HVAC Schematic for 96 F-350

Chuck_Biscuits · 07-03-2009, 06:24 AM

Hey there ElectrolysisDoc! I've been following several threads regarding HVAC troubleshooting and I noticed you have schematics for several vehicles. I'm trying to diagnose my 1996 F-350 and I haven't found any schematics online or on this forum. Would you happen to know where I could get them? My problem is that there is no power at the compressor. I don't own a repair manual for this truck yet(I just bought it). Thank you in advance!

ElectrolysisDoc · 07-03-2009, 08:09 AM

Of course you have checked fuses right? When you say you have no power at compressor, have you tried jumping the low pressure switch? Maybe you have no pressure? What testing have you done?

The most common causes for no power to compressor are 1) Insufficient refrigerant 2) Blown fuse

Chuck_Biscuits · 07-03-2009, 09:12 AM

All fuses are good. I did jumper the low side pressure switch and the compressor came on. I just bought a pressure gauge set and checked the pressures.
Engine off : high 123 psi. low 85psi.
Engine running/air on: high 250psi. low 85psi.
Engine running/air off: high:110psi. low 85psi.
The pressure on the high side went up to 275psi when I first cranked it, but after 5-10 minutes running it dropped down to 250psi and held steady.
The air blows cold with the pressure switch bypassed. Do my pressure readings seem correct? My truck has about 185,000 miles on it and it has the 460 in it with an automatic. I just fixed two vacuum leaks i found on the motor (small black vacuum lines) and that seemed to fix the air diverter from going to defrost. Thanks!

ElectrolysisDoc · 07-03-2009, 09:20 AM

The pressures seem excessive. I don't like the low pressure at all. Not sure how you can be getting cold air with a 85psi low side. If you have power at the low pressure switch, and jumping it makes the compressor engage, then replace the pressure switch.

I think you are over charged....

Chuck_Biscuits · 07-03-2009, 10:27 AM

I replaced the low side pressure switch and cranked the truck. The Compressor engaged. I had the gauges hooked up to see what was happening pressure wise, and the high side pressure went up,up,up to 450psi and then the compressor clutch disengaged. I guess because of the high side pressure switch. Now I'm wondering if the orifice is clogged? Why would the pressure climb like that on the high side just after changing the low side pressure switch? I noticed when i changed the switch flourescent green fluid came out of the valve. Is that freon,oil or UV dye? I also noticed a screw inside the switch. Is that so you can adjust the low pressure cut off point? I got the switch from NAPA, part #207404. I assume that it comes pre-adjusted?

ElectrolysisDoc · 07-03-2009, 11:00 AM

The switch does come pre-adjusted.
The big problem with your A/C is the excessive pressure. Sounds like somebody dumped in a couple cans of refrigerant. Might even have air in it.

Most common reasons for excessive high side:
1) Too much refrigerant
2) Air in system
3) Too much oil
4) No air flow across condenser
5) Restricted condenser

Since you just bought the truck, my money is on "too much refrigerant".
A little bit of oil and dye is pushed out by the refrigerant whenever a switch is replaced. No worry. The pressures given do NOT suggest an orifice tube problem.

The environmentally correct way of dealing with an overcharged A/C is to have the excess recovered by a technician certified to handle refrigerants, using an approved machine. You may only need 8-10oz recovered from the system. It's just a guess.

However, the text-book correct way to deal with this situation is to recover, evacuate, recharge.

P.S. Thanks for providing gauge readings! It makes a huge difference.
You can order a factory repair manual from Helm: https://www.helminc.com/helm/Dealer....XTHT5K9D7U43G3

ElectrolysisDoc · 07-03-2009, 11:05 AM

I found this folded up in my back pocket. You may not need it.

Chuck_Biscuits · 07-03-2009, 01:55 PM

It's all fixed! I had to take it to a friend of mine to evac & refill. Low press is now 47. High side is 187. She's ice cold in the cab. Thanks for all the help and tips and thanks a million for the schematic too! I think I've got a handle on this HVAC diagnosis now and that's the best part of this whole experience. The internet is such a beautiful thing.