Run AC with a toggle switch?

[PlayingWithTBI]

If I do this, below is what I plan to order. It looks like the oem low pressure switch shuts off at 22 psi. I haven't found the high pressure switch yet - I assume this would go on a port on the back of the compressor?

What do you think?

Your link goes to my shopping cart which is empty but, the LPS looks good. The HPS should be somewhere around on ,200 PSI and off >430 PSI or something like that and, it goes on the back of an R4 compressor, the newer ones may have it in the line somewhere, YMMV

 

[1998_K1500_Sub]

Thanks. I will look into that if I do this. I can't decide as it sure is a nice, simple set up with no AC stuff in the way!

Simple, yes, but you'll likely kill the compressor.

Oil circulates with refrigerant and the refrigerant flow (and hence oil flow to lubricate the compressor) won't always be sufficient if the compressor's running constantly, particularly if the evaporator's frozen-up. Thus, the low-pressure "compressor cycling switch" is essentially mandatory for compressor life.

The high-pressure cut-out switch is almost as important as its low-pressure counterpart, but for different reasons. The HP cutout switch is mounted on the HT6 compressor. RockAuto should have them.

Use them both.

There's also another high-pressure switch mounted on the hose assembly, very near the compressor. All I've known this switch to do is force the AC system into recirc mode and turn on the aux radiator electric fan (if equipped).

Compressor engagement when in "Defrost" mode can be eliminated / disabled with a simple "snip" of a conductor in the dash-mounted control head.

The ECU has the ability turn off the compressor to protect if from overspeed, e.g., when the engine's at high RPM, and the engine RPM at which the compressor is disabled / re-enabled is tunable. IIRC, the crank pulley is 8" and the compressor's is 4.4", so the compressor sees an 1.8:1 overdrive. I don't know the HT6's max continuous RPM rating, but the Sanden replacement for the HT6 (p/n #4440) has a max continuous RPM rating of 6000RPM (from here https://www.sanden.com/productlibrary/schematics/4440.pdf) which corresponds to 3300 engine RPM. Sanden also quotes a "max allowable" RPM of 8000 (4400 engine RPM) but they don't say how long it can be tolerated. I personally have tuned mine to disengage at 6000 compressor RPM.

(edited May21) I made a mistake above. The Sanden #4440 has a 130mm (5.1") diameter pulley. I believe the stock HT6 on the GMT400 is the same. So, my numbers above should be ~1.56 compressor overdrive, with a compressor RPM of 6000 when engine RPM is ~3800, and compressor RPM of 8000 when engine RPM is ~5100.

 

[1998_K1500_Sub]

You won't have to worry about it freezing either since your pressure will go up as it starts to freeze so, your high pressure shutoff switch with start cycling it automatically. Given everything else is correct - proper charge, air flow, etc.

@PlayingWithTBI, I respect your comments but I (might) disagree this (one) time. Read this and comment.

IMHO when the evap starts to freeze the low-pressure side will see sustained low pressure (<25psi corresponding to an evaporator temperature of <32degF for R134a), but I don't believe (and do not yet see any reason) for the high-pressure side to rise provided the system isn't overcharged.

I actually believe (but haven't checked in practice, so... ignorance) the high-side pressures would be typical or perhaps lower since:

- the refrigerant flow in the system will be very low (as little refrigerant is evaporating in the frozen evaporator) and so

- the liquid in the high-pressure side will be resident in the condenser for an atypically longer period of time, so thus atypically cooler (closer to ambient temp anyway), so thus at atypically lower pressure.

 

[PlayingWithTBI]

- the refrigerant flow in the system will be very low (as little refrigerant is evaporating in the frozen evaporator) and so

- the liquid in the high-pressure side will be resident in the condenser for an atypically longer period of time, so thus atypically cooler (closer to ambient temp anyway), so thus at atypically lower pressure.

Yes, it'll be cooler coming out of the condenser, but I'm thinking, extremely hot (and consequently higher pressure) exiting the compressor since it's not moving enough cooler refrigerant, from the evaporator, through it.

 

[letitsnow]

Your link goes to my shopping cart which is empty but, the LPS looks good. The HPS should be somewhere around on ,200 PSI and off >430 PSI or something like that and, it goes on the back of an R4 compressor, the newer ones may have it in the line somewhere, YMMV

Sorry - I can't figure out how to link it to here. I'll figure it out and try again...

 

[1998_K1500_Sub]

Yes, it'll be cooler coming out of the condenser, but I'm thinking, extremely hot (and consequently higher pressure) exiting the compressor since it's not moving enough cooler refrigerant, from the evaporator, through it.

I think what you're saying here is the refrigerant (which also functions to remove heat from the compressor, that's known) may have a atypically high exit temp from the compressor on account of the heat removed from the compressor and the relatively low refrigerant flow. I can see that, but I would suspect the long residency in the condenser would keep the high-side pressures atypically low regardless (since the pressure in the high-side between the compressor and the orifice tube is the same throughout... as it has to be, as there's no restriction in that part of the system to cause a pressure difference).

