1999 F250SD 7.3l

Symptoms:
AC is not coming on

Testing:
Checked Air Gap (.024) -Seems ok?
Applied power directly to compressor/clutch - clutch engages fine, A/C Blows Cold in cabin
Jumpered Low Pressure Switch - nothing happened
Hooked up a refill hose with a gauge on the low side- showing ~35 (when I manually start the compressor)

Jumpered the High-pressure switch - compressor comes on. I had to really wiggle the jumper around inside the plug to make contact, and, had to hold pressure on it.

Tested voltage to compressor - 14v while unplugged, drops to 0 as soon as it's plugged in



So based on my testing I'm leaning towards a bad high pressure switch plug. Does that sound reasonable? When can I find a replacement pigtail? Is the high pressure switch a possible problem? I can test it with a multimeter but I'm not sure what it should be reading.

Thanks for your help. I have a long trip on a hot day this weekend and really need to get this fixed.

 

Gap and low-side pressure are okay.

The high pressure switch is supposed to be normally closed. It's only supposed to open if the pressure goes way high (450-475 PSI), usually due to a restriction in the condenser or a failed cooling fan.

You're probably correct in your diagnosis of a bad harness plug for the high-pressure switch. In a pinch, I'd simply bypass it.

You might want to check with your dealer's parts department to see if a pigtail is available. If not, you'll probably need to get one from a salvage yard.

 

Thanks for the help. I bought a new High pressure switch and that fixed the problem.

I checked the ohms from pin to pin in each sensor and the new one showed a good circuit, the old one showed a few thousand ohms indicating to me that something inside had shorted out or degraded to the point that it could no longer carry enough current to operate the compressor clutch. I'm just glad to have A/C back!

On a side note...the schrader valve under the sensor was stuck partially open. I was quick and didn't lose much freon, but I thought that was strange.

 

I have the same problem with my F550 7.3 Powerstroke. However, mine has been repaired several times and the problem keeps coming back. The last time I realized that the high pressure sensor connector was burnt and a little melted. I replaced the switch and the connector, but after a couple of weeks, the connector is burnt again. Seems like something is drawing too much current in the high-side pressure switch circuit. Anyone have any ideas? Anyone have a schematic?

 

What model year is your F550?

 

Oh yeah, I should have noted that. It's a 1999.

 

The circuit should be pulling about 4 amps when the clutch is engaged as the clutch is directly switched by the series circuit of the low and high pressure switches. You can measure the clutch coil draw with most multi-meters on the 10A scale.

If the connector wasn't making good contact with the switch terminals, the current would quickly heat up the contacts and damage the terminals. I'd replace it again and use some dielectric grease in the connector to improve its life.

 

Here's an update. I measured about 3.2A with the compressor on, so it seems like it should be within range. Sure doesn't seem like enough to burn that connector, but I guess it's possible if there was contamination. I cleaned and greased the terminals, and it's working for now, but I still plan to get a new connector and switch. Thanks for the help.

 

Replaced the connector (new harness from Ford) and the switch (from NAPA since Ford only has it in a package with an accumulator!). All seems to be working now. Thanks for the help!

 

Good news! Thanks for updating with results.