Hello everyone,

I have a 96 F350 with 46K miles on it. The other day my AC just stopped working. The AC had been used rarely over the 3 years I've owned the truck. The fan works but the compressor clutch does not come on. I discontected the power lead to the compressor, grounded my test lamp, turned on the AC, but no power appeared to be getting to the compressor. Any ideas of what I should check next? I did a search on this issue yesterday and thought I saw something about an AC pressure sensor that might go bad. If the system loses pressure, won't the clutch still run? My manual also indicates there is a clutch relay between the fuse and the clutch. The AC was working great before this happened. Any help would be much appreciated.

 

It may have just leaked the refrigerant. If there is not enough pressure in the system, the pressure sensor will prevent the compressor to engage. I would go to walmart and buy a Interdynamics refill kit that includes an charging hose with a pressure gauge. (the hose can be bought by itself at autozone for around $25).
Using the hose/gauge on the the low pressure side port, you will be able to tell if it lacks refrigerant (the reason I suggested the Interdynamics brand is bacause I use that one with good success, and it has a very clear readout, color coded, about if it is undercharged , charged , or overcharged) .
A fairly small ammount of refrigerant escaping the system can be the difference between a working and non working AC. If you indeed find that your AC has leaked, I would suggest you buy at walmart a couple of Interdynamics leak sealers and put them in the system before adding the refrigerant. I used this procedure in my 1995 with a rather large compressor seal leak, with excellent results. Mine cools the best a little on the "Overcharged" side.

 

You should have a low pressure switch on one of your lines. This prevents compressor damage by opening the power circuit to the a/c clutch. You can see if this switch is open or closed with a multimeter. The truck does not need to be running.

If it is open, i.e. no continuity, you will need to have it checked by a service facility. I do not think that you can replace the switch without opening up the system, which would cause whatever R134 you have in there to leave. Also, if the pressure is low, you might have a leak somewhere and that should be fixed.

So, no, if there is no pressure, the A/C clutch will not run. You can jumper the low pressure switch to test the clutch, but I would not run it very long.

 

Thanks for the suggestions. Pantera-nm, is checking the pressure with the Interdynamics set-up a very easy operation? I'm reasonably handy with tools and basicly know my way around an engine. I have never worked on an AC. All the manuals I've read say "don't touch the AC, high pressure" etc. Does the Interdynamics set-up walk you through the procedure i.e. which is the low pressure port side? Thanks again.

 

Working on an automotive AC system has its perils, but not more so than working, for instance on a pressurized fuel system. It is just a matter or being cautious and knowing a little about what you are doing.
An automotive AC system has two sides : the low pressure side (which can be identified by the hoses getting cold when the AC is operating) and the high pressure. In my 1995, the Low pressure port is on the accumulator/dryer assembly (the accumulator/dryer is the usually black vertically mounted cylinder next to the blower fan shroud on the passenger side of the engine bay, close to the firewall.) This port is one on which you will connect the hose/gauge. In every kit I have seen, they warn you about the dangers of connecting it to the high pressure side, (high pressure port is located close to the radiator on the drivers side) and of course you should not connect it there, but the fact is that since the low pressure port and the high pressure port and physically different, the chances are slim to none that you will ever have the chance to confuse them, let alone actually being able to connect the charging manifold to the high side, since it will not fit.

BEFORE YOU BEGIN!!!! Disclaimer:
Fully check your AC system for obvious preblems, such as cracked hoses or worn out components. Any component wet with oil very likely has a big leak. If you add refrigerant to a system that has an obviously faulty component you will lose the refrigerant again and in a worst case scenario could blow up if a damaged hose can not whitstand the pressure.
Proceed at your own risk.

Now with that out of the way....

The process to check the charge of the system is very simple. Connect the manifold to the Low side port, and turn on the engine. The interdynamics kit has a check valve built in that wont allow refrigerant to escape from the system through the hose, even if you have no gas bottle screwed to the other end.
With the engine on, you will be able to read the pressure inside the system. If the needle is on the green section (0 to 25 psi) you definitely lost the refrigerant.
To add refrigerant, you simply screw the fawcet-like adapter that comes with the kit on the refrigerant bottle (just open the lever to the max first or you will puncture open the can/bottle prematurely. You will understand how it works once you see it) .
Once you have the adapter screwed on the refrigerant bottle, screw the adapter + bottle on the end of the hose that is not attached to the car, puncture open the refrigerant bottle by closing the lever on the adapter, and when you open the lever back up again, the refrigerant will flow into the system. ALL THIS SHOULD BE DONE WITH THE ENGINE ON AND THE AC SET TO MAX COOL / MAX FAN.
While the refrigerant flows through the maniforld, the pressure reading will jump well into the red zone, but will drop back as soon as you close the flow. Check the actual pressure closing the adapter valve often, to prevent overchaging. You should also check the temperature of the air coming out of the vents of the cab while you have the bottle closed. try doing this precedure befroe 10 AM or after 6 PM , because pressures vary (rise) with the midday heat.


In my truck, the best cooling efficiency was between 45 (end of blue/charged zone) and 55 (middle of the yellow/Alert zone) PSI. Your mileage may vary. Get a thermometer, place it on the rightmost dash vent, and stop adding refrigerant once you get to those pressures or before, if you get air temps around 40 F.
From Zero pressure (or close to it), this trucks take around two 13 oz. cans or R-134a refrigerant to cool properly, but I would buy 3, because if you have a large leak, the leak sealer will not be fast enough and will allow some refrigerant to escape the system, and you may need to to the top the refrigerant off again in a couple of weeks.

Also, try to drive the truck with the AC on for at least an hour or two after adding the leak sealers and refrigerant to ensure that the sealer is evenly distributed through the system.

All this I am telling you is what I learned through my own experiences, learning by doing, and I did not manage to hurt myself, so I would say it is fairly safe if done with care. I hope you can benefit from this, and let me know if you need more info.

Pantera-nm

 

Thanks for the info Pantera-nm. I'll be checking it out this weekend.