98 GTP A/C problems

I am a happy owner of a 1998 GTP with the 3.8 SC engine that just turned 60K miles. I have had NO problems with this car until today. I went to town and my a/c worked. I got back in the car and it just blew hot.

I am HOPING that it is just low on freon. How do you jump the 3 wires on the low pressure switch in order to kick the compressor on? Are there any other things I should check? The mechanics around here tend to create more problems than they solve (and then charge outrageous prices) so I won't take my car to them...I love it too much.

Since it looks like you plan on DIY, why not just check the R134a refrigerant charge on the low pressure port. It will range from 35 - 55 psi depending on the actual ambient temperature. Unless it's prohibited by state or local law, Autozones, Advanced and even Walmart have re-charge kits with a pressure gauge or sell the gauge by itself. It's cheaper than taking it to a HVAC mechanic. Good luck!







http://i5.photobucket.com/albums/y186/lizzywiz/gtp.jpg
Thrasher CAI, DHP v1.0 PCM, SLP Headers,
3.25 SC pulley, CAIT GMAF, 2.5 DP/Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 72mm TB,
TB spacer, MSD 8.5mm wires, Autolite 103,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials
1/4 ET: 13.210 sec. @ 104.490 mph
0 - 60 ft: 1.945 sec.

Basically what I need to know is how to bypass the 3-wire pressure switch. I have freon on-hand as well as guages...but have only bypassed the 2 wire low pressure switches in the past.

Basically what I need to know is how to bypass the 3-wire pressure switch. I have freon on-hand as well as guages...but have only bypassed the 2 wire low pressure switches in the past.

I see what you mean. The two wire low pressure switches are self explanatory. The 3 wires are a different story. On the wiring harness connector plug of the LPS are your wires color coded red/black, black, gray?

Even the GM GP service manual wiring diagram doesn't show the LPS circuit for some unexplained reason. But I'll check again.







http://i5.photobucket.com/albums/y186/lizzywiz/gtp.jpg
Thrasher CAI, DHP v1.0 PCM, SLP Headers,
3.25 SC pulley, CAIT GMAF, 2.5 DP/Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 72mm TB,
TB spacer, MSD 8.5mm wires, Autolite 103,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials
1/4 ET: 13.210 sec. @ 104.490 mph
0 - 60 ft: 1.945 sec.

You can check the static charge and it should read about 50 psi (both sides). If it does, check the electrical circuit (A/C fuse and relay).
The compressor clutch coil is energized through the compressor control relay. This relay is activated by the PCM in A/C modes. The PCM provides cut-off of the relay under certain operating conditions, including wide open throttle.
The A/C Pressure Sensor (3 wire sensor)is an input to the PCM and does not directly control compressor operation. Because this is a Variable Displacement Orifice Tube (VDOT) refrigeration system that is comprised of a variable displacement compressor and a fixed expansion (orifice) tube, the system can match the air conditioning demands under all conditions without cycling.
You can check the static charge and it should read about 50 psi (both sides). If it does, check the electrical circuit (A/C fuse and relay). You can jumper the relay and if the compressor runs, you need to check the relay and for PCM command.

Basically what I need to know is how to bypass the 3-wire pressure switch. I have freon on-hand as well as guages...but have only bypassed the 2 wire low pressure switches in the past.

The accumulator/refrigerant low pressure switch is part of the PCM circuits, so here's the following from what I can see:

Jump Pin B (Gray) to Pin C (Red/Black) and the switch should be bypassed.

Pin C (red/black) is the output of the LPS switch feeding the PCM module at Pin 73.

Pin B (Gray) is the 5v ref out signal input from the PCM Pin 17.

When pressure is good the 5v ref out signal is passed directly back to the PCM as an input.

BTW - Pin A (Black) appears to be a low or ground. Let us know how it turns out. Good luck!




http://i5.photobucket.com/albums/y186/lizzywiz/gtp.jpg
Thrasher CAI, DHP v1.0 PCM, SLP Headers,
3.25 SC pulley, CAIT GMAF, 2.5 DP/Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 72mm TB,
TB spacer, MSD 8.5mm wires, Autolite 103,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials
1/4 ET: 13.210 sec. @ 104.490 mph
0 - 60 ft: 1.945 sec.

