Hello, I need help in powering up my a/c compressor. I added a/c system to my 1998 f-150 with a 4.6l. I pulled a stock a/c system out of a donor truck and installed everything into my f-150. All the necessary connections were already in my trucks wiring harness so I connected everything. All I need now is help in how to power my a/c compressor, when I turn on my hvac controls the blower runs but the a/c clutch doesn't engage. Do I need to change my throttle body to add a idle sensor? How do I get power to the a/c clutch? The clutch connector was already there and is connected. What am I missing?Please help me cool off.

 

Can you please be more specific?

 

Making a few assumptions here.

1. You verified that you have connections in the fuse / relay box under the hood for the AC fuse and relay.

2. That you have taken a test light and made sure they are hot and acting accordingly.

If not...

I don't have a wiring diagram so I can't tell you if the PCM is involved in the AC circuit on the 98, but my memory says no.

Regardless of your AC controls or PCM, you should have power through the fuse to one pin of the relay.

Then it's some diagnosis. DIVIDE AND CONCUR!

You have the two sides of the relay coil controlled by the AC controls , and maybe the PCM.

Then depending on if it's a four or five pin relay you have the common, and then the NC / NO pins. ( you will only have the common and one other for a four pin ).

As a quick test you can bypass the relay by jumping from the common to the wire going to the AC clutch ( check your wiring diagram ). I believe that side of the circuit is positive switching.

When you jump it you should hear the AC clutch kick in letting you know that part of the circuit is working.

At that point you will know if it's a problem under the hood or with the controls.

Hope this helps

 

I agree with everything @vigness said.


No one has mentioned - and I 'assume' you have charged the system AT LEAST enough to close the low pressure switch.


If you are relying on the compressor suction side to charge the system, you will need to jump out the low pressure switch until you get a little refrigerant in there.


Also - (I do not know about 1998) my 2004 PCM will shut the A/C compressor down (via the relay) under WOT conditions -AND- for about 30 seconds if misfiring. Do not know if NON-AC model would need to do something different to drive A/C relay.


( Just for a funny story - My A/C would shut off and not run for 30 seconds or so after hitting a railroad track or rough bump. FINALLY found I had a flacky/bad COP connector - discovered totally by accident that when I wiggled it and caused a few misfires I could hear the A/C click off!!)

 

You shouldn't need to jump the low side switch.

If it has enough to even try it will at least cycle until the low side has enough pressure to stay locked in. Even if it's totally empty the low side switch is VERY low pressure. I've had some that will engage just from the fill can pressure if it's close enough to the switch.

As long as your fill setup has some kind of gauge you should be able to tell if you have any pressure at all.

All that being said, if the electrical side isn't functioning it won't make a difference. BTW, all the electrical testing on by passing the relay can be done without the engine running, and probably should just in case. All you're looking to find out is if the system is activating ( Pulling in ) the AC clutch.

Once you know the clutch will pull in with bypassing the relay, then you're into what is or isn't functioning to do it under normal conditions.

Don't bother getting fancy on trying to bypass the relay, just open up a paper clip! It's just for a quick check, but DEFINITELY check the relay connections. Many of them have the diagrams as part of the case molding.