How to you jump low pressure switch on AC

[hal keiner]

The compressor on my 2002 Sport Trac does not ever move when the AC controls on the dashboard are turned on an any postion. After reading several posts, I understand that one troubleshoot method is to 'jump' the low pressure switch for a second or two to see if the compressor turns on for those few seconds. I want to perform this test but am not completely sure how to do it correctly/safely and thought I would ask so I don't make things worse.



I believe this is what I should do:

- Start the motor

- Turn the AC controls to Max

- Disconnect the connector attached to the AC cycling switch (located near the AC low side port valve)

- Use a paper clip or similar method to short the two female ports on the removed connector.

- See if the compressor starts to turn/move.





Have I got this right?

 

[scott degroot]

That's how you do it. For the record the cycling switch almost never goes bad. They may leak but I don't think I have ever replaced one in 25+ years because it went bad. If you suspect that the compressor clutch is defective check for power at the compressor clutch with the ac turned on. If you have power then the clutch is bad. More than likely your system is low on refrigerant.

 

[Duane Foster]

You might unplug both the high and low pressure switches, and check the plugs. The a/c in my '01 wasn't kicking on at all. I checked the low pressure switch, and had no power there. On a whim, I checked the high pressure switch on the driver's side and saw the O ring that's supposed to seal the plug was actually in the plug. Pulled the O ring out, put the plug back together, and the compressor kicked in. I added some refrigerant, and I had a/c!

 

[Vic Sorlie]

I'm with L1tech; check refrigerant first. Go to Walmart or parts store and buy a can of R134 with built-in pressure gauge for about $35 or so. That was the fix on my former '02 with weak AC.

 

[hal keiner]

The reason I wanted to jump the low pressure switch connector was to determine if the reason for the compressor not working was in fact insufficient coolant in the system. I thought this test would do that (i.e., the compressor runs when the connector was jumped) and I thought it a good test to do before adding more R134 to the system if insufficient coolant was not the actual problem. I didn't want to overfill the system and cause more problems.



I did initially hook up an AC recharge kit (can of R134 and hose/fitting assembly with dial indicator) to the low pressure port with the motor running and the compressor not turning. The arrow of the gauge sat in the red and did not seem to move when squeezing the trigger on the refill kit/assembly for about ten seconds. It did not seem that any coolant was being transferred from the can of R134 to the AC system so I stopped. I kind of recall from prior experience recharging AC on other vehicles that the arrow does sit in this position all the time when the compressor is not running regardless of the coolant level in the system. Maybe I recall wrong.



Anyway, I jumped the low pressure switch and the compressor did NOT turn so I have the following additional questions:



- Does the compressor NOT turning while the low pressure switch is jumped indicate that insufficient coolant level is certainly NOT the problem?



- Will coolant transfer from the can of R134 to the AC system while the compressor is NOT turning? I thought the compressor has to be turning for the coolant to transfer.



- Is it possible that I just didn't pull the trigger on the refill kit long enough and that there was some coolant transferring (even if imperceptible to me) from the R134 can to the AC system? If so, should I try again for a longer time?



- How do I check the power at the compressor to test if the clutch is bad? Would I use a multi-meter to test for 12v on a particular connector/pin on or near to the compressor? If so, where is that connector?



- The AC was working fine and was cold until it suddenly stopped working. It did not gradually lose its cool indicating to me that the coolant has not gradually leaked out over a relatively long time. What clue does this provide in regards to what the problem is likely to be and likely NOT to be?





(The below picture shows the AC kit I am using)









[Broken External Image]:

 

[Richard L]

My guess is that you have a leak in the system. The leak is sufficient that it leaked out rather quickly...not a slow leak.



I never was very fond of those refrigerant recharging kits since if freon leaked out, air leaked in. That means that moisture in the air also leaked in and that can freeze and block orifice tubes and H-valves.



I always recommend that you take the vehicle to a knowledgeable AC Tech and let them fix it. Yes, it's more expensive, but the job will be done right. The leak needs to be located and repaired. Then the AC system needs to be evacuated of all air and contaminates with a vacuum pump before fresh refrigerant and compressor oil is added back into the system.



If you don't have a vacuum puimp and a R134 manifold gauge set, you are doing a half-a$$ job. And jobs done on the cheap, always end up being more expensive in the end.



...Rich

 

[Gavin Allan]

+1 what Richard L said ^.



A/C repairs done cheaply will cost so much more in the long run. You need a vacuum pump to get the system down to at least 40 micron for 30 minutes to dry out the moisture. The proper compressor oil needs to be added in the proper amount.



Buy a can of refrigerant recharge and get ready for the really big bills coming.

 

[scott degroot]

If the arrow on the gauge was in the red when you hooked it up that indicates that there was enough pressure in the system to close the switch and allow the compressor to operate. What you need to do is to plug everything back in, turn the ac on even though it doesn't operate, and then check for voltage at the connector to the ac compressor.



I will backup what Richard said, if you don't have the proper tools for servicing ac then don't do it. I get a lot of people in the shop that have tried to do their own ac work only to end up costing them more money due to improper practices and repairs that they performed.

 

[hal keiner]

I am just trying to troubleshoot what is wrong/broken. Once I reliably determine this I can decide what to do about it. I can handle replacing a blown fuse, replacing a faulty relay, correcting a bad wiring connection or similar. I don't do AC system repairs. When something like this breaks my wife tells me to just bring it in to a shop (to a pro) to be fixed and pay whatever it takes, rather than messing around with it myself. After 30+ years of marrage I still don't follow her advice.



Anyway, I'll next check the voltage at the compressor and see what that tells me.

 

[hal keiner]

I fixed my problem with the AC. For those that come across this thread trying to solve a similar AC problem, I thought I would explain how I found and solved it:



I checked for voltage at the AC compressor connector with the AC turned on at the dash and there was no voltage. I then checked for voltage at the female ports/pin outs inside the Battery Junction Box where the AC clutch relay is plugged in. I found that there was voltage at the appropriate ports/pin outs and so I suspected that perhaps the AC clutch relay is bad. I didn't know how to readily test a relay but as it happens, the horn relay located in the same Battery Junction box is exactly the same part. I switched them out and found that the horn stopped working and the AC compressor started working. For the cost of a new relay (about $13 at my local auto parts store) I got my AC working again. Yeh!