Need a/c help

[keysersoze]

Hi all - 1998 Ford Ranger XLT 4wd that I recently obtained. A/c does not work - compressor does not engage. No power at compressor or either of the pressure switches. Tested both switches with ohmmeter as suggested by RonD. High side was closed but low side was open. Could it be as simple as replacing the low side switch? But the strange thing is testing for power at the a/c relay. With key on/selector off, I get 12v on pin 3 AND pin 2. Same thing with selector on. That doesn't seem right based on what I've read here. Also, there is no receptacle for pin 4 - just an empty hole. Is that normal?
Any help would be greatly appreciated.

 

[lowspeedpursuit]

Hold up: did refrigerant pressures check out before you started focusing on electrical? If it's broken because your refrigerant leaked out, the low pressure switch being open is expected. If pressures are good, then yes, it seems the low pressure switch is bad, but you'll need to remove the refrigerant to replace it.

If you google the wiring diagrams for the relay, 3 is hot, 4 is no contact, and 1 and 2 are controlled by the PCM, so if it activates by switching ground on 1, those voltages are reasonable.

 

[keysersoze]

Thanks, I'll check pressure tomorrow

 

[JoshT]

"High pressure", or A/C Pressure Cutoff Switch should be closed, it opens if pressure goes too high to stop A/C compressor. "Low pressure", or A/C Clutch Cycling Switch, will open andclose depending on pressure in the system. As mentioned above, if system pressure is too low, it will never close.

That doesn't sound like the first problem you need to diagnose though.

No power at compressor or either of the pressure switches.

The compressor clutch does not get power until the relay is commanded closed by the PCM. When A/C is turned on, the PCM sends power to the low pressure swtich, if closed the power continues to the high pressure switch, if that is closed it continues to ground. When the PCM detects this ground signal it commands the clutch relay to close.

So, if the A/C is turned on you should be getting power to the low side switch. It sounds like that isn't happening, so first thing to do is find out why. I would start by checking to see if you have power on pin 41 of the PCM when the control panel is set to A/C, MAX A/C, or either defrost setting, as all of these command the PCM to power the A/C system.

You can check the system pressure, but there's no point in draining or recharging the system if there are other issues that will prevent it from being able to function. Besides the pressure is supposed to be checked with the A/C on and running.

Also, the switches are on a schrader valve, so draining the system is not required to replace the switch.

 

[JoshT]

Maybe this will help on the relay, and the electrical diagnosis in general. It's for a 99,but electrically and functionally it should be exactly the same.

You could really delete the PCM from the equation and jump those pins and the AC would function perfectly fine. Only thing you would be lacking is the WOT cutout, and the idle adjustment for the extra load which are both provided by the PCM. If we ignore the internal workings of the PCM, when the control panel is turned on, power is sent from pin 41 to pin 71 and the clutch relay. Then the power returns to PCM at pin 69 and over to pin 86, where it goes back out to the pressure switches. When both switches are closed the power is sent to ground, completing the circuit and closing the compressor relay.

Since we're looking at solid state electronics in the PCM, the pins aren't actually connected. Contrary to what I said in previous post, the fact that you get 12v at relay pin 2 suggests that your PCM is receiving the signal from the control panel without issue. Most likely the PCM energises pin 2 on the relay when it recieves power from the control switch, then controls it through ground @ PCM pin 69. The PCM looks for ground on pin 86 to tell it what to do with pin 69. If pin 86 circuit is closed then pin 69 is grounded, if the circuit is open or hit WOT the ground is broken.

Your post above sounds like there is no power getting to the low pressure switch. If there was a short between PCM pin 86 and the switch, pin 69 would be grounded and the clutch should be running constantly. If there is a break in the wire, or the PCM is not sending the signal, then the PCM will detect an open circuit on 86 and never engage the clutch (just like an open switch).

IMO, first thing to do is figure out why there is no power at the low pressure switch. After that, diagnose why the switch isn't cycling. Even if low on charge, you need the compressor clutch engaging and cycling to recharge the system.

 

[rubydist]

On most systems, the low side switch has a schrader valve under it, so when you remove it you do not lose all the refrigerant.

 

[keysersoze]

Confirmed that there is a schrader valve and no problem removing the low pressure switch without draining, but it makes sense that the issue is incoming power rather than a bad switch. I'll test the PCM this morning.
Thanks for all the help.

 

[keysersoze]

No power at pin 41 of the PCM, so I tested at the A/C-heater switch and got power there. So I then tested at C124 and no power. I then moved to C135 to try to narrow down further and that's when I must have done something really dumb, because I lost all power everywhere; no lights, no sound, nothing. Battery is new and fully charged. I have power going into the power distribution box in the engine compartment, but there's no power at the interior cab fuse box. I'm not sure where to go from here. Anyone have any ideas? I'm not an electrical whiz by any stretch but I do have a decent enough multimeter. Thanks for any help.

 

[RobbieD]

You probably blew a fuse. Visually inspect, or test both sides with a voltmeter, each and every fuse in the power distribution box.

 

[JoshT]

Sorry, I ain't got schematics for the rest here. I happened to have the AC one saved on my PC, but my books are in the truck at my parent's house where I'm doing the work on it. To knock out all power to the interior, I'd start looking for loose connectons and blown fuses.

 

[keysersoze]

Thanks, I was able to fix the no power at all issue and here's where I'm at. I have power at pin 2 on C135. I have no power at pin 37 on C124. Based on a schematic I found, there's nothing else between those two points. When I tested the resistance between both points, I get zero ohms, so I've got the right wire. So does it sound like a short to ground? I tried to test between each point and ground, but got widely varying readings between zero and OL. I'm not really sure how to approach this. The connectors look ok from what I can see, which isn't much. Do I have to try to take out the harness and inspect the wires inside? Please tell me there's an easier way.
Thanks.

 

[JoshT]

Ohms is a measure of resistance. Zero ohms means zero resistance, is a good connection. Infinite ohms would indicate an open circuit, like a broke wire or unplugged connector.

Can you check continutiy (resistence/ohms) between C234, at the back of the control switch, and C124?

If you had a short to ground, I would be expecting for the fuse to blow. If you get power at C135 you are good between the switch and there. You say that you have continuity between C135 and C124, so that wire should be good. If you don't get continuity between C234 and C124, I think you likely have a bad connection at C135.