AC works intermittently

[99ranger]

My AC works for about 15min and then shuts off and if left off for about 5 minutes it works again when restarted. It has the correct refrigerant and charge. Opinions?

 

[55trucker]

What pressures are you seeing on the manifold gauges as the system is running?

 

[RonD]

AC works by compressing the refrigerant then letting it expand back to "normal" pressure which causes the cooling effect

There are two pressure switches in the system, high pressure switch which will be located after the compressor, and low pressure switch which will be located after the expansion.
These switches are closed when system is working correctly.
High pressure switch keeps lines/seams from blowing out
Low pressure switch prevents compressor from running dry, running out of refrigerant to compress

On older models the compressor's 12volt activation voltage ran from AC ON switch thru each pressure switch and then to compressor, so if either switch was open compressor would shut off(no AC)

Later models used the Ground for compressor voltage, but same result, if either pressure switch opened then no AC

Your 1999 uses the Ground thru the pressure switches but........it goes to the Engine Computer(PCM)
The AC on switch goes to PCM, PCM then passes the pressure switch Ground to the AC compressor Relay in the engine fuse box
When AC relay closes 12volts is sent to the Compressor

Ground--------pressure switch-----------pressure switch-----------computer------AC relay

Reason computer(PCM) was added to the AC system is so it can shut off the compressor at WOT(wide open throttle), and if engine should start to over heat.

I would test if switches are closed key off, use OHM meter
Then start engine and run AC until it shuts off on its own, then turn off engine and retest switches to see if one is now OPEN

Could be the AC relay is going bad, long shot, but if both pressure switches are closed and AC won't come on then that would be next stop

 

[99ranger]

160/35

 

[99ranger]

Thats kinda what I was leaning towards but didn't know why it would work for awhile and then shutoff and then be able to come back on shortly afterward?

 

[RonD]

If there is a blockage then pressure builds up until high pressure switch opens
Once pressure drops down switch closes and AC works again.

Blockage can also cause low pressure side of system to run out of refrigerant so low pressure switch opens, then closes when enough refrigerant comes in, past the blockage.

Also the condenser in front of radiator should have nice even heat, slightly warmer on side where compressor hooks up, but no cooler spots, thats a blockage.
Compressor can vaporize the refrigerant, if it gets too hot, the condensers job is to cool it down and turn it back into a liquid, this also lowers the pressure in the high pressure side of the system

 

[99ranger]

I should mention that I had it hooked up to gauges for about 45 minutes and it stayed working. It only quit on its own when going down the road. The only way I could get the AC to fail in the shop was buy turning the AC off and trying to turn it back on, it would not turn back on until I unhooked the connector from the compressor and hooked it back up again.

 

[Denisefwd93]

160/35

meaniless without knowing ambient temp.

 

[55trucker]

160/35

When the system is running the pressures are always changing, if inside a shop what is the room temp? The discharge side will be constantly building as the suction side drops. The suction side low pressure valve cuts out at approx 26lbs, then the system will attempt to *stabilize*, the low side rises, the switch will close at approx 42lbs. and the cycle begins again. So what are both gauges reading thru-out a cycle?

Ford uses a *clutch time* on, *clutch time* off, *clutch cycle rate* per minute to determine system performance.

The static pressure when the system is not in use should be approx 100lbs. The system should hold 30 oz.

 

[99ranger]

When the system is running the pressures are always changing, if inside a shop what is the room temp? The discharge side will be constantly building as the suction side drops. The suction side low pressure valve cuts out at approx 26lbs, then the system will attempt to *stabilize*, the low side rises, the switch will close at approx 42lbs. and the cycle begins again. So what are both gauges reading thru-out a cycle?

Ford uses a *clutch time* on, *clutch time* off, *clutch cycle rate* per minute to determine system performance.

The static pressure when the system is not in use should be approx 100lbs. The system should hold 30 oz.

The gauges didn't really move much they stayed pretty much 160/35. approx ambient temp was 70F
The sticker on the rad support said 1 lb 14 ounces so that is the charge it has.

 

[55trucker]

The gauges didn't really move much they stayed pretty much 160/35. approx ambient temp was 70F
The sticker on the rad support said 1 lb 14 ounces so that is the charge it has.

1 lb/14oz is 30 oz.

160/35,

that doesn't seem quite right, at 70F there should be a continued cycling, you'll see that on the gauges moving up & down constantly, if the suction side is holding at 35psi the low pressure switch will not open & the system will not cycle as it should do, possible sign that the system is not building discharge side pressure because of possible low refrigerant or the orifice tube is not creating a restriction or the compressor is struggling.

At suction side 26lbs. when the low pressure switch opens the discharge side should be over 200lbs.

 

[99ranger]

1 lb/14oz is 30 oz.

160/35,

that doesn't seem quite right, at 70F there should be a continued cycling, you'll see that on the gauges moving up & down constantly, if the suction side is holding at 35psi the low pressure switch will not open & the system will not cycle as it should do, possible sign that the system is not building discharge side pressure because of possible low refrigerant or the orifice tube is not creating a restriction or the compressor is struggling.

At suction side 26lbs. when the low pressure switch opens the discharge side should be over 200lbs.

When it is working the air is a solid 40F so it works well when the compressor is on

 

[55trucker]

When it is working the air is a solid 40F so it works well when the compressor is on

That I can readily see..

if the system is literally running 100% *clutch on* then the vents will get cold, but the system should cycle. If above 85F the system will indeed run *clutch on* far more intensely than at 70F. I'd be concerned with the evaporator core freezing up seeing as the compressor not being a variable displacement item just keeps pumping.

Here the wiring schematic for the Ranger's system.

On the non re-engagement of the compressor...... if when this anomaly occurs you bypass the low pressure switch by bridging the contacts in the harness connector *while the system is running* if the switch is suspect the compressor should engage, there is also the high pressure cut-out switch on that same circuit, these two pressure switches are in series, they are simply ground paths for the ECM that controls the circuit. Check them both. If the system does not re-engage run a ground wire directly from the low pressure switch (BK/V wire) straight to the ground side of the battery. There may be the possibility of a poor ground on that circuit.

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