Automotive Air Conditioning Information Forum (Archives)

dannyual767 on Wed July 18, 2012 4:48 PM

Year: 2001
Make: Ford
Model: F250SD
Engine Size: 7.3l
Refrigerant Type: 134a
Country of Origin: United States

My diesel truck cools well at low engine speeds (1100-1500) during slower town driving (35-45mph) but doesn't cool as well at highway speeds.

I only use the ac on Max. Two weeks ago on a towing trip with 107° ambient temps, it seemed to cool fine on the interstate. However, last night on the interstate it was cool (maybe 75° ambient) and I noticed my vent thermometer reading temps between 55° and 60°. By the time I got into slower, in-town driving, the vent temp was down to 45°. It's almost as if the blend door was letting outside ambient air in, i.e. normal ac instead of max.

On the other hand, could these higher vent temps at highway speeds be caused by a low refrigerant level? I've got a manifold and gauge set as well as a scale and about 20+ lbs of R134a left in my 30lb bottle. Thanks for any help you can give me.

TRB on Wed July 18, 2012 9:18 PM

What are your pressure readings at idle and 1500rpm? Do you know what the charge level is at this point?

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Leggie on Wed July 18, 2012 9:25 PM

Snake the manifold gauge hose under the hood and put it up against the windshield in such a way that it won't damage the paint.
Close the hood at least enough to let the hood catch, but you don't have to close it all the way if you couldn't.
Have someone else drive the car and study the pressure reading from the passenger seat.


Edited: Wed July 18, 2012 at 9:26 PM by Leggie

dannyual767 on Wed July 18, 2012 10:25 PM

Quote
Originally posted by: TRB
What are your pressure readings at idle and 1500rpm? Do you know what the charge level is at this point?

I haven't connected my gauges to it yet. I've only owned the truck since January and I've never done anything to the ac system. I've always suspected that the truck might be a little low on refrigerant because the ac system never seems to be "freezing" cold.

Last night was the first time that I've driven it on the highway using the ac when the ambient temperature was cool and moist. In the daytime heat (85°-100°+) it seems to do just fine. However, last night the vent temperature was near 60°F on the interstate. By the time I got into town, vent temps went as low as 45°F.

fireguywtc on Wed July 18, 2012 11:16 PM

I keep up on the super duty stuff pretty good since I have one. The pros here will help you with all of your refrigerant charge questions.

Super dutys have a few issues that are easy to address and check.

Since yours is an 01 and new to you, you may want to consider cleaning and inspecting the inside of the HVAC box. I did a write up on the dieselstop as a general guide. It is pretty easy. HVAC clean and inspect

The super duty's also commonly have a clutch gap issues after years of use. Here is a tutorial to guide you on how to check it and fix it. clutch gap

Finally, once you get everything in good working order you may want to consider getting the heater hose shut off valve to get cooler A/C. The newer super duty's come with them (03+). Here is a place that sells them as a kit, they are supposed to be very nice with good instructions. dieselsite

Just a thought, you heater core may be reheating the recirculated air more when you are going down the highway because of the increased load. But I think its more likely a low charge. Good luck on you repairs!

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1970 Ford F250 4x4
2007 Ford F350

Edited: Wed July 18, 2012 at 11:25 PM by fireguywtc

dannyual767 on Thu July 19, 2012 2:42 PM

fireguy, that's a lot of really great information. I just ordered the heater hose shut off valve from Dieselsite. I'm sure that it is going to help. Later when work allows, I'll install it, check the clutch gap and get my gauges on this ac system to see what readings I get.

Edited: Thu July 19, 2012 at 2:44 PM by dannyual767

Dougflas on Thu July 19, 2012 7:35 PM

Ford had bad cycling switches back then. You may also be low on charge.