Car won't take A/C refrigerant, compressor won't start.

The deal is you need to pull a vacuum on the system before it will allow you to put any more freon in it. That oil is called pag oil, and make sure you dont lose any of it, or youll have to buy some and add it in there.

Your compressor isnt shot, it has a pressure switch on it that automatically turns the compresor off if there isnt sufficient freon in the system. If you can, take your car to a place that will pull a vacuum on your a/c system, tell them NOT to drain the oil in the system unless yours looks like it needs changed. If you have never changed it, go ahead and tell them to add new oil.

Once they have pulled a vacuum on the system, call ford and find out how many ounces of oil and how many pounds of freon its supposed to take so you dont overcharge your system and possibly blow anything up.

also, is your car still r-12, or have you converted it to 134a? If its r-12 still, good luck finding a supplier that will unload any for ya... its you tried putting 134a into a system that takes r-12 you more than likely killed some of your seals (o-rings)

The A/C system has a low pressure cutout switch mounted on top of the accumluator on the firewall. Its purpose is th shut down the system when the refrigerant charge drops below a safe level. This keeps the compressor from burning up due to lack of circulating lubricant. You can disconnect the switch wiring connector and jumper the two terminals inside to test the system. Do not try to operate the system for more than 1 minute with the jumper in place unless you are recharging the system.

See http://forums.stangnet.com/showthread.php?t=705606 for more information and some tips on replacing the low pressure cutout switch if it is defective. Do not replace the switch unless you know the refrigerant has a full charge and the rest of the A/c's electrical wiring is in good shape.

R134a Air Conditioner Conversion and recharing instructions


R134a = $7-$10 a can – takes 2 - 2 ½ cans.

R134a compatible oil = $5-$7 for an 8 oz bottle – better get 2 bottles.

Gauge set for recharging = $20-$120 – check out the pawn shops for a bargain before you pay retail. See http://www.harborfreight.com/for A/C charging gauges that you will need.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92649 is the A/C charging gauges that you will need.


My current vacuum pump is a refrigerator hermetic unit (motor & compressor in one unit). It was a freebe from a local used appliance store. I bolted it down to a piece of plywood & soldered a 1/4" flare fitting on the suction line. It works great, pulls 29" of vacuum in about 10 minutes or less. I have used it several times and have had excellent results.




Pump to force cleaning fluid through the system $20-$50 (may use compressed air to do the same thing).

O ring seal kit = $8.

R134a charging adapter = $13 ( I cut mine up to use it with the R12 gauge set that I have had for a long time).

Plastic tools to disconnect refrigerant lines - 1/2" & 5/8" = $4 each.

Flushing agent - Discount Auto Parts has some flushing solvent in a 1 gallon plastic bottle - try that first. Or use Mineral spirits = $4 a gallon, tetrachloroethylene =$5-$10 a gallon, takes 2 gallons of either one.

Miscellaneous hoses and fittings to adapt the flushing pump to the system, and the R134a adapter to the R12 gauge set = $15.


R134A refrigerant charge is 20-22 Oz of R134 refrigerant. Oil charge is 6-8 Oz of oil for a system that has been flushed of oil. For a recharge add 2-4 Oz of oil.

I did a R134a conversion on my 89GT, and used all stock parts. You will need to replace the dryer/receiver (about $75 if you get the one with the hose made as part of the unit), and should replace all of the rubber "O" ring seals as well. You will need to drain all of the oil out of the compressor and replace it with new R134a compatible oil.

Keep in mind that to fulfill the requirements of the EPA, you are required by law to recover any refrigerant that still remains in the system. How (or wither or not) you accomplish this is up to you. Connect the charging gauge hoses to the service ports on the A/C (red gauge = high pressure, blue gauge = low pressure) and open the valves on the gauges to dump the remaining refrigerant (if any) into your "freon recovery system", whatever it may be. Disconnect the charging gauges since you are finished with them until you are ready to fill the system with R134a. Next comes the nasty part – in order to get all the old oil out of the system, you will need to flush it with special flushing solvent, or mineral spirits (ok) or tetrachloroethylene (better, but may be hard to get). If you leave the old oil in place it will congeal and reduce the heat transfer in the condenser and evaporator (read that it won’t cool good) and possibly damage the compressor.

