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a/c will not take charge?

57dave on Mon July 14, 2014 1:57 PM User is offline

Year: 1987
Make: Jeep
Model: Cherokee
Engine Size: 4.0L
Refrigerant Type: r134a
Ambient Temp: 95
Pressure Low: 97
Pressure High: 0
Country of Origin: United States

87 Jeep Cherokee, 4.0L with OEM A/C r12
A/C has not worked for over 4 years
retrofitted for r134a, flushed lines, replaced evaporator dryer and o rings, drained compressor and filled with PAG 100 oil.
pulled vacuum of 29 for 45 minutes and held lowside vacuum at 29 for another 45 minutes.
tried to add can of r134a freon with manifold gauge set, open lowside valve reads 97 psi and highside reads 0 psi, compressor does not kick in, but if I jump the switch compressor will kick in with no change in highside pressure. The system does not suck in a hole can of freon. If I close the freon can tap and open the highside valve the lowside drops to 5 psi and high stays at 0 psi.
Any ideas why the system pulls a good vacuum but will not suck in a can of freon?

TRB on Mon July 14, 2014 11:13 PM User is offlineView users profile

Retro fittings have scharader vales. If so did you remove the ones in the the service ports.


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57dave on Tue July 15, 2014 9:28 AM User is offline

Yes, I replaced the service ports with new r134 style ports so the gauges would connect easily.
I purchase the ports specificaly made for my Sandex U508s65 compressor from .
So the orginal service ports have been replaced.
Do you think the schrader valves are not working correctly or maybe they are bent?

I was thinking of pulling out the lowside schrader valve and trying to pull a vacuum on the system to see if I am getting a true vacuum reading.
Any ideas on how to troubleshoot this issue?

mk378 on Tue July 15, 2014 9:38 AM User is offline

An old Jeep may have isolation valves. With those you unscrew the big metal or plastic cap from the other side of the fitting on the compressor and then turn the valve stem a couple of turns to the halfway point to open up the service port.

What TRB was saying is that you want to remove and discard the car's original shrader cores before you screw the adapters onto the ports. Otherwise you have two valves in series and the original ones may not open. Isolation valves typically do not contain shrader cores in the first place, instead it is possible to remove the service hose with the system charged by closing the port off with the valve stem.

Have zero pressure in the system. Hook up the manifold and vacuum pump normally and turn on the vacuum pump. At first, open only the high side wheel on the manifold. The low side gauge should show a vacuum. This proves that you have an open connection at the ports and really are evacuating the car and not just the manifold hoses. After this test passes, open both wheels and pump for a while to evacuate fully.

Edited: Tue July 15, 2014 at 9:51 AM by mk378

57dave on Tue July 15, 2014 10:41 AM User is offline

Yes original schrader valves are gone along with original service ports.
Replaced with new r134 ports from

I can pull a vacuum with the highside port, but the lowside port seems to be not opening the schrader valve.
Inspected and even changed schrader valve with no change.
This is a new gauge set from Harbor Freight. I believe that the lowside connector is bad.

wptski on Tue July 15, 2014 1:04 PM User is offline

What makes you think that the low side valve isn't opening? If you think that it may be the gauge set, swap the lines just for test purposes. Your not using a gauge set that has manually operated valve core depressors are you?

I have seen valve cores affect vacuum readings twice before but this is when using a micron gauge something you'd never see using analog gauges.

I'm not sure why you used PAG 100 for a conversion either? If there is a chance that mineral oil is still in the system because of lack of a full proper flush, POE oil is a better choice. I don't remember what happens when the PAG mixes with mineral oil.

Happen to just read that mineral oil turns to glue and can plug up a system if mixed with PAG oil but I imagine that depends on how much mineral oil is still in the system.

Edited: Wed July 16, 2014 at 9:35 AM by wptski

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