ac diagnosis

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The ac in my windstar works fine going down the road, but blows warm when the car is idling at a stop. I thought it would be a leak, but when I put the gauge on it, it has very high pressure - I think on the low pressure side, because it is the smaller fitting. If the system is plugged with very high pressure, why would it work with increasing rpm's and/or vehicle speed. I did not check if the ac fan is working.
 
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If it works when you increase the RPM, I would look at having the Compressor checked out. Does it blow cold when in park with higher RPM? If it does more then likely the compressor, if it is cold when moving only then I would believe the Condenser. Also, check to see if the Condenser is clear of bugs and debris. If it is only hot at idle i would check the cooling fan as well, assuming it has 2 fans.
 
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Originally Posted By: Burt
I thought it would be a leak, but when I put the gauge on it, it has very high pressure - I think on the low pressure side, because it is the smaller fitting.
The low pressure side has the big cap, and the high pressure side has the small cap. The high side should rise to about 200 PSI, and the low side should reach 3-40 PSI. When the car is cooled to ambient temperature, it should be about 1 PSI per degree of ambient temperature. When the compressor fails, the difference between the two sides is too small.
 
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Perhaps you have a vacuum driven blend door problem that leaks enough at idle, but higher engine RPM's masks But that's a rarity
 
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Low pressure on the low side and high on the high points to a plugged orifice tube, high pressure on the low side is either a bad compressor or a severely over filled system, you can't really diagnose it without knowing what the high side pressures are at a given low side pressure. A blown out orifice tube (fairly rare) would give you high low side and low high side. Generally with the system running, the low side pressure is fairly close to the surface temp of the evaporator core for a 134a system, if the doors are sealing properly and the evap core isn't dirty it should be reasonably close to the output temp at the ducts
 
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Google scroll compressor control valve. My friends son had the same issue....cooled great on the road but blew warm in stop-go traffic. I replaced the control valve on his Ford Freestyle (after reading up on it via internet) and it repaired his issue. The control vale was $38 and includes the valve and o-ring type gasket.
 
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Did you check low side pressure when the engine was running and AC on? Or did you check it when the engine/AC was turned off? The low side should be between ~20-40 psi when the AC is running. When you turn the AC off, the high and low side pressures equalize to roughly 60-70 psi.
 

Burt

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Originally Posted By: exranger06
Did you check low side pressure when the engine was running and AC on? Or did you check it when the engine/AC was turned off? The low side should be between ~20-40 psi when the AC is running. When you turn the AC off, the high and low side pressures equalize to roughly 60-70 psi.
With the engine running and AC on. Pressure is very high in the 'red' zone. Indicates fan is not operating or for some other reason, heat is not being removed because low side is not low. It can't be overfilled, b/c I have not added anything.
 

Burt

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I'll have to check the fan. I'm curious if there is enough fan capacity to cool the engine, but not the ac. Seems like it would pull air through both. both should be equally hot.
 
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Squirt some water on the condensor while the ac is running....if its a condensor-cooling issue, the suction pressure will drop and the air will be noticably cooler at the ducts. Based on your original post, I'm thinking it probably is a control valve problem....many, many Fords have this issue.
 

Burt

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all it took was to replace the bad fan. I thought it would be more parts to replace as I didn't think they would design it so one bad fan would knock the other out, but it did. Low side pressure is now 25 psi, low end of acceptable. Not sure if I should add any.
 
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