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Old 08-11-2011, 04:32 PM
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A/C stops when towing

My A/C works very well when I'm not towing. After approx 20 minutes of towing, my compressor clutch disengages and the A/C blows warm air. When the A/C is working, the manifold guages show 240/40 high/low PSI with the AC on max at idle RPM. Turning the A/C off shows 120/110 High/low. Anyone know why the compressor would stop after I put a heavier load on the engine (towing) for 20 minutes?
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Old 08-11-2011, 06:26 PM
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This is reaching but here goes. I have seen many compressors have their air gap on the clutch wear to the point that when it gets hot and everything expands that the magnet is not strong enough to pull the clutch together. While towing you may be introducing enough extra heat to push your clutch over the edge. A simple test is to physically push the clutch in when the air quits. Do not do this with your hand, use a tool of some sort. Make sure to push in a manner that your tool can't be taken out of your hand and shot into your hood, fender or worse-your head or chest. Make sure your tool is long enough so that you don't lose any fingers. Yes, this is as dangerous as it sounds. I have tested dozens upon dozens of compressors this way and I have to take a deep breath every time. The thing to remember is that the part you are going to push on is not moving, until this diagnosis is correct, and then it is going to engage and spin your tool off of it's comfy resting spot. The scary part of this is that you will need to do this test 20 min away from home with your complete rig on the side of the road. Maybe someone else will chime in and say, "yeah, these all need compressors at X miles. Don't do that scary test." Update what you find out, and be careful.
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Old 08-25-2011, 01:48 AM
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more clues

Thanks Sargar,

I wasn't brave enough to try your test. My alternator went out the following day, and I thought this may have something to do with the A/C not working. With the truck running, I'd see 11.5 V at both of my batteries. You mentioned the compressor was controlled by an electromagnetic clutch. So, I thought maybe my voltage was too low, causing the clutch to disengage at times.

I replace the alternator and now see around 14 volts at both batteries. But, that didn't help my A/C problem as it still stops randomly. The last leg of our trip was a 6 hr drive, and for some odd reason it worked that entire 6 hrs. So, the wife was happy.

Here's another clue that I just noticed. When my A/C compressor stops, I also lose my vent thermo control. No matter what temp I have my dial set at, when I'm on "vent", I get really hot air. We noticed this because one of the days the A/C quit it was around 74 degrees outside. It was a sunny day, so you still needed the A/C. However, my A/C stopped working around 20 minutes after we took off. So, we tried just running on "cold" "vent". The air coming out was VERY warm. So, we ended up rolling down our windows to stay cool. Not fun in I-90 with all the noise.

But, the next leg of the trip, the A/C worked fine for 6 hrs straight. With the A/C working, I could turn the A/C off, switch it to "cold" "vent". The air felt the same temp as what was actually outside.

I've since made the test when the A/C compressor works, and the thermo control works then as well. When the compressor stops, my regular vent always blows hot.

Is there a vacuum line or something else that could be causing my grief here?
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Old 08-25-2011, 06:54 PM
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Quote:
Originally Posted by garya View Post
Thanks Sargar,

I wasn't brave enough to try your test. My alternator went out the following day, and I thought this may have something to do with the A/C not working. With the truck running, I'd see 11.5 V at both of my batteries. You mentioned the compressor was controlled by an electromagnetic clutch. So, I thought maybe my voltage was too low, causing the clutch to disengage at times.

I replace the alternator and now see around 14 volts at both batteries. But, that didn't help my A/C problem as it still stops randomly. The last leg of our trip was a 6 hr drive, and for some odd reason it worked that entire 6 hrs. So, the wife was happy.

Here's another clue that I just noticed. When my A/C compressor stops, I also lose my vent thermo control. No matter what temp I have my dial set at, when I'm on "vent", I get really hot air. We noticed this because one of the days the A/C quit it was around 74 degrees outside. It was a sunny day, so you still needed the A/C. However, my A/C stopped working around 20 minutes after we took off. So, we tried just running on "cold" "vent". The air coming out was VERY warm. So, we ended up rolling down our windows to stay cool. Not fun in I-90 with all the noise.

But, the next leg of the trip, the A/C worked fine for 6 hrs straight. With the A/C working, I could turn the A/C off, switch it to "cold" "vent". The air felt the same temp as what was actually outside.

