No Power to A/C

[JCrank]

Has anyone had a problem where they are not getting power to the a/c compressor ? I installed my new compressor last night after flushing the system and installing a new drier and expansion valve. Pulled a vacuum for an hour and when preparing to charge the system I found that there is no power getting to the compressor. Any suggestions ?

Thanks,

Jon

 

[Greenleaf]

Yes. I encountered the same problem but never figured it out, quite... ... ... .

It was winter and I noticed that in defrost mode, the clutch would not engage :{ I fooled around with it, tested EVERY component including the computer and found that there was no signal from the computer to the relay. So, anyway, come spring it all began to work again



The FSM has a nice diagram of what the system is all about in terms of electrical function. All I can recall is the brown wire at the computer has a major part in the system.



The relay sends power to the clutch, which is controlled by the computer, controlled by the pressure switch, contolled my the temp switch, controlled by the A/C selection/function switch. If you jump the relay you can get the clutch to engage if the clutch and wires are good.



I'll shut up now and simply say you need the manual



GL

 

[PToombs]

If you had a vacuum on it the low pressure switch cut out the power. Draw a vacuum, let that suck in some freon,then the switch should trip on the compressor. If not, power the 'presser with a jumper wire until it runs on its own. I think the switch cuts out at 20-25 psi.

PS by the time you fix it, it will be cool again :-laf

 

[JCrank]

PT,

You are probably right about it being cool again by the time I get things operational... .



GL,

I have several service manuals but not a FSM... . I seen a pretty good schematic in two of them, but none of my manuals tell me which pins on the PCM plug are for what.



I was think if the problem is at the PCM, there might be a way to bypass the PCM like we do with the external voltage regulators to solve charging problems...

Thanks you all !!

 

[Greenleaf]

Exactly!! That's what I WAS going to do. When I get home I may look-up the location of the wire for you @ the ECM





Scott

 

[JCrank]

Scott,

Did you have a chance to take a lookat the FSM about PCM pin location for the a/c ?



Thanks,

Jon

 

[Greenleaf]

When I said "git home" I ment "drive home from Southern VA"... ... ... ... . not drive home from the bus garage. I had been using Tugboats computer.



The input to the computer for a/c function is the a/c switch on the inst. panel. You simply are grounding the wire by opeating the switch. You cannot ground the wire (activate the switch) unless the a/c pressure switch is closed. (has to have pressure) and the clutch cycling switch has power and see's a need for the pump to circulate freon. (It has a sensor bulb that senses the temp of the line it's attatched to)



The input control wire is pin # 27 brown wire.



The other trminal at the computer is # 34 dark blue/orange it goes to the wide open throttle cut-out relay. It's also a ground wire. Will activate the relay. The relay has four terminals used. #85 is the signal ground, # 86 is signal power (which is grounded by way of # 85) then #30 is battery voltage and #87 is power to a load (in this case the a/c clutch.





There is an electronic clutch cycling switch that takes control over the manual a/c switch. The brown wire that is at the computer goes to this control switch. The middle wire on the clutch cycling switch is battery power (dark blue) and the third wire at the cycling switch is the contol wire (ground) that goes through the pressure switch and to the manual a/c selector switch on the inst. panel. This wire is dark blue/orange.



Clear as mud, right?



You can manually activate the the clutch by grounding the proper wire or wires and/or jumping the cycling switch and pressure switch.



-S

 

[JCrank]

Clearer than mud. Thanks for looking things up, I really appreciate it.

Jon

 

[JCrank]

Done a little checking last night. It seems the problem is that I have lost one of the grounds for the system, maybe the dash switches aren't working (they provide a ground when pushed ? ). I used a jumper wire from the battery negative to the relay (blue/orange wire in relay plug) and activated the compressor clutch with this ground. I was then able to charge my system. The only problem seemed to be that I also bypassed the pressure switches in the system.

 

[Greenleaf]

Yes. That would bypass the cpmputer and the switches. The dash switch is grounding the circuit.



-S

 

[JCrank]

I think my next step is going to be pulling the switch panel out of the dash and see what I have there..... such as checking to see if I'm creating a ground when depressing the switches.

Jon

 

[Greenleaf]

I checked my (dash) switches by testing them at the pressure switch, to see if they were grounding.



-S

 

[JCrank]

I'm think you done this by checking the continuity of the circuit ?

J

 

[B.G. Smith]

I'm think you done this by checking the continuity of the circuit ?

J

Disconnect the connector at the pressure switch on the receiver/dryer and check for voltage with the AC switch on, one terminal should be hot. If you have 12 volts on one terminal and refrigerant in the system then the switch is probably bad. The switch is screwed onto a schrader valve and can be replaced without discharging the system. bg

 

[Greenleaf]

I don't know if there is a valve behind the switch, but it's connected onto the expansion valve right up tight against the fire wall.



I have made varoius checks with continuity and voltage. I don't recall where I used which method. I generally test the circuit wires with a Ohm meter.



-S

 

[JCrank]

With B. G. speaking of the pressure switch, I'm wondering now if my problem could be coming from there. When installing the new compressor, I also installed a new expansion valve and drier. The old expansion valve had the switch screwed in on the side facing the fender. The new valve had the switch on the opposite side, causing me to be unable to install the switch with the wire connector & boot because of lack of clearance. I made jumper wires and was able to make the connection this way. I do know that I have no voltage to either wire connecting to the pressure switch whether checking at my jumper wires or the original connector.

Jon

 

[Greenleaf]

Power is "comming" from the clutch cycling switch I thought(?) Then through the pressure switch then to the dash panel switches. The cycling switch has battery voltage too it right? Isn't that what I told you? Can't remember. Check your clutch cycling switch. There is a method in the FSM that gives directions. I think it has battery voltage to it. I don't think the computer's brown wire supplies the voltage. It's ground signal only. Now I'm getting confused... ... ..... I'd have to look at the book again.





-S

 

[JCrank]

The only place I can get battery voltage is at the relay on the left fenderwell. No voltage at the compressor connector or at the pressure switch connector. Didn't have a chance to work on it yesterday evening, maybe the weekend.

 

[B.G. Smith]

I'm not sure when they switched over from expansion valves to capillary tubes?? If the switch is mounted on the expansion valve, it is a low pressure cut out only, not a CCS. It,s function is to prevent the compressor from running and self destructing when the charge is too low. If it is open it will prevent the compressor from operating at all. Also, I don't believe the computer will power up the AC circuit until the engine is running so your tests need to be with engine running. bg

 

[Greenleaf]

The CCS is mounted next to the expansion valve. Has three wires. I don't recall which wire does what. I posted that on the front (first) page a few days ago. It has the capilary tube mounted to it and goed into the suction line to sense the line temp.



-S