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  #1  
Old 06-04-2019, 03:19 AM
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W123 Diagnose Failed AC Low Pressure Switch

How does one diagnose a failed AC low pressure switch????

Supposedly OE spec is that it kicks "on" at 30 PSI. Does that mean that with over 30psi of static pressure (low side measurement, engine off) it will flip on?

The system had 85 psi at the low side, with the motor off. Should the low pressure switch be "on"?? I jumped/bypassed the switch and the compressor came on.


This is a 6 yr old Behr branded switch. Kinda find it hard to believe that it died, but i think it's dead.

Thanks!
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  #2  
Old 06-04-2019, 07:49 AM
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Quote:
Originally Posted by gottarollwithit View Post
The system had 85 psi at the low side, with the motor off. Should the low pressure switch be "on"??
The switch should be closed. 'Twas me, I would just jumper the switch and carry on. Just don't continue to operate your a/c if it doesn't cool.
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  #3  
Old 06-04-2019, 03:27 PM
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So... In other words, my switch is dead.

Is there a way to replace the low pressure switch without draining the system?

I don't want to take it to a shop with a machine.... Usually very expensive....

That said, i have gauges, more R12, a vac pump, and an empty r134a tank.

Would it work if i pulled a vac on the empty tank, stuck it in the freezer, then, drained the benz's system into the empty tank?

Any residual in the system i'd vac pump into the tank i guess? Terrible idea?
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  #4  
Old 06-04-2019, 11:34 PM
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It's been confirmed, this doesn't work the system must be purged...Confirm this...anyone???...IIRC the low pressure cut out switch can be changed without evacuating and recharging. I think it is over/on a schrader valve fitting.

Reading 85 lbs static charge would close the switch; but, the pressures that need to be known are the low side pressure with the system running and the high side. The switch opens and interrupts the circuit when the system is on and the low side pressure drops below the switch on/off point or approx 30 psi.

You could put gauges on and jumper the switch, if the low side drops below 30 it is low on refrigerant or there is a blockage and it should not be allowed to continue as the low refrigerant is also likely low oil level from leakage and low oil circulation causing damage.

Wear eye protection and gloves while doing AC work!!! A mishap could cause severe burns and eye damage.

Good luck!!!
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Last edited by Sugar Bear; 06-05-2019 at 07:19 PM.
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  #5  
Old 06-05-2019, 01:51 AM
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Well, according to the Pelican Parts tutorial, removing the low pressure switch will vent the system. Pretty sure it's not on top of a schrader valve - that'd be too easy!

That's a good idea to put the gauges on while jumping the low pressure switch. That way i can definitively confirm/deny the low pressure situation.



What are your thoughts on my R12 draining idea??
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  #6  
Old 06-05-2019, 08:36 AM
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Quote:
Originally Posted by Sugar Bear View Post
Confirm this...anyone???...IIRC the low pressure cut out switch can be changed without evacuating and recharging. I think it is over/on a schrader valve fitting.
Not correct.

Quote:
Originally Posted by Sugar Bear View Post
The switch opens and interrupts the circuit when the system is on and the low side pressure drops below the switch on/off point or approx 30 psi.
The pressure switch is located on the "high side" and cannot possibly be affected by "low side" pressure.
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Old 06-05-2019, 08:41 AM
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Originally Posted by gottarollwithit View Post
That's a good idea to put the gauges on while jumping the low pressure switch. That way i can definitively confirm/deny the low pressure situation.
If the system cools properly with the pressure switch jumpered, you already know what you need to know.
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  #8  
Old 06-05-2019, 05:04 PM
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Originally Posted by tangofox007 View Post
The pressure switch is located on the "high side" and cannot possibly be affected by "low side" pressure.
Our guy specifically stated he was diagnosing the pressure switch on the low side.

For AC in general, some use both. The low side sw cycles the compressor when max cooling isn't needed or if the system has a low charge.