Well, I'm shooting from the hip here. I don't know everything about AC systems, but I just wanted to comment that I didn't envision the high-side pressures becoming excessive during a frozen evaporator situation.

If, of course, the system's overcharged, then there can be excessive pressure in the high-side because all of the refrigerant's been evacuated from the low-side and stuffed (literally) into the high side... and if there's too much refrigerant / too little volume on the high-side, well... one can't put 10# of crap in a 5# pail and pressures go through the roof.

I'll go back into my hole now.

 

[PlayingWithTBI]

I think what you're saying here is the refrigerant (which also functions to remove heat from the compressor, that's known) may have a atypically high exit temp from the compressor on account of the heat removed from the compressor and the relatively low refrigerant flow. I can see that, but I would suspect the long residency in the condenser would keep the high-side pressures atypically low regardless (since the pressure in the high-side between the compressor and the orifice tube is the same throughout... as it has to be, as there's no restriction in that part of the system to cause a pressure difference).

Keep in mind where the High Pressure Shutoff switch is located (normally), in the back of the compressor where it sees the highest pressure in the system so, it'll start short cycling which will, in turn, help warm up the evaporator too.

We're talking about compressing a gas into a liquid, in a perfect world but, there's still compressible gasses in there. If the evaporator is restricted due to freezing, the "high side" will build more pressure until flow throughout can be somewhat normalized. Once it stops/slows down flowing, it's gonna take more pressure to get it moving again.

If, of course, the system's overcharged, then there can be excessive pressure in the high-side because all of the refrigerant's been evacuated from the low-side and stuffed (literally) into the high side... and if there's too much refrigerant / too little volume on the high-side, well... one can't put 10# of crap in a 5# pail and pressures go through the roof.

Yepper, been there done that, especially with R134A.

I'll go back into my hole now.

Hey, don't go away, discussions like these are educational for all concerned, I'm learning and enjoying it all the time

 

[letitsnow]

I have learned a bunch from all of you - thanks for joining in.

I still can't decide if I want A/C back or not. My truck/camper set up is balanced really well right now. I just can't decide if the power loss/complexity is worth having the A/C, as it would only be there to eliminate the noise of driving with the windows down in this truck. If the windows are up, then my wife might take that opportunity to talk. You have to consider all things that can be affected...

 

[1998_K1500_Sub]

You have to consider all things that can be affected...

Umm, isn't this called "Be careful what you wish for"?

 

[L31MaxExpress]

Simple, yes, but you'll likely kill the compressor.

Oil circulates with refrigerant and the refrigerant flow (and hence oil flow to lubricate the compressor) won't always be sufficient if the compressor's running constantly, particularly if the evaporator's frozen-up. Thus, the low-pressure "compressor cycling switch" is essentially mandatory for compressor life.

The high-pressure cut-out switch is almost as important as its low-pressure counterpart, but for different reasons. The HP cutout switch is mounted on the HT6 compressor. RockAuto should have them.

Use them both.

There's also another high-pressure switch mounted on the hose assembly, very near the compressor. All I've known this switch to do is force the AC system into recirc mode and turn on the aux radiator electric fan (if equipped).

Compressor engagement when in "Defrost" mode can be eliminated / disabled with a simple "snip" of a conductor in the dash-mounted control head.

The ECU has the ability turn off the compressor to protect if from overspeed, e.g., when the engine's at high RPM, and the engine RPM at which the compressor is disabled / re-enabled is tunable. IIRC, the crank pulley is 8" and the compressor's is 4.4", so the compressor sees an 1.8:1 overdrive. I don't know the HT6's max continuous RPM rating, but the Sanden replacement for the HT6 (p/n #4440) has a max continuous RPM rating of 6000RPM (from here https://www.sanden.com/productlibrary/schematics/4440.pdf) which corresponds to 3300 engine RPM. Sanden also quotes a "max allowable" RPM of 8000 (4400 engine RPM) but they don't say how long it can be tolerated. I personally have tuned mine to disengage at 6000 compressor RPM.

Sanden markets that compressor for the vans too as a drop-in. Vans have an 8" crank pulley and the 4261 has an 4.11" compressor pulley. Factory PCM shuts the compressor off at 4,500 crankshaft rpm or WOT. I have a March 25% underdrive crank pulley that is 6" diameter. With the 6" crank and 4.11" compressor pulley it has a 1.45 drive ratio and the engine can turn nearly 4,800 rpm before the compressor exceeds 7,000 rpm. I left the shut-off at the factory 4,500 rpm.

Pressure switches are a must. My van was built with a compressor mounted combination high side low and high pressure cutout and an accumulator mounted low pressure cycling switch.

The high side pressure switch needs to be there in hot weather with a heat soaked interior. When the inside of the vehicle is starting out over 120°F and sometimes as much as 150°F, it drives the head pressure stupid high even with the windows down.