bannerd

Anyway to engage an A/C compressor? I put a new one on and it does not engage, so I put the old one back on thinking maybe there is something else? There is no gas in the system, is there a pressure sensor in the system?

cabtruck

N. Besonderes

I think you have to unplug the two-wire connector for the low pressure cut-out switch at the firewall and jumper it with a short piece of wire to get the compressor going when trying to fill an empty system.

-Niemand

maybe368

Yes, that is the low pressure cutout switch. One thing is that if you are charging a fresh system,or one that is in a pumped down state, you really don't have to jump the compressor wires, it will come on when the minimum pressure is reached.That is assuming that the low pressure cutout switch is working...Mark

Angry Johnny

You need to have it vacuumed and then recharged. Do not jump the low pressure switch unless you want to put another new compressor on it.

edwinsmith

If you're intent on fixing it yourself, you have to have a set of gauges and a good vacuum pump. After you replace parts and have cleaned the other hoses and such you should pull a vacuum on it for an hour or so. Then you check for leak down by observing your vacuum gauge. If it holds vacuum then you need the oil charge. Once the oil is in you pull vacuum again and start letting in the refrigerant. You need PAG oil for R-134a. R-12 uses mineral oil I think but don't quote me.

If your system had R134a to begin with then you should put back the specified amount. If it originally had R-12 and you're putting R-134a back in the you only use 80-85% of the R-12 amount.

Be careful of exceeding about 600 psi on the high side or you can pop the over-pressure valve and lose oil and refrigerant.

You can carefully monitor the temperature of air coming out of the evaporator inside the cab while you're slowly letting in refrigerant and when it gets to the lowest and starts going back up then stop letting refrigerant into the system.

If you search the stickies you'll find a thread about "Refresh your A/C" which has a lot more info.

Edwin

bannerd

Okay, doesn't look like a bad job to do myself. I was thinking about getting something like this so I can pull vac, 3cfm more than enough?

edwinsmith

That's the exact kit I bought. It works fine.

Here's the charging procedure a friend who works in HVAC gave me. First pump the vacuum down and then close the valves and shut off the pump. Wait an hour or so to see if the vacuum is maintained. If not you have a leak. If you still have vacuum then you take the center inlet hose and dip it into the oil. You should have only the correct amount of oil in the can. Open the low side valve and allow the oil to be sucked into the system. Then close the low side and open the high side and pull a vacuum again. This will pull some of the oil into the compressor. Close the high side valve again and connect your supply of refrigerant to the center hose. Open the low side valve and allow the refrigerant into the system. You can start the engine now. You may have to submerse the supply can of refrigerant in some hot water to get the flow to proceed at a reasonable pace. At some point of adding refrigerant the compressor should kick in. While monitoring the temperature of the air coming out of the A/C outlet in the cab put refrigerant in the system until the air temp goes to it's lowest and just starts back up. Close the valves and disconnect the hoses to the system and put the covers on the ports, you're done. You may want to note the pressures on the high and low sides for reference.

Edwin

maybe368

A good description Edwin, but a couple of things. Be sure to bleed the air out of the gauge hoses by loosening the connector at the gauge end until a bit of freon comes out.Also, the best way to charge the correct amount is to use a scale and do it by weight. The extra degree up seems to affect a 134A system a bit. However it is done, feed the gas in slowly and watch the temp closely so as to not over charge it...Mark