AC high pressure cut out switch needed or not?

[DetroitIron]

Hi Guys, I'm replacing my entire stock AC system with new...getting ready to start installing and the one question I can't find a definitive answer to is whether I should add a high pressure cut out switch to the back of my new compressor. My stock system has the low pressure cut out on the dryer. Thanks very much for your guidance.

[hatzie]

For R134 I would do it. It protects the compressor.
Just wire it in series between the Low Pressure Cutoff and the compressor clutch.

[gchemist]

YES!! You'll blow a hose or low pressure switch. The switches run in series. Power to the low switch, then to the high pressure switch. If one trips, then the compressor is cut off. Install it!! Ground it really good too.

[DetroitIron]

Thanks both very much...I will pick one up.

[DetroitIron]

Another related question...I got the high pressure cut out switch mounted in the back of the new compressor....the switch has two white wires about 6 inches long...where and how do I splice/connect these in to get this powered up? Thanks,

[gchemist]

Connect one wire to a good ground. Splice the other wire into the "out"/ground of the low pressure switch.

A/C 12 volt power -> low pressure switch "power" --> low pressure ground out to compressor -----> compressor out to good ground

The path is a series 12 volt power connection.

[DetroitIron]

Originally Posted by gchemist

Connect one wire to a good ground. Splice the other wire into the "out"/ground of the low pressure switch.

A/C 12 volt power -> low pressure switch "power" --> low pressure ground out to compressor -----> compressor out to good ground

The path is a series 12 volt power connection.

Thanks a lot...I see two wires coming out of the low pressure switch...any tips on how I ID the out/ground wire of the two? Thanks Again

[gchemist]

Use a volt meter. One will have 12 volts coming into it from the body harness. Wiring colors changed through the years. My 83 has power into the switch with a light green wire. The dark green wire goes to the compressor clutch connection.

[UATahoe]

Hate to revive this old thread but i am trying to figure out the same thing here but still need a little help. Would the connections be as follows:

ac power=>low pressure switch power=>low pressure switch ground runs to high pressure switch power=>and high pressure switch ground runs to a good grounding point.

Is that right??

[UATahoe]

Which line from the low pressure switch goes to the new high pressure switch?


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[UATahoe]

Here is the compressor switch. I tapped into the green wire to trigger my electric fans to come on. I am using a painless wiring electric fan kit that kicks on off of a temp sensor or when the ac is turned on.



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[Sabaka454]

You don't need a high pressure switch. Most compressors have a high pressure relief valve if the pressure gets too high. If you don't have one just pick one up and install it. Help keep the clutter under the hood neatened up as well. Is there a specific reason you want to install the high pressure switch?

[Sabaka454]

But if you want to ...

[UATahoe]

Well I was under that impression as well that I didn't need one and then saw where others were saying I do need one. I am switching from the r4 compressor to the ht6 style found in the 96 and later trucks. I just want to make sure I hook it up safely and get it to work correctly.
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[gchemist]

It is possible to run a system without a high pressure switch. Only in a cool climate. Any warm and hot climate will build too much pressure. Sure the compressor valve will relieve pressure at a cost. The system will lose freon. Install the switch for operation.

[UATahoe]

Chemist, help me out with the wiring. The dark green wire runs from the low pressure switch to the compressor. If I install a high pressure switch it has 2 wires. I am assuming I run the dark green wire from the low pressure switch into the high pressure switch. The second wire from the high pressure switch then runs to the compressor clutch connector (same way it did originally from the low pressure switch) and then compressor to ground (mine is actually run to the back of the alternator currently). Is this correct?
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[hatzie]

Originally Posted by UATahoe

Chemist, help me out with the wiring. The dark green wire runs from the low pressure switch to the compressor. If I install a high pressure switch it has 2 wires. I am assuming I run the dark green wire from the low pressure switch into the high pressure switch. The second wire from the high pressure switch then runs to the compressor clutch connector (same way it did originally from the low pressure switch) and then compressor to ground (mine is actually run to the back of the alternator currently). Is this correct?
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You got it. That's how you do it.

[UATahoe]

Sweet! Thanks for the help! I am going to wire it up tomorrow.
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[Sabaka454]

Originally Posted by gchemist

It is possible to run a system without a high pressure switch. Only in a cool climate. Any warm and hot climate will build too much pressure. Sure the compressor valve will relieve pressure at a cost. The system will lose freon. Install the switch for operation.

I've never run a high pressure switch and they didn't come that way from the factory either. I've never had a problem or seen a problem due to the lack of a high pressure switch. If your system pressure's truly get high enough to where the system vents you have bigger fish to fry. Typically inadequate airflow at the condenser or blockage.

I'm not saying there's no point to having additional failsafe's but really just make sure the system is properly charged, no blockage at the condenser with adequate air flow and you'll be just fine/

"Any warm and hot climate will build too much pressure" Don't you run the AC in warm/hot climate?

[hatzie]

Alabama qualifies as a HOT climate New England does not. R134 runs higher pressures than hydrocarbon blend or R12 refrigerants. The R134 system on my 2000 GMC C2500 T400 chassis has a high pressure cutoff switch. If GM decided it was necessary on the factory R134 system it should be installed on a retrofit.

On my 2000 GMC 2500 T400 chassis the switch on the accumulator is the LPCO and the switch on the compressor is the HPCO. I mimicked that setup on the '98 6.5L with its' H series compressor that I installed in my '76. I put in a parallel flow condenser, variable orifice valve and late 80's accumulator/evaporator. I have yet to see high pressure cutout... but #1 I was extremely careful about charging using gauges and thermometers not assuming using pounds of refrigerant. #2 I was in Cincinnati and am now in Northern New England so summer isn't as rough as the Deep South. On the 76 My high side pressure is reasonable at 90°F ambient temp and My vent temps are 40-45°F.

Make sure the HPCO is on the high side and LPCO is on the low side.

[UATahoe]

Hatzie, not sure I follow the very last comment. If I install like mentioned I am good right?
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[hatzie]

Originally Posted by UATahoe

Hatzie, not sure I follow the very last comment. If I install like mentioned I am good right?
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Should be. The H series compressors have a HPCO on the high side and the stock squares have LPCO in the right place so...

[UATahoe]

Now I confused myself. Is the back post on the alternator a groundig point or power out? After looking, it seems like it is power not ground. So that would mean the ac compressor clutch gets power from the back of the alternator? And the green wire from the low pressure switch serves as the ground?
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[hatzie]

The big alternator post on the back is power out (hot) to the battery. It's usually marked BATT.

The light green wire is hot when AC is selected and maybe defrost. It runs from the heater/AC controls to the low pressure cutoff switch. If the AC is charged then the LPCO switch is closed and power runs through it to the dark green wire that's directly tied to the compressor clutch.
I believe the H series compressor clutch has a short ground wire that runs to the engine or accessory brackets. There are no switches in the ground wire and it shouldn't be tied to the alternator BATT terminal.

[UATahoe]

Ok yeah I am an idiot. Death by my own labeling. The ground wires were grounde to the alternator brackets. Not the alternator. That would have been a bad mistake. Lol
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