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I have a 1999 F-350 7.3 Diesel with 150K on it.

A.C. started blowing warm today at idle. Driving down the road, it blows cold. I'm scheduled to bring it to a local mechanic tomorrow, but I'm looking for any tips you can suggest as to what the problem may be. Also, any idea of the cost to fix any problems you may be able to suggest would be appreciated. I figure the more I know, the less likely that the mechanic will be able to take advantage of me...maybe.

 

The AC wheezing out at idle or low speeds is usually due to an airflow problem over the condenser.
The best way to pinpoint it is to look at the High and Low side pressures. A down and dirty test is to mist water over the condenser and see if cooling improves.
If so, clean the condenser and radiator from the center (between the 2 parts) outward. A strong soap like Purple Power cleaner works good.You will be surprised at how much grundge will collect in there.
If that doesn't solve the problem, your fan clutch is probably weak. Replace it with a new MOTORCRAFT unit. Aftermarket fan clutches are often not up to par, especially on the Diesels. The MC fan clutch will be about $175 (ouch), but is money well spent.
There's no good DIY way to test a fan clutch. A weak clutch will affect AC performance at idle long before the engine even notices.

 

While the truck was in the shop the A.C. worked flawlessly. The mechanic did tell me how I can perhaps diagnose a weak fan clutch as the problem when it presents next time. He told me that if at idle it's blowing warm, open the hood to see if the fan clutch is pulled in and spinning (I hope I have the terminology correct, I'm kind of ignorant on this stuff), if not, press against it with a dowel or the like and see if it then pulls in and starts blowing cold. If so, that's likely the problem

The other items he mentioned that may be the problem are an intermittantly failing high pressure or low pressure switch.

 

The Fan Clutch is the viscous coupling between the water pump pulley and the engine cooling fan. Your mechanic was talking about the AC Compressor Clutch, also a good suggestion.
Just like he said, when it stops working, look at the front face of the AC Compressor. It should be spinning along with the pulley. If it's not, tap it with a dowel or screwdriver handle.
If it kicks in, the clutch gap is too wide and needs to be adjusted.

While that problem doesn't "quite" fit your symptoms, it is a possibility and is very easy to diagnose and fix. Check that first, then try what I suggested.
If it worked perfectly in the shop, my suggestion doesn't "quite" fit either, though.
Here's some questions that will help:
-Does the loss of cooling only happen at idle?
-Will it always cool better when going down the road at about 35mph or better?
-Will the AC cool well initially and then stop once the engine is up to full temperature?
-Once it starts happening, can you repeat the results, ie: Stop and idle=AC warms up. Drive down the road=AC starts cooling.

 

Clutch gap is a good place to start, especially with the age of the truck (0.014-0.030" gap). Check the fan too, as noted and clean out the condenser. If that checks out, you need to get AC gages on it. Depending on the readings from them, you will know what the issue is. It could be as bad as the compressor dying. You still get some cooling and it is better with improved airflow, but not enough to keep it happy at idle or low speeds.

I'd bet clutch gap or dying compressor, but you need to get gages on it to know if it is the compressor. The pressure switch guess is really not likely.

 

"...but you need to get gages on it to know if it is the compressor. The pressure switch guess is really not likely."

Yeah,...what Dave said.

Knowing the physical state of the compressor clutch is foremost. No engagement=no cooling. Your truck is old enough that the clutch gap certainly can be an issue.
High Pressure Cutout Switches don't generally get flaky. They work or don't work.
Low Pressure Cutout Switches (Cycling Switch) on Ford products often fail intermittently, but your symptoms don't fit the "usual" symptoms.
You don't likely have a "pressure switch" problem. (But at 10 years and 150K, it wouldn't be the worst $12-15 you ever spent "just in case" to replace the LPCO switch).

To truly know what is going on in the AC system when it's operating, you need the manifold gauge readings.

However, to disagree just a bit with Dave, I've never seen an FS-10 (Nippondenso 10P-xxx) compressor get "weak". They usually either work well, or internally self destruct rather quickly. Their "dying" period is usually shockingly short IMHE. Some other compressors do get "weak" with age and wear, but..... It's very possible, but not proable.

