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1987 - 1996 F150 & Larger F-Series Trucks 1987 - 1996 Ford F-150, F-250, F-350 and larger pickups - including the 1997 heavy-duty F250/F350+ trucks

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  #1  
Old 03-16-2005, 09:20 PM
drivewizard drivewizard is offline
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A/C Air stops blowing when accel.

93 F-250, 460, 4X4. 92K miles.

When A/C is set on MAX, it will quit blowing when I accelerate. When I let off on the gas it starts to blow again.
When A/c is set to NORMAL, it works fine.
It appears that the fan stops or air is diverted to another spot, and doesn't come out the vents as supposed to.

I had it into the repair shop and they replaced a vaccum chamber up by the heater assembly. they said that should solve this problem.

Well it didn't, so I took it back again, talked to the mech this afternoon and he had a vac gage on it and said all was tight, no leaks.

Has anybody experienced this problem, or know what the problem might be.

Thanks for your input.
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Old 03-16-2005, 09:47 PM
Steina Steina is offline
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You need to determine if the fan really stops, or air is going somewhere else; "Defrost" is the fail-safe position in case of a failure, so if the air is going there when this happens it sure sounds like a vaccuum problem.

Steve
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Old 03-16-2005, 10:04 PM
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tex94F250 tex94F250 is offline
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I'm not sure why, but when I am hauling a load and running the AC if I "nail it" the AC seems to stop for a second until I am back in the normal acceleration range. It's like a high load cutout on non-essentials or something like that I thought. It has never bothered me and has done it since 3/31/94, the day it rolled off Akins lot. It is more like the compressor clutch disengages than the fan in my truck. Someone else is going to have to explain it if it's not normal. Oh, and it does the same thing with the defroster on full though I don't really notice it that much.
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Old 03-16-2005, 10:20 PM
Steina Steina is offline
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Tex -

I think you're right, the compressor clutch is designed to cut out during heavy acceleration; Ford probably figured you might need that extra "oomph" to avoid a head-on when passing!

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Old 03-16-2005, 10:38 PM
95 F150 95 F150 is offline
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Mine is the same way, like mentioned it is designed that way to give you all the power you can get at heavy throttle situations.
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Old 03-16-2005, 11:10 PM
cadman cadman is offline
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I just went through a similar problem on my '95. My AC and heat would stop when under heavy throttle going up a hill. There may be an AC compressor cut-out, but the fan and vents should still operate. If you get the same problem when the heater is on and you are pulling a hill, it is likely a vacuum problem. In my case it was a check valve. A new one was about $9 from the local dealer. It is black and tan plastic, has three ports, and is in the engine compartment. It is near the heater core and hangs on the two vacuum lines that go through the fire wall near the heater. The third line goes to the black vacuum reservoir on the right fender liner. I changed this valve first since it is a lot easier than looking for a leaking line or vacuum motor under the dash.

If you think about it, you need vacuum to move the various doors that direct the ventilation flow. When you floor the gas pedal, engine manifold vacuum drops to next to nothing. If directly connected, the vacuum in the ventilation system would also go away. The check valve allows the intake manifold to pull a vacuum on the reservoir and vent system when manifold pressure is less than what is in the ventilation system. But under heavy load the check valve closes and prevents the vacuum from bleeding back into the engine.
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Old 03-16-2005, 11:22 PM
drivewizard drivewizard is offline
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Thanks for the replys.
Some clarification: I've had the truck 10 years and it just started doing this.
It is the fan that stops blowing, don't know if air is diverted or fan stops.
I understand about the clutch dropping out if you are passing or pullin a big hill. That makes sense. Although I have never noticed air temp rise when passing or climbing a hill, but that is not my problem.
Also, my MPG dropped from about 10.5 to 11 to around 8.5 to 9 when this air flow problem arose. Mechanic said he was going to drive it and see if he could duplicate symptoms, he said he hooked a vac pump and guage to lines, and they held a vac. so he feels there are no leaks.
Maybe it has a intermittent leak or only leaks when driving? I don't know about automotive Vaccums and how they affect engine operation or preformance. My expertice is Commercial/ Indust. HVAC.
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Old 03-17-2005, 06:59 AM
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If it is the fan that stops, I don't know, mine keeps blowing, I just notice that the compressor clutch is freewheeling, so to speak. If the fan stops it might be electrical, don't know, your mechanic is in a better position to diagnose.
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Old 03-17-2005, 08:12 AM
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There is small diameter white plastic vacuum tubing under the hood that runs to the vacuum motor at the blower housing. On my '93, after a few minutes driving on max, the air flow would be diverted to defrost and not come back to the dash registers. When I looked under the hood, I noticed the above mentioned tubing had deteriorated and was disintegrating. I cut out the damaged section and slipped a piece of rubber vacuum hose over the remaining ends of the plastic. Been working fine since. Hope this helps.
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Old 03-17-2005, 09:21 AM
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You can test this by removing the connector on the AC clutch, running two wires to it and a 12V lightbulb you stick through the hood. Then turn on the AC (which won't blow cold, but the light you just added will light up). Then accelerate hard, and you'll find the light goes out.

Most fords are this way because the AC robs a fair amount of horsepower, and under hard acceleration they designed in a "give back" system which is a vaccum actuated switch that is in series with the AC switch on your dashboard.

As the vehicle ages, and acquires more vaccum leaks, this switch appears more "sensitive" because its getting less vaccum to start with. Could also be the switch has aged and should be replaced. in the F-series, I believe its on the passenger side of the AC lines. Its been a while. Certainly, check your manifold vaccum at idle as well as at speed (run a vaccum hose off the T, through the firewall, to a gauge on your lap).

Remember that most everything on your truck runs on vaccum... the brakes, the emissions equipment, the heater/AC controls. Acquire a leak in any of those places and you'll have all sorts of interesting problems. Could also be a leaking intake manifold gasket, a leak between the upper and lower EFI intake pieces, could be one of the many fittings on the intake itself (remove, wrap in plumber's teflon tape and reinstall), etc.

Many, many sources for vaccum leaks. Could even be piston rings for that matter.
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Old 03-18-2005, 12:35 AM
drivewizard drivewizard is offline
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I think we have a winner!!!!

Firstonraceday, by a nose. Just jeard from my mech this afternoon. He found a very small diameter vac line in vicinity of heater that was starting to degrade and had a seeper leak. Took him a while to find it I guess.
Thanks all for the response and ideas.
Sorry, no prizes.
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Old 03-18-2005, 01:58 AM
rdw516 rdw516 is offline
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I had the same problem..."92 f150 300/6 4wd. There is a vacuum line that controls the air diverter. On acceleration, the diverter would loose vacuum and the spring loaded door would change the air flow and it would come out the defroster. I discovered a small pin hole in the line and that was all it took to cause this problem. It was near the heater plenum under the hood...very easy to get to and fix. THOSE CONFOUNDIT HARD PLASTIC LINES ARE THE WORLDS WORST WHEN THEY AGE A BIT!
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Old 03-18-2005, 01:58 AM
 
 
 
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