Disconnect the compressor and remove it from its mount to flush it with cleaning solvent. Pour about a cup of solvent into the suction port and turn the compressor center hub about 10 turns while shaking the compressor to move the solvent around inside the compressor sump. Drain the flushing solvent out and continue to turn the center hub by hand to force out any remaining solvent. Then fill it with oil: add about 6-8 oz of the new oil to the compressor large suction fitting. Turn the compressor center hub about 20 turns as you turn the compressor face up and face down to distribute the new oil inside the compressor. Catch and replace any oil that comes out of the compressor.

Connect the pump (I had an electric sump pump I bought for $20) to the hose from the high-pressure side of the compressor. Alternately, you could use compressed air to force the cleaning fluid through the system. I didn’t like to do this since compressed air has lots of moisture in it, which is death to A/C systems. Pump the cleaning fluid through the system and let it come out the hose that was attached to the old dryer/receiver. I used 2 gallons of mineral spirits and pumped it all through the condenser and evaporator. The expansion valve is located near the firewall in the high-pressure line of the evaporator, and may cause the cleaning fluid to trickle through the lines at a very slow pace. You may want to pump cleaning fluid through the evaporator and condenser separately to speed up the process.

Next comes the changing of all the old "O" rings so that the chances for leaks is minimized. Use the plastic connector tools to separate the lines, place the extended collar part of the tool so that it faces the large part of the connector and push inwards: this expands the spring so that you can pull the tube apart. You may need a helper to push on the tool while you pull on the tubes to separate them. Install the new "O" rings: be sure to coat them with new oil when you put them in. Install the new dryer/receiver, R134a service port adapter, compressor, add about more 4oz of oil to high pressure line and tighten up all the lines. Close the hood, start the engine, let everything get warm under the hood, but don’t add the R134a or turn the A/C on. Connect the charging gauge hoses to the service ports on the A/C (red gauge = high pressure, blue gauge = low pressure) and open both valves, then connect the center hose to the vacuum pump. The purpose of this exercise is to heat up the system so that when you vacuum it all down (yes, you will need a vacuum pump- mine is an old refrigerator compressor), that all the air, vapor and moisture from the cleaning fluid vaporizes and is removed from the system. Vacuum it down for about 30 minutes, this should give you about 28" of vacuum or more inside the A/C system. I have a vacuum gauge "T" connected into the vacuum pump line so that I can accurately watch the vacuuming process. This is a good time to take a soda and sandwich break since it doesn’t go faster if you watch it.

Install the R134a service fittings on the system: the red goes on the high side and the blue on the low side. This will help others identify that a R134a conversion has been done on the system.

Remove the electrical connector from the dryer/receiver and jumper the two connections inside the wiring harness side of the connector together: this allows the compressor to engage in spite of low pressure/no gas in the system. Close both charging gauge valves, and then disconnect the center hose of the charging gauges from the vacuum pump and connect it to the R134a can tapper. Put the R134a can in the can tapper and screw it down with the can tapper valve closed, then open the valve. Loosen the hose at the center connection of the charging gauge set until the R134a squirts out: this purges the line of air and moisture. The refrigerant is added through the low pressure side of the system, so open the low pressure gauge valve to add the R134a. Start the car and take note of the idle speed, then set the idle speed up to about 1200-1500 rpm, and turn the A/C on inside and set the fan speed on high. Watch for the pressure on the low side to drop off as you are filling, and the R134a can will get warm and stay warm. This tells you the current can is empty and needs to be changed for a fresh one. Before you disconnect the can, be sure to close the valve on the R134a can tapper.

Watch the high side pressure on the charging gages and regulate the adding of gas to keep the high side pressure under 350 psi. You will probably need a fan in front of the car to keep the readings below 350 psi. I had to put the R134a can in hot water while I was charging the system with it, or else the can got so cold that it quit flowing. Use caution when you do this so that you don’t get water in the charging adapter when you change the cans. When you have added the 2 cans of gas, the high side will read about 250-300 psi and low side about 28-38 psi. Turn the idle speed back to where it was, turn the A/C off, disconnect the charging gauges, and re-install all the caps on the service ports. Remove the jumper from the low pressure switch harness and plug it back on the switch connectors. Then put the R134a Service Sticker on, secure all the loose wiring on the system and you are done. I hope it cools good, mine doesn’t get quite as cold as it used to driving around town.

