2nd Gen Non-Engine/Transmission AC Clutch Always Engaged?

[Bunyan]

Any thoughts? My ac compessor clutch stays engaged continuously, while control is set to on max or regular. Air temp on max, 45 degrees, on regular ac 62 degrees. Any help appreciated. I have dye in the system and cannot find any leaks. Bunyan

 

[Bob4x4]

In hot weather that can be normal

 

[HBarlow]

Doesn't the ac compressor remain engaged 100% of the time when the unit is in the MAX Cooling mode?

 

[lil red cummins]

Bunyan

Have you connected a set of gauges to the system while it is running?

you need to see what pressures the system is running at. this will give you a good idea of what is going on.

it is possible, based on the temps you mentioned, that the system is low on freon and the gauges would be able to indicate that.

Jim

 

[Bunyan]

I don't have a set of gauges, wish I did. I can't remember, but I think you may be right about the clutch remaining engaged while on max. It's been 100-105 here in central Tx for a few weeks now. I know with the ac on, not max that the clutch had been cycling. Thanks guys for helping. I'll see if I can get some gauges. Bunyan

 

[RCorbeil]

Is the control set to defrost, that will keep the clutch engaged if the system is somewhat low on charge.



Ron

 

[B.G. Smith]

Your truck has a clutch cycling switch which is mounted in the side of the receiver/dryer. The switch should open (stop compressor) at about 23# and close (start compressor) at about 43#. It may be doing what it is supposed to do in this weather and it may be cycling when you are driving. I would say you are definitely not low on refrigerant. You will need a set of gages to determine if the cycling switch is working, the engine needs to be turning at 1000 rpm to check the operation of unit. If the cycling switch is inoperative, it can be replaced without evacuating the system, it is screwed into a schrader type fitting. bg

 

[fest3er]

Doesn't the ac compressor remain engaged 100% of the time when the unit is in the MAX Cooling mode?

No. On MAX, the recirc door is opened. Power to the A/C clutch goes from the battery, to fuse, to in-cab control, to PCM, to low-pressure cutoff switch, to high-pressure cutoff switch, to the clutch. Any one of the items between the battery and the clutch can disable the clutch.



Normally when the clutch is engaged, the compressor draws down the low pressure side. If not enough coolant flows from the high pressure side through the evaporator, then the low-side pressure can drop below about 23 PSI. At this point, the low-pressure switch opens and disables the clutch. When the low-side pressure rises to about 45 PSI, the low-pressure switch closes and enables the clutch. Ad infinitum.



If the compressor never cycles off and on, then there are two possibilities. The first is that the system has the exact amount of R134A that keeps the low pressure side between the low limit and the high limit; this may be true only in specific temp/humidity ranges.



The second is that the low-pressure switch is faulty.



A cheap parts store low pressure A/C gauge will show if this aspect of the system is functioning properly.



OK, there's a third possibility: the wiring was modified to bypass the PCM and low-pressure switch, and possibly the high-pressure switch.

 

[PToombs]

Easy test, unplug the compressor and run the truck. :-laf

 

[Bunyan]

No. On MAX, the recirc door is opened. Power to the A/C clutch goes from the battery, to fuse, to in-cab control, to PCM, to low-pressure cutoff switch, to high-pressure cutoff switch, to the clutch. Any one of the items between the battery and the clutch can disable the clutch.



Normally when the clutch is engaged, the compressor draws down the low pressure side. If not enough coolant flows from the high pressure side through the evaporator, then the low-side pressure can drop below about 23 PSI. At this point, the low-pressure switch opens and disables the clutch. When the low-side pressure rises to about 45 PSI, the low-pressure switch closes and enables the clutch. Ad infinitum.



If the compressor never cycles off and on, then there are two possibilities. The first is that the system has the exact amount of R134A that keeps the low pressure side between the low limit and the high limit; this may be true only in specific temp/humidity ranges.



The second is that the low-pressure switch is faulty.



A cheap parts store low pressure A/C gauge will show if this aspect of the system is functioning properly.



OK, there's a third possibility: the wiring was modified to bypass the PCM and low-pressure switch, and possibly the high-pressure switch.

Sounds good. The third would not happen here. Anything that needs replaced gets it. I've seen what can happen when altering the wiring. Once I saw I corvette melt to the ground. Thanks for everyones help. Bunyan

 

[fest3er]

Easy test, unplug the compressor and run the truck. :-laf

Don't laugh! It's a valid fourth possibility I overlooked: is the clutch seized?

 

[Bunyan]

Don't laugh! It's a valid fourth possibility I overlooked: is the clutch seized?

No. When you turn the ac control switch off it will disengage. Only when the ac is turned on the clutch engages, but does not cycle.

 

[fest3er]

No. When you turn the ac control switch off it will disengage. Only when the ac is turned on the clutch engages, but does not cycle.

Ah, good. You've finished 25% of the analysis.

 

[E7mack96]

ok, does this system not have a relay somewhere supplying power to the ac clutch circuit?

 

[fest3er]

ok, does this system not have a relay somewhere supplying power to the ac clutch circuit?

Now that you mention it, it does, in the PDC, but since the compressor works when turned on and stops when turned off, the relay's working OK. It's that the compressor doesn't cycle on and off. My apologies for steering y'all wrong.



I did describe the wiring backward. In a stock system, the PCM controls whether the clutch engages or not based on its 'sense' inputs from the A/C control switch (both directly and through the low- and high-pressure switches). The control switch closes a circuit to ground. If A/C is called for and the pedal ain't to the metal, the PCM activates the relay, thus engaging the clutch. If either pressure switch opens, the one sense input is no longer grounds and the PCM deactivates the relay. If the control is turned to a non-A/C position, that sense input is no longer grounded and the PCM deactivates the clutch.



The A/C shop rewired my truck years ago, taking the PCM out of the loop. The control/pressure switch circuit now directly controls the clutch relay, reducing the complexity of the system. The A/C suddenly stopped working one day; I guess the PCM went on strike. A scabbed-in wire bypassed the PCM.



So another possibility is that the PCM is no longer responding to changes in the pressure-switch circuit. I'd sooner believe the low-pressure switch just isn't working. Unplug the connector from the low pressure switch (near the turbo and the A/C dryer); this will open the circuit. If the compressor disengages, unscrew the switch from the system and plug it back in, if the compressor engages, it's definitely the pressure switch.

 

[Bill Stockard]

In the discussions above about functions of the different A/C compressor controls, there has been no mention of the Evaporator Fin Sensor, aka the evaporator temperature sensor and is part of the system that controls the compressor clutch.



Bill

 

[Bunyan]

I'll be testing the switches today. System is charged and pressures are correct. Hoping it's just the low pressure switch. Now the clutch will disengage when applying heavy throttle. It may be that the 102-106 here in Texas is the culprit. Some mornings its as warm as 90 at 10:30 AM. The machanic who tested the system said customers have been coming in daily complaining the ac is not cooling. After testing each system, he said it's most likely the heat. (A very few were low on freon. ) I keep everyone updated. Thanks again for the help. Bunyan



ps: I think yesterday was the twenty-seventh or eighth day above 100.

 

[fest3er]

In the discussions above about functions of the different A/C compressor controls, there has been no mention of the Evaporator Fin Sensor, aka the evaporator temperature sensor and is part of the system that controls the compressor clutch.



Bill

I didn't include it because my SM makes no mention of said sensor. Did '98 and earlier pickups have an evap temp sensor?