I've since made the test when the A/C compressor works, and the thermo control works then as well. When the compressor stops, my regular vent always blows hot.

Is there a vacuum line or something else that could be causing my grief here?
I'm on my way out of town now but send me the year and model of your truck and if it is manual or automatic climate control and I'll cruise my wiring diagrams next week when I get back and see what I can find out for you.
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Old 08-28-2011, 04:50 AM
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vehicle model

It's a 2004 F350 Super Duty 6.0 with manual climate controls.
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Old 08-29-2011, 05:03 PM
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I printed the wiring diagram for both the manual a/c circuit and the heater circuit. The heater circuit is just a simple diagram for the blower motor. In looking at the a/c diagram I can see no point where temp and a/c compressor operaton are linked together. The control panel looks to be 3 seperate circuits inside. The temp control potentiometer (knob you dial the temp with) shares nothing with anything else. The mode selector switch and the blower motor switch share a ground and nothing else. The temp changing to hot was a good clue but now we need to figure out why. Does the truck have a heater valve under the hood to shut off the flow of hot water? A little hot water will cover any and all a/c function. Is it possible that the compressor clutch is still engaged but the blend door is allowing the heater to offset the a/c performance? See if you can get a visual of the front of the clutch when the air quits. If the clutch is still engaged and you get hot air I think you need to look at either the blend door motor or the control panel. It may be easier to wire up a temporary light that you could zip tie to your antenna as a visual that the air is getting power outside of the firewall. It still seems odd that this only happens when towing. Are you running a Scangauge or insight? Any chance of the pcm turning the air off to save the engine because it is hot? It might be nice to snag a cheap thermometer to stick in the a/c vents to verify air temp. I'm afraid this doesn't help much. Send me your email address if you would like me to email the diagrams to you. PM it to me if you don't want it out there for everyone to see. I'll help all I can but this one almost needs to be seen.
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Old 08-30-2011, 10:21 AM
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more info

Well, yesterday the cold vent was blowing hot air. I switched it over to A/C, and it worked fine. So, it may have been a coincidence that they both failed together last time.

I have looked at the A/C clutch, and I'm certain it disengages. I also put manifold guages on it while the A/C is on (and not working), and the low/high runs around 110/120.

I'm not running Scanguage or Insight (don't know what they are). I'm also not familiar with the PCM. But, the AC does fail only when the truck is working harder than normal. So, it could be related to a higher working temp.
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Old 08-30-2011, 04:16 PM
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if the truck gets to hot when towing they a/c will shut off this is part of trucks strategy so check the temp at time the a/c shuts off! seen this many times with pendot trucks and others
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Old 08-30-2011, 04:33 PM
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If the pressures are 110/120 with the air working then something is not right. Those numbers should not make cold air. At idle on a hot day I would expect to see something more like 50/250, or in that neighborhood anyway.

A Scangauge or an Edge Insight is an electronic monitoring system that plugs into your OBD diagnostic port and reads information directly from the PCM (powertrain control module). If you don't have one and are running a 6.0 you need one. I had a 6.0 Excursion towed in the other day. After getting it running I got approval from the customer to drive it home to make sure it was right. I grabbed the Insight out of my truck and put on it and low and behold, the oil cooler was junk and the customer didn't even know. I had a 32 degree delta, which would have been bad news with a camper in tow under heavy engine demand. Bottom line, if he had been running something like this he would have known. You need to get you one.

If you have determined that the clutch is disengaged then you need to check for power at the low pressure switch on the accumulator when the air is dead. If you have power into the switch then you have eliminated a problem inside the truck and need to search further under the hood. If you have no power then the have an inside the truck problem.

You may want to start asking around town about who is good on odd a/c problems. You may be able to find someone who is interested your problem and offer up some free information. Where are you anyway?

I'd like to know what you find. Hope this helps.
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Old 08-30-2011, 04:34 PM
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Quote:
Originally Posted by JUGERNAUT View Post
if the truck gets to hot when towing they a/c will shut off this is part of trucks strategy so check the temp at time the a/c shuts off! seen this many times with pendot trucks and others
This is where the Scangauge or Insight would be a lifesaver!
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