The above technical note is for informational purposes only, and the end user is responsible for any damages or injury. The end user bears all responsibility for proper recovery/disposal of any refrigerant.

I have an EPA 609 MVAC certification. And yes, you can shortcut the process, but there are negative factors if you do. Sooner or later, something will cease to function like it should. Shoddy work is a time bomb ticking away, waiting to explode.

Yeah. Better watch out for the epa when you discharge your sytem in your garage. They could be in your rafters watching you. Then you will have to pay a big fine.

88 Fox GT said:

The car already has R-134a and was converted before I bought it. I am guessing that low pressure switch went bad or something.

All I want to do is get the system to accept coolant again and just fill it up, because it was working perfectly.

Click to expand...

get someone to pull a vacuum on the system, then add your freon. it will not take any more freon until you pull a vacuum on the system. right now you are trying to force freon into a system that if probably full of air, hence the reason it will only take 1/2 a can.

The low pressure switch probably sensed that there isnt enough freon in the system and its shutting the compresor off

The air and moisture come out because they vaporize in a vacuum. The oil will not vaporize when you vacuum down the system. The oil is available in 4 Oz cans packaged with R134a refrigerant so you can easily add more when you recharge the system if it is needed. Normally the only time more oil is needed is when you replace a major component or you had a ruputure of a hose or line that dumped all the refrigerant out very quickly.

Autozone EF Products / Pag 100 Oil Charge P/N 310


I suggest that you go back and read my R134 conversion & recharge post. It will help you understand the recharging process.

Check the A/C WOT relay. Here's a diagram...

Diagram courtesy of Tmoss & Stang&2birds


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

check out that WOT relay - they are a problem area. The wire insulation on mine was burned up on the wires to the clutch due to a poor connector crimp which also ruined the socket. I suggest that all connections to and from that relay block be soldered.

sure, get a jumper with aligator clips on both ends or pull the relay and use a jumper wire in the socket and follow the diagram to be sure where to jump it

88 Fox GT said:

So I want to jump from Red to Orange/Light Blue and from Pink/Light Blue to Black/Yellow.

Click to expand...

Not unless you like to get high off the smell of burning wires.

Jump the Common and Normally Closed terminals: pink/lt blue to blk/yellow.

FWIW, often times just manipulating the relay and socket in your hand will create magic (make the compressor kick on till you move again. On, off, on, off).

My wiring and socket went south a long time ago (also with vulcanized insulation). Tom was kinda enough to help me ensure I was not making the fix more difficult than it had to be (I think you were down after back surgery Tom. But you were still willing to help. ).

88 Fox GT said:

Well it still doesn't work. Last night I noticed a blown A/C compressor fuse, so I replaced it with no luck. Today I tried the A/C and it was working for quite awhile and just quit working again. I jumped the wires for the relay and nothing. The relay is working fine, the low pressure switch is working fine, the connector is connected to the top of the compressor, and the fuses are all good. I really don't know what to do now.

Click to expand...

You have to test when the thing is acting up.

When it is not working: ensure there is power reaching the inlet of the LPCS. If there is, see if there is power leaving the LPCS. If there is go to the WOT relay. See if there is voltage going into the relay (pink/lt blue). If there is, see if there is power leaving the WOT relay (Blk/yellow). If there is, see if power is reaching the clutch. If there is, check the clutch's ground.

I have the feeling that there is still a wiring issue that's intermittant (going back to the WOT relay possibly).

If you have 12 volts going into the LPCS but not leaving it, put your gauges on and see what's going on.

That's what comes to mind for a random rehash.

Good luck.

ok after searching i ran across this thread.

my a/c died over the winter and i didnt realize it until last week. i just recharged it with r12 but the compressor still doesnt come on. after jumping the LPSS the compressor switched on but still blew warm air. does the compressor have to be on to recharge the system? the gauged showed that the system was low and when i finished recharging the gauge had gone up into the fullish range.