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Old 06-11-2007, 09:56 AM   #1 (permalink)
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Default p38 air conditioning

Hi Guys I know this has been done to death but I still have not got all the answers I need.

I have a couple of issues with my Hevac and air con. Firstly the air con compressor will not run. This is because it is not getting any feed. If I hot wire it then it runs fine and ice cold air comes out. I have read and believe it could be to do with the pressure switch. The system was re gassed a few weeks ago and in order to fill to the correct weight the engineer hot wired the compressor.

I have just had a diagnostics done and told there was no voltage to the compressor. So reading many article relating to this I feel it could be the pressure switch. If so is there a way to find out if this is working or not. If it is not working can I replace it with a new one without affecting the gas or do I need to re gas after?

Could this be a relay? Where can I test for a voltage to th air pressure switch.

Thanks for any Help.

Paul
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Old 06-11-2007, 07:27 PM   #2 (permalink)
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There is the possibility that the pressure could be malfunctioning, but there is also the possibility that it is functioning. Whenever there is an issue with the switch "not working", it could always be that the expansion valve is letting the high side bleed over and actually the switch is doing its job. That is what the switch is actually for. However, since an AC tech regassed the system, they probably put a manifold gauge to it.

The three way switch is: one turns the compressor on, one activates the condensor fans at low speed, and one doubles the speed of the fans.
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Old 06-13-2007, 09:45 AM   #3 (permalink)
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I have the same problem and could never figure it out. But from what I've read in the manual and on here, the Becm may be the culprit. Let me know if you find the problem though.
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Old 06-14-2007, 01:42 AM   #4 (permalink)
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Hi Im not sure about the becm this was replaced recently. To cure some other problems. Had this problem with A/C before and since replacement. When I had diagnostics the report only showed no voltage at compressor cost 80 to know what I already did.

I would like to know where to test the voltage to the pressure switch and after. also what pins do what on that switch. I did short them out to get the fans working but nothing I did made the compressor come on. Where would this be from.


PAul
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Old 06-14-2007, 08:24 PM   #5 (permalink)
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OK, your problem could certainly be the switch. Here in the US, it is a relatively inexpensive part. If you do replace it, you would do well to replace the receiver dryer as well. I know that isn't fun to hear after having it recharged, but that would be your best bet. The other problem you could be having is your HVAC control unit. When I have seen those bad, they did more things wrong than not cut the compressor on. Good luck.
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Old 07-11-2010, 04:19 PM   #6 (permalink)
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did we ever figure out what was causing the proble? my car is doiing the same thing and i would love to know.
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Old 07-14-2010, 05:19 PM   #7 (permalink)
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Talking A/C Repair?

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did we ever figure out what was causing the proble? my car is doiing the same thing and i would love to know.
bsbllfit7 and p38paul,

Do you still need some guidence with your vehicle? If so let me know what years your vehicles are and I'll provide some help. ibis1
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Old 07-14-2010, 09:37 PM   #8 (permalink)
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yes i do its a 1997 range rover 4.6 hse
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Old 07-15-2010, 10:34 AM   #9 (permalink)
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Talking A/C diagnoses

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yes i do its a 1997 range rover 4.6 hse
bsbllfit7,



I am going on the assumption that your compressor is not turning on. If that is not the case, please let me know. Bear in mind there are many things that can prevent your compressor from coming on. We need to eliminate them one at a time starting with the simplest ones first. I'm going on the assumption that you have checked all possible fuses. To run these tests the compressor has to be in the non functional mode (not working) (1) Turn your A/C on (engine running) If you have a "feeler gauge" (not brass) hold it close to the front of the compressor to see if it gets pulled toward the clutch, this indicates that the clutch is receiving power, but not engaging the compressor. If this is the case, you may need a relay installed or worst case a new clutch assy. Years ago there was a TIB for a relay kit that we installed at the dealers. Yours may have already been updated. If you did not have power in step one, we move to step two. Please remember to be careful around moving parts and belts! (2) Trinary switch bypass. Locate the trinary switch. (by receiver-drier) You need to get under the vehicle (drivers side NAS spec) Behind the bumper at the top of the receiver-drier you will see two switches. One 2 wire and one 4 wire. Remove the 4 wire connector. You should have a black/yellow, white/light green, black and black/blue wires. Connect the first two b/y and w/lg using a jumper wire. Do not worry about the other two at this time. If your compressor runs at this time, you have one of two things causing this. (1) No refrigerant or (2) A faulty trinary switch or connection to it. We will stop here. Check these things and report back your findings. ibis1
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Old 07-15-2010, 08:30 PM   #10 (permalink)
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cars ac runs fine when i give it a direct 12v source from the batter, car blows cold as ice. thats whats making me wonder what it is because the compressor engages and the low side port says it has the proper amount of refrigerant so i am so lost on what it could be, i heard the pressure switch but also heard hevac controls can do this
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Old 07-16-2010, 07:08 PM   #11 (permalink)
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Similar problem last week, turned out to be the trinary switch. Local LR shop replaced it and no probs since!
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Old 07-16-2010, 07:19 PM   #12 (permalink)
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right on, where is that located, ill do it myself haha
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Old 08-14-2010, 10:31 AM   #13 (permalink)
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Sorry for the delay. Located Driver's side in front of the dryer. There are two in the area, you need the one in front of the dryer.
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Old 08-14-2010, 10:40 AM   #14 (permalink)
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turned out being a tiny little shim of a washer between the clutch and ac compressor. Took the clutch off pulled the washer put it back on and voola! I guess it wasnt getting a close enough clearance so the voltage wasnt really hitting hard and when i removed that it turns on every time like a charm!
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Old 09-29-2010, 08:51 AM   #15 (permalink)
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Default same ; compressor doesn't come on, low side high

Ibis, I have same problem, 1998 4.0... I can figure it out based on what you said previously, but a schematic would be nice....

Possibly related, my bigger problem is if I hot wire the compressor, my low side is 100 psi, high side 125.

First: I found a pin hole in high side line at firewall. Wet foam Pad had been in contact with bare aluminum hose = corrosion on line, causing hole. Removed line, welded hole, cleaned, reinstalled, Pulled vaccuum for 45 min, no leaks. Added 2 oz. oil, almost 2 cans of 134a... System won't take any more 134. Compressor wouldn't turn on; hot wired it, got above readings. Figured bad compressor... Changed w/ compressor from my '97 which was a pressurized system (parts car - I don't know if it worked, only that there was full charge on system)

Anyway, same readings....high side actually goes down 5 psi when compressor first kicks on.... then goes up to 125psi or so...only 18 oz of 134a, but low side goes past 125 psi if I heat the can of 134a to add more. Sounds like compressor? or expansion valve to me...Stumped, please help... Gauges are harbor freight, but are hooked up correctly, charging from low side only w/ engine running, compressor on.... full charge is 44 oz?

please help!
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