I'm still thinking poor condenser airflow (dirty condenser or bad/failing fan clutch) judging by the symptoms originally given by firemediceric.

 

... and don't overlook the possibility that the blend door is flopping around in there...

The clutch gap is the most probable.

 

Thanks again for all of the tips.

I guess the mechanic did put the guages on the system. Wouldn't he need to to judge freon level? He asked if freon had ever been added, as it was just slightly low. I told him that the system had never been serviced prior to now. He added a little freon and a dye in case there's a leak.

To answer the questions posed above as best I can going on my recent memory of events,
-Does the loss of cooling only happen at idle? Yes.

-Will it always cool better when going down the road at about 35mph or better? For the most part, yes. There were a couple of instances where even above 35 MPH it seemed to continue to blow warm.

-Will the AC cool well initially and then stop once the engine is up to full temperature? The AC seems to always cool well initially. It is only occassionally following a stop at a traffic light or the like that it starts blowing warm. When that happens it will either blow warm the rest of the time regardless of speed, or as happens more frequently, blows warm at a stop and begins cooling again once we're traveling down the road. It never completely resolves and works right on a drive where it starts malfunctioning.

-Once it starts happening, can you repeat the results, ie: Stop and idle=AC warms up. Drive down the road=AC starts cooling? As stated in the previous answer, yes. Once it starts happening it always continues as described above. If the truck is shut off and allowed to sit for a few hours the problem may or may not be present once I start it again.

FWIW, the problem has not shown up again in the little bit of driving I have done since I got the truck back on Friday. Also, my son reminded me that this happened to him while he was driving the truck almost a year ago, but after that one isolated incident it didn't happen again until now.

Thanks again guys, and don't forget to call your Mothers today
__________________

 

The AC started blowing warm. I popped the hood to see that the disc on the front face of the compressor was not spinning. I touched it with a dowel and low & behold it started spinning and the A.C. started cooling.

Is this something I can adjust myself being as ignorat as I am about this, or how big of a job should I expect the mechanic to tell me it is? I will likely call him tomorrow; it's getting to be the time in Florida where AC is a necessity.

 

Clutch for this truck through FTE Parts guy under 99 F-350 7.3 under AC and Heating is $25 and some change

 

That was exactly what our '99 V8 explorer did. Maybe it's a different model compressor - dunno. It was working well enough that with airflow from moving you would get decent cooling, but in stop & go it would get warm. New compressor, all solved. But then I put gages on it and saw that the high side pressure was low and the low side pressure was not much below it at idle. => compressor not compressing

Bingo - clutch gap. You can usually adjust the gap at least one time. Pull the clutch off (one bolt) and there will be shim on the compressor shaft (thin washers). You can pull the clutch by prying at a couple of spots at the same time if you don't have a puller. Measure the gap at rest, and then remove one or more of those washers to tighten up the gap (measuring them helps). Gap should be 0.014-0.030", and you would be better off getting towards the 0.014 end of that. You may need to dig out the shims as they may be buried down in one of the parts. Torque on the retaining nut is 10 ft-lbs.

-Dave

 

I just measured the gap (I think I measured the right area) and it is indeed on the wide side. My thickest feeler gauge is 0.026 and that slides in easily with plenty of room to spare.

In your experience, how much time do you expect a shop will charge me to do this?

I think I would rather pay someone experienced with this that will get it right the first time rather than to try getting my big hands in there, have things not go smoothly and then let my frustration get the better of me.

 

No idea. I do all my own work. It's a ~15 min job, realistically, if access is decent (I don't know what the 7.3's look like in the engine bay). ...If you have a good shop and can get them to do what you want them to and not try to sell you anything extra. Not all shops will be willing to take your diagnosis and run with it.

 

All you need is 15 minutes and an 8mm wrench.

It's tough to screw up.

 

You may have emboldened me to give this a shot on my own. Do I have it right that I use an 8mm wrench to remove a single nut on the front and then things slide straight off? The serpentine belt can remain in place as everything is in front of that pulley?

As things come off, the washers/shims are exposed and I remove them as needed to reduce the gap to 0.014, which will take some estimation and trial and error?

Any diagrams you can direct me to that shows a breakdown of how everything comes apart and goes back together?

Thanks again for all of the input.