SSminnow

Ok, here is the deal. Got the gauges on, Freon level good, equalized @ 78Lbs low and high side, today's temp is 70* outside. Compressor will not turn on most of the time, every now and then, it will. I was just checking it, and what happened is it pulled down to @ 60 LBS of pressure on the low side, and then it would shut the compressor off, build back up to 75 LBS or so, turn the compressor on, pull back down to @ 60 LBS, and shut the compressor off, did this a few times, and now the compressor will not turn back on.

What do you guys think? Low pressure switch? High pressure switch? Something I am missing (that is my assumption).

Now, coming home from the store, A/C was not working driving down the road, and then all of the sudden, compressor kicked on, startign blowing real cold, working just fine for about 10 minutes or so, and then shut off again.

What are we working with here, what is the deal?

vzdude

Have seen the connector at the low pressure swich burn. Look at it closely and probably will have to replace low pressure switch. Short term fix- use a paper clip to "jumper" across the 2 terminals in the switch and see it it kicks on and stays on. If it does- great- need switch. If not try and replace the relay with another one from the box- I typically try to use the horn one, because if my horn doesn't work I just use my 9mm! LOL no, really though, just seems to be the least likely relay you will miss having until you can get a new one. By the way! You're a jerk you know! It was -8* here this morning and forecast 3-6 inches of snow tonight and tomorrow! To heck with you and your a/c and 70* weather!!! LOL Just kidding you know!

SSminnow

Got love the South!!!! My family is from Chicago an Minnesota (I defected when I was born down here to the south, I was smart )

Now, where can I locate the Low pressure switch on my truck? I will not be able to check it until after 6:00, o it will be dark, so knowing where it is at will help. Thanks for your help, even though you did call me a jerk I guess at the same time, I called you stupid for living in such cold weather, so maybe we are even

SSminnow

Anyone give me a heads up on where the low pressure switch is???? Thanks

79warlock

No low pressure switch, it's a pressure transducer located on the discharge line near the a/c compressor. It would be difficult to diagnose with out a scan tool looking at sensors, but I would say it's more likely a bad evap fin sensor.

SSminnow

OK, thanks. Can you elaborate on the Evaporator Fin Sensor?

vzdude

Beside this you can backprobe pin #3 the dark Blue wire at the transducer and compare voltage to the chart.
A/C PRESSURE TRANSDUCER
The A/C pressure transducer is tested using a DRBIII® scan tool. Refer to the appropriate diagnostic information. Before testing the A/C pressure transducer, be certain that the transducer wire harness connection is clean of corrosion and properly connected. For the air conditioning system to operate, an A/C pressure transducer voltage reading between 0.451 and 4.519 volts is required. Voltages outside this range indicate a low or high refrigerant system pressure condition to the Powertrain Control Module (PCM) or Engine Control Module (ECM) depending on engine application. The PCM/ECM is programmed to respond to a low or high refrigerant system pressure by suppressing operation of the compressor. Refer to the A/C Pressure Transducer Voltage chart for the possible conditions indicated by the transducer voltage reading.

A/C PRESSURE TRANSDUCER VOLTAGE CHART Voltage Possible Indication
0.0 1. No sensor supply voltage from PCM/ECM.
2. Shorted sensor circuit.
3. Faulty transducer.
0.150 TO 0.450 1. Ambient temperature below 10° C (50° F).
2. Low refrigerant system pressure.
0.451 TO 4.519 1. Normal refrigerant system pressure.
4.520 TO 4.850 1. High refrigerant system pressure.
5.0 1. Open sensor circuit.
2. Faulty transducer.

A/C PRESSURE TRANSDUCER (DIESEL) - BLACK 4 WAY CAV CIRCUIT FUNCTION
1 K55 18LB/WT SENSOR GROUND
2 K72 18DG/OR 5 VOLT SUPPLY
3 C18 18DB A/C PRESSURE SIGNAL
4 - -

SSminnow

Ok, great info. Now, may sound like a dumb question (even though I am an opponent of the only dumb question is the one that does not get asked) where is the A/C tranducer located?

Can I check voltage at the plug on the compressor? Or is that where it is referring to?

Today, the AC did not work at all, until 5 minutes from the house, then finally came on. But shut off shorty there after. Around 70* again today...

simadownow6988

you will not be running 60 psi at 70 degrees ambient. Thats too much. Your probably overcharged and the compressor is cycling out on high pressure.

simadownow6988

you need to be running about 37 to 40 on your low side while idling. you will be running around a 42 degree coil. I have been in the heating and air buisness for 15 years, so trust me. Thanks

SSminnow

I understand that, just read a little closer. With the system off (compressor) it equalizes @ 75 LBS on a a 70* day, like today. Ok, when the compressor does decide to kick on (which is rare), the pressure starts to drop, and once it hits 60 LBS, it shuts the compressor off.

Make more sense? Also, I have owned the truck for over a year, and since it just now started acting up, it is IMPOSSIBLE for it to be overcharged, since someone would have had to add freon to it for it to be overcharged, and that has not happenend.

Also, since I have had this problem over the last couple of days, and Ihave had my gauges on it, when the compressor stays on, the low side drops down to @ 30-32 LBS, and that is when it is working good.

When it act's up, the compressor will not turn on at all, and like I said above, when it does, it will start pulling down from the eqaulized pressure of 75 LBS, it will hit 60 LBS and shut off. But when it works fine, it will pull down to the 30-32 LBS.

So, it being overcharged is definitely not the problem.

My wording above may have confused some, sorry about that.

SSminnow

How about this:

I am going over my TST R49AP manual, and it says that I may get a fault code for AC clutch problem.

Maybe that is what is happening????

simadownow6988

ok. I might have not read it right. After reading so many forums, your eyes start to hurt.
You might have one of two problems.
The low pressure safety, which is electrical on these trucks (like a thermister), might be bad, sending bad signals like it has low pressure.
HAve you checked your high pressure readings by any chance?
Garbage in the orfice will also cause a similar problem, causing gradual lowering of the low side, like its working fine, but what you dont know is that your high pressure is getting higher and higher. I have run into many like this. Sometimes it will work fine because the debis is not blocking the orfice, but sometimes it will act up when the debris does.

SSminnow

I have hooked up my gauges quite a few times (high and low) but seems that when I do that, the compressor never comes on, so I am only getting a equalized pressure reading on the high and low @ 75LBS. Now, when the compressor DOES kick on, I have the low side hooked up only

Call it a coincidence, but that is what is happening. I can not get the compressor to come on (or at least call it bad luck) when I have the high side line hooked up, so I have not been able to get a reading on the high side when the compressor does kick on .

I was reading the TST manual, and it says if I have a A/C compressor clutch problem, to take the ground off the Neg terminal on the battery (from the TST box) and run the ground inside the truck.

I called TST to elaborate on this, and of course they are closed now

simadownow6988

Dont know about that much (TST Box). See what they say I guess. If that dont work, try what I said (hopefully you can get the compressor to come on again).