A/C hi/low pressure cut off switch mod?

[911st]

I am about to have my A/C repaired and I am interested in what a "hi/low pressure cut off switch" is?

Where it is installed?

Is there a kit for my 85 911?

Or, how do I achive this?

Than you in advance.

[mthomas58]

Required on all 134a systems. I don't recall the typicall high and low cutoff pressures, but the switch will cut power to the compressor clutch if pressure is too low (low on coolant such as with a leak - A/C oil will not circulate properly leading to possible compressor failure) or too high when a hose or other componant may burst or spring a leak.

The switches are usually located on the high side line between the compressor and first condenser. However, some systems have the high/low switch on the receiver/dryer.

[mthomas58]

This one cuts out (opens) at 28 PSI and at 384 PSI then closes at 256 - 340 PSI

http://www.ackits.com/c/conversionitems/R134a+Conversion+Items.html

[GH85Carrera]

It is something that makes a lot of sense. It will turn off your compressor if the pressure is so low that there is not sufficient refrigerant and lubrication to keep the compressor from eating itself. It will also shut off the compressor if some problem causes the pressure to go so high that the seals could blow. It usually goes in the hose from the high side of the compressor to the rear condenser.

As I understand it is required for a change to R-134a. It comes as part of the system if you upgrade all your hoses to barrier hoses which is another good idea. When I upgraded my A/C system the switch was just part of the system. I feel better knowing I have one in my system.

[911pcars]

This is a best attempt; not definitive.

A/C pressure switches are installed in the high pressure side of the system, between the compressor and the receiver/drier.

The objective is to control the compressor according to system pressure and avoid/minimize compressor damage. The switch shuts the compressor OFF in case system pressure is too low (indicating low refrigerant level and/or leak) or if system pressure is too high (expansion valve stuck shut or other blockage in the high side of the system).

A mid-pressure setting conforming to a "normal" system pressure range allows the compressor to energize and circulate refrigerant.

As for specifics and installation tips, that's a DIY project for your particular system. Check your system specs with current vehicles that use a pressure switch. Denso provides many A/C components in current vehicles (domestic and foreign).

Hope this helps on an elemental level somewhat.

Sherwood

[dentist90]

Why did R12 systems not have pressure cutoff switches? Are they not subject to the same problems of high/low pressure as 134a?

My system has had a 134 conversion, or more likely a partial conversion. The high/low side connectors have been replaced, the old oil drained out and R/D and expansion valve replaced. Whomever did it did not include a pressure cutout switch. I would like to install one before the next recharge, but 375psi for high cutoff sounds pretty high. What operating pressure ranges should one be looking for with 134a? Should the system take more or less 134 than R12, or should my mechanic just go by high/low pressures as he adds more 134?

[911st]

On a Porsche 911 where dose it get plumbed in?

Dose it go hear the compressor for ease of wiring?

Is there an existing port to add it to?

[911pcars]

While this project can be a DIY project, I think you'll need more background on A/C than what can be offered here. Not to minimize the information you'll receive from forum regulars; someone might come up with Plan A that outlines the exact procedure for you, but you should have a fairly basic knowledge of the vehicle system you plan to modify. Doing it by the numbers is one approach, but only if every contingency is accounted for.

I would enlist the aid of some basic texts on A/C theory and service as well as an enthusiastic and experienced Porcshe/AC repair shop to assist.

As a start, please review the responses/answers related to your first and last posts from the top, again.

Sherwood

[GH85Carrera]

Originally Posted by 911st

On a Porsche 911 where dose it get plumbed in?

Dose it go hear the compressor for ease of wiring?

Is there an existing port to add it to?

On my system I changed all the hoses. The hose that goes from the compressor to the rear deck condenser had the place to screw on the switch. Your local A/C shop can probably fabricate something for you. If you want your A/C system to really work search the site for information on how other guys have done the upgrades. A/C work is not that difficult but it does require some special tools and a understanding of how A/C works.

[911st]

Thanks guys.

Sounds like this switch is mostly a safety item that can work w old or new gases.

Dose not seem that is would add anything to performance.

[jeff1hughes]

Here's my question: how do you wire a "new" pressure switch in so that it DOES cut off the compressor? Do you need to run the power lead instead of into the clutch into the switch and then go back to the clutch?

[Scott98]

Originally Posted by dentist90

Why did R12 systems not have pressure cutoff switches? Are they not subject to the same problems of high/low pressure as 134a?

Some do. My '82 Mercedes has a high pressure cut off switch mounted on the receiver/drier.

Scott

[Brorag]

• you'll probably have two wires sticking out the top of the dryer--mine's located in the front drivers wheel well.
• run the same gauge wire to the engine compartment to the wire controlling the compressor clutch.
• hook the two wires from the drier in series with that wire.
• if the switch in the drier trips, it breaks the connection to the compressor clutch, and the compressor turns off.

[Hugh R]

It should go in a new R/D that has a port for it. It should go through the relay wires that control the mag. clutch on the compressor.

[squelch]

I just ordered this switch:


I haven't gotten it yet, but from what I understand, it mounts inline right off of the high pressure port and wires right into the compressor, so there is not running long wires from the drier. I haven't gotten it yet, so I'm not certain, but will report back if it does. Here's a link to the supplier: http://www.ackits.com/merchant.mvc?Screen=PROD&Store_Code=01&Product_Code=57807-14MD&Category_Code=conversionitems

Squelch

[jeff1hughes]

very cool set up, no pun intended