Here we go guys, I have a 1999 F250, 5.4 lt, automatic trans, manual shift transfer case, warn lockout, no vaccume hubs. The truck is just an XL with tilt, cruise, and AC as its only options.

Here's the problem, I bought the truck used, 140,000 miles or so on it, an the AC doesn't work. I had the guy who does all my AC work check it out for me, and he tells me that its fully charged, and the compressor is ok, but the reason it won't work is because of how it is wired. Something about the PCM tells the compressor when to lock up and when to release. Then only answer that he could give me is that I would have to get a different PCM.

What I am wondering is, has anyone else had the same \ similar problem , and is there a way to wire around the PCM????

Any help would be greatly appretiated.

 

What ?????????????
I think I ask you this before
Does the compressor run at all? short cycle?
Does the fan/blower work? in all positions
Its a xl model so its a manual ac setup
Did he jumper out the high/low presure switch?(ac cycling sw)
Have you chk all fuses?

The only part that I know of controlled by the pcm is if you hold it at wot it will cut out the ac.

I will verify it and get back

Rich

 

I belive the compressor only cycles when he jumps one of the pressure switches. The fan changes speeds at the different spots on the dial, I have heat when i turn it to red, and it blows out the right spots when you change it via the dial [ dash, floor, defroster] All the fuses are good, but also remember him saying that he couldn't find an AC relay. Is there an AC relay??? Mavbe that is the problem???

hope this helps

 

Crew at this point I don't have my 99 5.4 manual in front of me.
But I have a 6.8 manual I don't think there was a difference in 99

Unless someone else can verify any differences
I will keep looking
Rich

 

Okay there is no ac relay .
you have 3 switches in the circuit for your ac
one is the ac cycling switch and the other is called a ac containment switch.
and the sw on the dash
At your compressor plug you should have a tan with lg green tracer wire
this provides the 12v for the compressor clutch
There should also be a black wire this is gnd.

In looking at the schematic ,your tech may be right .
The pcm is tied into the 12v supply to the compressor at a diode.
if the pcm has a internal relay that pulls the wire feeding the clutch to gnd the clutch will not activate.
The test will be if you have 12v at the diode or the clutch.

But I need to verify the schematic on a 99 5.4 first.

If he jumpered a switch and the compressor ran then he had a bad sw or low pressure.

But if he put 12v to the compressor and it ran he could be right that the pcm is at fault.

Rich

 

If this is correct then while the jumper is in the system should get cold. If it does then that pressure switch is the problem.

In my experience pcm failures are very rare.

 

I'll call him today and see if he remembers if he put power to or grounded the switch

Thanks

 

When you said he jumped out the switch that would mean that the switch was unplugged and the harness pins were bridged together using a piece of wire.

If he sent power or ground into the harness it would be totally different meaning not a bad switch.

 

Crew ,why don't you just do the tests yourself.?

Do you have a volt/ohm meter?

As rwillimo and I have said this system is pretty simple

Rich

rwillimo,I haven't worked on this system , with the diode in the circuit .
Only the newer ones with relays

Is what I thought about the pcm ,possibly pulling the line to gnd
Correct???

 

[QUOTE=FortyFords]Crew ,why don't you just do the tests yourself????

I don't do the tests myself, cause after puttin in 12 to 14 hours a day working on customer cars all day long, i just don't feel like working on my own.

I am a mechanic by trade, work at one of the auto auctions in suburban Chicago, and not real familliar with ac work. I mainly stick to steering / suspension, and drive line work.

Not that I couldn't figure out what is wrong with the AC, but figured it would be a whole lot easier on me, if I could short cut it, and go off some else's knowledge that has done this before.

Thanks