How to recharge an A/C system that has a pressure switch

[Duncan]

I have a 2001 ML320 W163 Benz which lost a fight with a deer. Now I have repaired the front end its time to charge the A/C with R134a. Ive got the manifold gauge etc with DuPont Suva cans and the system is holding a vacuum, but I discovered the car has a low pressure switch to prevent the compressor running when there is no refrigerant. Its a digital pressure switch so not easy to override and I have searched the web (including mbworld.com), but no answers as to how to pre-pressurize the line short of renting the special $4k machine or taking to a dealer. Do you have any suggestions on how I can do this, such as a brand of refrigerant can that will also add pressurize to the line? Many thanks

Topic

[Richard Kirshy]

Before you can add refrigerant you’ll need to evacuate the system using a vacuum pump and manifold set.
At which point after you pull a vacuum you should be able to add refrigerant through the manifold set.

[Duncan]

Yup, I hear ya, I sucked down a vacuum, which holds, but I cant charge it with the manifold gauge because the car electronics prevent the compressor from running due to the A/C low pressure switch, so it will not suck in the refrigerant.
Ive read you can override the compressor to make it run but only for a few seconds to prevent it burning out. The pressure switch is on the high pressure line by the dryer, and Im sure the benz is not the only manufacturer that has this setup, so hoping someone else has dealt with this type of A/C setup before

[Rahul Jones]

The pressure difference from the Vacuum in the system and the pressurized bottle will pump the initial few pounds of refrigerant after which the compressor should kick ON. If not, then there could be problem that needs further diagnosing.

[Duncan]

ok, thanks for the help,I wasnt sure if the bottles added any pressure or if the vacuum causes the refrigerant to be sucked in until the pressure equalizes with the armosphere.

[Bluesnut]

I’ve always disconnected the wire connector at the switch and used a jumper across the connector. (jumper wire, paper clip, etc)

Once you get a can or half a can in that should be sufficient enough to engage the compressor so remove the jumper and plug it back onto the switch.

[Duncan]

Thanks. Ill see if this works.

[Author]

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