A/c works at idle but not towing or highway

[lariat 7.3]

My AC works good just idling but when i start running down the highway itll intermittently blow room temp air. When towing heavy it doesnt work at all. Just the fan. I have already removed the shim because the air gap was huge. Cleaned the coil best i could when it was still in the AC box. Have yet to throw some real gauges on it. But the cheapo auto store ones says the gas level is good.

 

[PWRstroke_smoke]

a weak fan clutch can affect air flow through the condenser but that normally means warm ac air at idle which you dont have. still might check the fan clutch out
is the front condenser clear of debris behind the front grill
If the engine gets too hot the computer triggers the Ac pump to shut off
check high and low side pressures

 

[Redman920-06 350]

could be a bad Expansion valve (orifice tube) causing too HIGH head pressure causing the high pressure sensor to kick it out. I've seen this quite often. I know you said you haven't hooked up gauges but you should, that'll rule out a lot. Also if the system is low on Refrigerant.

 

[zilla68]

we've had the same damn issue on a buddies truck, still not fixed, no orifice tube debris, pulled it down, held a vac great, truck has 60k on a 2002 7.3 and still not figured it out.

 

[lariat 7.3]

yeah im planning on it, trucks in paint right now though. Condenser is clean, pressure cleaned it out last week and it wasnt too dirty

 

[Redman920-06 350]

we've had the same damn issue on a buddies truck, still not fixed, no orifice tube debris, pulled it down, held a vac great, truck has 60k on a 2002 7.3 and still not figured it out.

What were the pressures on the gauges? (If you've hooked them up)

 

[zilla68]

yes, we had gauges, it was last summer so I don't remember, but they were within spec I remember that, pulled the orifice tube and it was clean also. with 60k on it, you'd think you wouldn't have any issues that's for sure.

 

[Redman920-06 350]

Keep in mind guys AC systems are pretty simple systems. (I found this online as to save me from typing it out) There could be lots of causes. I'll run through a few common ones.
1. The air conditioner compressor is broken - to fix it you'll need a new compressor, schrader valve, and possibly the evaporator.
2. The belt isn't on the compressor or isn't tight enough - to fix it, you'll just need the belt and have it properly tightened.
3. The freon has leaked out of the system - to fix it, you'll need to find the leak, repair it (could be a hose or the condensor, thest two are the most likely, but not the only places that could leak.) Then you'll have to have a vacuum pulled on the system and new freon.
4. The low pressure cut off switch is broken - just a new low pressure cutoff switch and a freon re charge.
5. Relay could be out - just a new relay.
6. The AC switch could be broken - just install a new switch will fix it.


Most of these repairs will have to be made at a shop with an A/c system handy to remove/recharge the sytem

 

[Redman920-06 350]

yes, we had gauges, it was last summer so I don't remember, but they were within spec I remember that, pulled the orifice tube and it was clean also. with 60k on it, you'd think you wouldn't have any issues that's for sure.

It's A/C Nothing surprises me XD. I work on Semi's and You would be surprised at what can happen in so few miles. As for "in spec" its hard to say with out seeing the system, it could be that your Compressor is bad. I have seen systems with good pressure but what happens is the compressor will...vapor lock and trick the system into thinking its working. It could also be a dirty air dryer. Its hard to diagnose with out seeing though.

 

[Lethalthreat7.3]

Have you rotated the compressor by hand to see if it has some compression? Does the compressor cycle on and off at idle or run continuously once the cab has cooled down? Does it start cycling when the engine is brought up to 1800 or so rpm? Is the accumulator sweating?
Actual pressures are needed to diagnose. I have alot of people tell me the pressure are "normal", but that is usually not the case. The last "normal" pressures were 75 suction and discharge. Pulled the orifice tube out and reinstalled it correctly. Pressures were then normal 46/260, for the current load in the vehicle.
Pressures will vary depending on the heat load in the refrigerant so saying pressures are "normal" can be completely different on a 100* day with a black truck in the sun, to a white truck sitting in the shade on a 85* day.

 

[Redman920-06 350]

:whs:

 

[Southmike]

have you removed the belt popped the clutch off and removed the shim..er washer?

 

[PWRstroke_smoke]

dont have to remove the belt to change shim, But i think OP said that had been done

 

[lariat 7.3]

yup, removed shim. Trucks still in paint. Ill get some real gauges on it soon

 

[Lethalthreat7.3]

With the engingine off you can rotate the clutch by hand and "feel" if there is compression. Leave the shim in place for the proper "air gap", and turn the ac on . The coil should engage the clutch/compressor and the accumulator should get cold.
This will tell you if the compressor is engaging(electrical) and accumulator getting cold will indicate refrigerant moving/flooding but guages will be needed to tell the charge condition.
If the compressor cycles off and on fast your probably low on charge.

It's a start without guages.

 

[NRHX2009]

I had the same problem. Replaced both pressure switches / sensors and got it recharged. Works perfectly fine now.