI and high side 250-300 PSI with it being 95F 100% humidity outside. Those pressures seem perfect according to the various charts I've read.

I found that the A/C simply cuts on/off repeatedly only when you start the truck with the A/C button ON. If I turn the A/C manually off and restart it, it blows as normal with the good pressures showing. Obviously leaning towards an electrical issue, but what exactly? The switch on top of the compressor? Or something behind the dash?

2 Replies

BMDOUBLE

MECHANIC
1,139 POSTS

I would begin by retrieving the trouble codes from the power train control module and the HVAC module, also check the battery voltage when cranking the engine.

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Thursday, April 15th, 2021 AT 2:32 PM

ASEMASTER6371

MECHANIC
52,797 POSTS

Good afternoon,

I am concerned with the pressures. They are not perfect.

The high side should be 2 times the ambient temperature plus 15%.

The low side should be 25-30 lbs. Rock solid. 50 pounds is way too much for the low side.
That pressure is not a temperature variant.

As far as the compressor cycling, when it starts turning on and off, can you monitor the voltage to the compressor connector with it running to see if the signal is dropping out?

The PCM grounds the relay, the relay needs to be checked for powers to be sure it is constant and the signal from the control head is what feeds the relay.

Roy

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