The high side switch is to turn on aux condenser fans when sitting stationary. Some systems use the high pressure SW as a safety compressor shut off it high side gets too high.
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Old 06-05-2019, 06:18 PM
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Quote:
Originally Posted by 97 SL320 View Post
Our guy specifically stated he was diagnosing the pressure switch on the low side.

For AC in general, some use both. The low side sw cycles the compressor when max cooling isn't needed or if the system has a low charge.

The high side switch is to turn on aux condenser fans when sitting stationary. Some systems use the high pressure SW as a safety compressor shut off it high side gets too high.
Sometimes being familiar with the car instead of generic GM knowledge is a useful thing. MB's of this era had no switch on the low/suction side. The switch was on the high side at the filter/drier. Early switches were low-pressure only to prevent operation if the system was out of gas, later ones (and modern replacements) were dual-function high/low switches to trip the system if head pressure got too high.
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Old 06-05-2019, 07:18 PM
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All, thank you for chiming in and clarifying that the switch cannot be changed without purging the system. I've edited my post.

Be well!!!
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Old 06-05-2019, 07:30 PM
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Originally Posted by 97 SL320 View Post
Our guy specifically stated he was diagnosing the pressure switch on the low side.
Not exactly. He said he was diagnosing the low pressure switch (aka low pressure cut-out switch) which happens to be located on the high side of the system in a W123.

Quote:
Originally Posted by 97 SL320 View Post

The high side switch is to turn on aux condenser fans when sitting stationary.
On the W123's, the aux fan is controlled by a temperature switch which is located a few inches from the low pressure cut-out switch. Both on the high side.
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Old 06-05-2019, 07:31 PM
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Originally Posted by Diseasel300 View Post
Sometimes being familiar with the car instead of generic GM knowledge is a useful thing.
Exactly. Sometimes great advice for the wrong car isn't all that great.
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  #13  
Old 06-06-2019, 03:03 AM
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Dang, i wish i had this info a few hours ago.

I put the gauge on the low side with the switch jumpered. It was about 85F out today. Static pressure was around 80 psi, and the AC was blowing cool. With the compressor engaged, engine at idle, it would stay at about 30psi. When holding the motor at around 3k rpm, the pressure would drop to about 10 psi.

I couldn't find an R12 pressure/temp chart like one uses for r134a. Anybody got one?? As the low pressure switch cuts out at 30 psi, i figure the system is low on charge. Thoughts???

I put a little more R12 in to the point where when at around 3k rpm, it is at around 15-20 psi. Plus, just for giggles i reconnected the low pressure switch. Now it works!

Without draining/evacuating the system, how can i tell what PSI my low side is supposed to be at given a specific ambient temp?
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  #14  
Old 06-06-2019, 06:34 AM
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When using a P / T chart, you need to measure refrigerant temp at the point you are measuring pressure.

Where exactly is the pressure sw you are jumpering?

Are yo measuring high side pressures as well?
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Old 06-06-2019, 08:33 AM
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Quote:
Originally Posted by gottarollwithit View Post
As the low pressure switch cuts out at 30 psi, i figure the system is low on charge. Thoughts???
Until you accept the fact that the low pressure cut-out switch is actually located on the high side of the system, your troubleshooting efforts are unlikely to be successful.

One more time: the low side pressure has no effect on the low pressure cut-out switch. If a static pressure of 80 psi is not closing the switch, the switch is bad, at least intermittently.

Quote:
Originally Posted by gottarollwithit View Post
... and the AC was blowing cool.
Which is a pretty good indication that the refrigerant pressure is adequate to satisfy a properly functioning low pressure cut-out switch.

Quote:
Originally Posted by gottarollwithit View Post

Without draining/evacuating the system, how can i tell what PSI my low side is supposed to be at given a specific ambient temp?
One thing for certain: draining the system is not going to provide the answer to that question.
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Last edited by tangofox007; 06-06-2019 at 11:31 AM.
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