I have *some* experience diagnosing AC troubles, but I am a little confused on my truck and the searches I have done have not really yielded anything consistent, so I am asking for some insight. This is another "not cooling like it once was" problem, but it seems weird to me. And it seemed to happen over night. My description might get a bit long, but please bear with me.

This is a 2002 F250 V10.

Symptoms:
If the truck has had enough time to sit and the radiator has completely cooled (like overnight), the AC is ice cold and operates normally. Once the truck gets to normal operating temp and the radiator is hot, the AC temps start to change. I have yet to feel it get warm but it goes from cold to cool and will return to cold if I get the RPMs up or if I have traveled far enough down the highway. So a typical example would be: Driving around town or down the highway with a cold AC. Truck gets to normal operating temp. Stop in to the office or a store for a bit. Come back out and AC is no longer cold until I either rev up the engine or get back on the highway. It will occasionally drop from cold to cool at a stop light if I am there too long. If it was low on refrigerant, I would expect it to cycle from hot to cool and not clear up so readily.

I still haven't located my gauges so I have no pressure readings. However, the lines are cold as I expect (not icy), water condenses on them, and the compressor is not cycling on/off excessively. The system has never been opened nor anything added to it. I do not see any oil anywhere to indicate a leak so far. So, until I can get my gauges, I am looking past low refrigerant for now. I will for sure get those readings eventually though.

It seems more like and airflow issue to me, but the truck is not running hot at all. If I spray water on the condenser at idle, the AC gets noticeably colder So I was going about testing the fan clutch. With the truck at normal operating temp, the fan will spin a few times when I turn off the engine. I can turn the fan with my finger but I can't spin it as many of the test procedures say. When it is cold, it is much tighter. I can still turn it, but it takes more effort. I have not done the newspaper test.

Does this sound like a fan clutch problem? Anything else I should look at?

Thanks for any info

 

Ok, so more confused now after running a little experiment just now.

Got the truck to normal operating temp and checked the fan clutch again. *IF* I understand the test procedures correctly, it seems ok. No forward/back play, I can turn it with my finger but not spin it and it turns smoothly. Granted, it is over 10 years old and everyone says "replace it, it's old" but I try not to just blindly change parts.

Experiment was to get the truck on the highway, see if it cooled once at speed and then get back into town to see if it lost it in traffic and at idle. So I got out on the highway and drove awhile. Air got cold but not like this truck has before. Cooled the cab ok, but not down to cold. Then suddenly, before I decide to head into town, the air gets back to the icy cold I am used to in this truck. I got into town and hit every red light I could and slowly circled a few blocks at idle to see if it would fail again, but it held the cold air.

So now I am stumped. I am going to let it sit for awhile and see if once the heat soaks the condenser it will struggle to cool again, but this sudden change is confusing. Almost as quick as it started.

Would it be possible to have desiccant loose in the system and have it clog up and then later free up? Never heard of one freeing up once clogged but I guess anything is possible.

I will note that I used to always hear the blend door thump when I started the truck, but I don't seem to hear that anymore. But turning the temp **** to hot then back does change the temp as expected, so I am ruling out a blend door/vacuum issue unless you guys can think of something I am missing there.

 

Put the ac on fresh air not max the compressor should stay on all the time in this heat. If it turns off more than likely it's the fan clutch. When you first start your truck in the am you should Hear the fan running. Check the front of the radiator to see if it's clogged. The evaporator could be clogged as well sounds like reduced airflow some where. Either across the condenser causing the high pressure switch to turn off the compressor or the evaporator clogged and getting too cold and the sensor turning the compressor off. Hope this helps.

 

Let me just verify what you are saying for my benefit:

If the system is operating normally, then putting it on Normal (fresh air) instead of Max should have the compressor running always. It's over 100 here.

If the airflow is, in fact, restricted then the system will build pressure and the high pressure switch will cut it off.

Correct?

If I do this and the compressor stays on but the air temp still fluctuates, would that indicate low refrigerant?

Thanks

 

In short yes There is a temp sensor in the evaporator. if the system is on recirculate or max ac it will cool the cab to a certain temp then cycle off so the evap doesn't freeze up. the temp will rise a few degrees and kick back on and cycle on and off like this. But it takes awhile to cool down to that temp.

The reason it will stay on when in fresh air mode is the evap is pulling in hot air from outside not cool air from inside the cab. It won't get cool enough in the evap to shut off the comp. now the temp in the evap is colder then the air coming out of the vents if the vent temp says 50 or 55 when the comp shuts off the evap is cold enough to freeze . sorry I'm kinda all over the place I am half asleep from swimming all day on vacation.

Just check the easy stuff first clogged evap clogged radiator fan clutch, does the big line coming out of the fire wall have condensation on it does it feel cold? An easy way to check the fan clutch or for a clogged radiator is to take a garden hose ans spray the condenser and see if the ac stays on. If it does and the radiator looks clean prolly a fan clutch

 

No, not necessarily. Compressor cycling is controlled by the cycling switch. However, at 100F, 100% duty cycle would be common.

Yes. The high pressure cutout will activate around 450 psi to protect the rest of the system. Insufficient airflow through the condenser or an internally clogged condenser will cause this.

No temp sensor in his truck. Evaporator temp is inferred by the low pressure cycling switch mounted on the inlet to the accumulator can.

See if you can find your gauges. They will remove a lot of guessing.

 

Thanks for the input guys. I'm going to have to just watch this thing for a bit more. As I mentioned, it started cooling properly again without me doing anything. I have been monitoring it today and it may take a little bit to catch up to the 100 degree heat, but once it does it cools fine.

To follow up, I let it run on Normal AC for a long time in the driveway and it never cut off the compressor and the air cooled. Switched to Max with the doors open and the compressor stayed on then as well while blowing cold air. Low side lines are cold and have water on them. I cant stop the fan using the rolled up newspaper test, so I think that clutch is fine. All I can think of is that I may have an issue with my blend door or maybe my evap coil is starting to clog.

 

Is there a valve for the hot water line to the heater core. If so it may not be closing..Or a blend door not closing..

Dick

 

No valve on this truck. I am starting to suspect the blend door. However, my initial symptoms would seem strange for that to be it as the temp would change as I revved the engine. But maybe it was just overcoming the heat from an open blend door?

The sudden change both ways it what has me scratching my head.

 

Well, I tried all weekend and again this morning to try and get this AC to act up again. Still blowing cold. No idea why it would seemingly overnight start acting as I described, do it for about a week, and then go back to normal.

I guess I'll keep an eye on it and revive this thread when/if it starts again. Thanks guys!