I have just replaced my ac compressor on my 90 coupe along with the accumulator, o-rings,and pressure switch since I]m converting to R-134.I've flushed the system . I just bought a set of guages and I was wondering if someone can direct me to a sight that will give directions on proper useage ,hook up ,etc, and what the diferent pressure reading mean and should be before I start.

Thanks

 

Service Manual has Pressure/Temp Charts for your vehicle, though it's going to show R12 numbers only for a '90. R134 is a couple of lbs less, but you can use those charts as a guide. You might try www.ackits.com - see the FAQ section. If your gages are R134, they use a quick connect fitting. If they're R12, they simply thread on, but you'll need an adaptor for the GM high side. Blue is low; red is high.

The '90 has self diagnostics built into the Processor meaning that if it detected a low charge or no charge condition before you replaced parts, it isn't going to run until you clear memory. So start by disconnecting and reconnecting the battery.

When you pull your vacuum, open both valves. That will speed it up. When done, close both valves

For charging and with both valves closed, attach a can to the center or yellow charge hose. Crack the yellow hose at the gage set momentarily to purge air. Then open the Low Side Valve (ONLY) and allow the vacuum to pull in the first can. When empty, close the Valve and attach a second can and again crack the hose momentarily to purge air. Before you do anything else and to keep the Processor from recognizing a low charge (which will shut it off), disconnect the Low Pressure Switch Harness and jumper the connector with a paper clip. Then start the motor and block the throttle so that it's running at 1200 rpms. Turn the a/c on max and verify that the compressor is engaged. Hopefully, at least one radiator fan is running, because that will enable you to charge it faster and give you a steady read on the High Pressure Gage (both should be running when the second can is in it). Because of the low charge, the Suction Line may, momentarily frost over. Open the Low Side Valve (ONLY) and let the the system pull in the second can. Repeat for a 3rd can (if needed).

When done, turn everthing off and reconnect the Low Pressure Switch. Restart the engine and again keep it running at 1200 rpms. With the system on Max, disconnect the blower motor harness and observe your low pressure gage. The lack of air flow across the Evaporator is going to reduce pressure and the Low Side should cutoff at 25 psi (R12); 22.5 psi (R22). If it's too high or too low, your switch should have an adjustment screw between the terminals. Turn it counterclockwise to lower the threshold, clockwise to raise it. Start with 1/2 turn adjustments and recheck.

Main fan should come on when the high side reaches somewhere around 200 psi. Secondary a number above it. I don't know the exact specs for the '90 but they will be in the Service Manual. Observe the high side and make sure they're coming on.

If you're converting to R134, only use 75 to 80% of the R12 fill. For a conversion and as a guideline, you can also charge by high side pressure which shouldn't exceed 2.2 to 2.5 times the temp of the air across the Condensor (usually 10 degrees higher than ambient on a bottom feeder), engine at 1200 rpms, both fans running; so, if it's 80 degrees at the condensor, you stop charging when the high side gets to 176 - 200 psi. This can be a little tricky because your ECM won't be turning on the fans until it's above this number. In other words, your high side will show 176 psi, but because the fan(s) aren't moving any air across the condensor, the pressure is going to rise and by the time the fans are running, it may be at 225 psi. The pressure should then drop and I'd use the lowest number achieved when or after the fans cutoff. As a side note, reprogramming the ECM to turn on the main at 175 psi and the secondary at 190 psi, is a better way to control your pressures with a conversion.

 

SunCr
Thanks for your detailed reply. It has cleared a few things up for me. One question I have that you could help with...I understand the low side gauge's fuction, but am unclear on the high side. At what point do I get a reading from the gauges. When the compressor is engaged or not engaged? I know not to open the high side valve when charging the system, but at what point do I get a high side reading? gauge open or closed,comp. on or off?
Thanks

 

I posted an A/C thread a week or two ago, with a few pics of plumbing hookups.

Try searchin'...

 

Gages read the moment they're attached - you don't need to open up either valve - and you only open them up during evacuation or charging. At rest - nothing running, the gages should be equal and the pressure in the system should be the same as the ambient temperature for the gas (refer to your chart). Once running, the compressor discharges a high pressure gas which becomes a high pressure liquid in the condensor. It's then metered into the Evaporator as a low pressure liquid, where it boils and absorbs heat. Your gages, with it running, are going to show these different pressures, but the low side when the valve is open to allow for charging, is only going to be accurate with the gage valve closed - so after you it get a couple of cans in it, take a moment to look at the high and low sides to see where it's at; ie, keep the low side closed. For the low side, R12 is 32 degrees at 30 psi. Refrigerants do not change temperature until fully saturated, but will climb in temp (depending on the metering device) once vaporized. That's called superheat (automotive systems usually allow 6 degrees over the saturation temp) and it's designed into your system to prevent the gas from condensing back into a liquid after it's boiled. That also prevents icing when operating at saturation pressures a tad below freezing. You'll want a range of 28 to 35 psi for most outside air temps 70 to about 95 degrees, relative humidy 70%. Your FSM will provide a chart for what Chevy expects from your vehicle.

 

SunCr
Thanks again for the info...Just to make sure, both the low side and high side reading are taken with the valxes closed, correct? In other woeds, add freon, close valve and take reading. Then open and add more freon and close valve again and take reading . Do this until your at the proper 45-50 lob. reading

 

Yes - always keep the valves closed except for evacuation (open both high and low) or charging (open low only). Observe pressures with both closed. Low should be somewhere around 28 to 35 psi up to 95 degrees, 70% humidity - High should average somewhere around 200 psi. These pressures are for fully charged, both fans running, engine at 1200 rpms, system blowing MAX and yours might vary a bit depending on how GM designed it, gas you're putting in it. Obtain these numbers and you should have mid 40's out of the vent; maybe low 50's if you switch to R134.

 

Thank you sir