The A/C refrigerant pressure sensor signal indicates high side refrigerant pressure to the PCM. The PCM uses this information to adjust the idle air control valve to compensate for the higher engine loads present with high A/C refrigerant pressures and to control the cooling fans. A fault in the A/C refrigerant pressure sensor signal will cause DTC P0530 to set.
The PCM will enable the A/C compressor clutch whenever the engine is running and the A/C has been requested unless one or more of the following conditions are present:
· Throttle angle is greater than 96%.
· A/C head pressure is greater than 440 psi (4.6 volts) or less than 33 psi (0.3 volt) as determined by the A/C refrigerant pressure sensor.
· Ignition voltage is less than 10 volts.
· Engine speed is greater than 4400 RPM for any amount of time.
· Engine speed is greater than 4392 RPM for less than 1 seconds.
· ECT is greater than 124°C (255°F).
· IAT is less than 5°C (41°F).
You can backprobe the red/black wire at the sensor..if the voltage is between .3 and 4.6, the compressor should run.
Jumpering the Grey 5v wire to the Red/blk signal wire will cause the PCM to read 5 volts (thinking there is a high head pressure situation) and command the compressor off. (but Baylor3400 should have known that)

·You can backprobe the red/black wire at the sensor..if the voltage is between .3 and 4.6, the compressor should run.
Jumpering the Grey 5v wire to the Red/blk signal wire will cause the PCM to read 5 volts (thinking there is a high head pressure situation) and command the compressor off. (but Baylor3400 should have known that)

First off I do not purport to be an expert on HVAC. But if the guy needs help I'll do the best I can considering it's a Sunday and I have better things to do. If the original poster is intent on jumping the connector to troubleshoot that's his perogative and his car. I'd go about it a different way. I'm just providing info that he specifically requested below:

Basically what I need to know is how to bypass the 3-wire pressure switch. I have freon on-hand as well as guages...but have only bypassed the 2 wire low pressure switches in the past.

Nonetheless, the pins provided are correct as you've pointed out. If a voltage drop below 5 volts is needed all it takes is a simple resistor in series to put it in the range specified of .3v to 4.6v.

And if you don't like it then it's too bad. Go screw with somebody else.






http://i5.photobucket.com/albums/y186/lizzywiz/gtp.jpg
Thrasher CAI, DHP v1.0 PCM, SLP Headers,
3.25 SC pulley, CAIT GMAF, 2.5 DP/Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 72mm TB,
TB spacer, MSD 8.5mm wires, Autolite 103,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials
1/4 ET: 13.210 sec. @ 104.490 mph
0 - 60 ft: 1.945 sec.

Go screw with somebody else.
I’m not screwing with anyone, just trying to give a member some advice on how to properly diagnose his A/C concern.
If kelshm follows your advice, he will not get satisfactory results because the PCM will read a high-pressure condition and disable the compressor.
If he has a meter, he can back probe the signal circuit and if the voltage falls within the parameters previously stated (as well as the other parameters), then the PCM should command the compressor on. (This is a lot easier then trying to locate a resistor and create a voltage drop, and if this is the only resistance in the series circuit, the resistance should consume all the available voltage and the voltage at the PCM should be 0 v and the compressor will still not be commanded on)
If there is at least 50-psi static pressure like I stated, then if it does, he should check the electrical circuit (A/C fuse and relay). You can jumper the relay (circuit 30 to 87) and if the compressor runs, he will need to check the relay and for PCM command.
But this is just some simple diagnostic advice from a professional technician without a real education.


Basically what I need to know is how to bypass the 3-wire pressure switch. I have freon on-hand as well as guages...but have only bypassed the 2 wire low pressure switches in the past.
Back probe the signal wire first..if the voltage is in the ranges specified above, then jumping the switch will not give you the results you are looking for.

Here is the Description and Operation of the A/C Pressure sensor
The A/C system uses an A/C refrigerant pressure sensor mounted in the high pressure side of the A/C refrigerant system to monitor A/C refrigerant pressure. The PCM uses this information to turn ON the engine coolant fans when the A/C refrigerant pressure is high and to keep the compressor disengaged when A/C refrigerant pressure is excessively high or low. The A/C refrigerant pressure sensor operates like other 3-wire sensors. The PCM applies a 5.0 volt reference and a sensor ground to the sensor. Changes in the A/C refrigerant pressure will cause the A/C refrigerant pressure input to the PCM to vary. The PCM monitors the A/C refrigerant pressure signal circuit and can determine when the signal is outside of the possible range of the sensor. When the signal is out of range (high or low) for a prolonged period of time, the PCM will set DTC P0530. When DTC P0530 is set, the PCM will not allow the A/C compressor clutch to engage. This is done to protect the compressor.

(This is a lot easier then trying to locate a resistor and create a voltage drop, and if this is the only resistance in the series circuit, the resistance should consume all the available voltage and the voltage at the PCM should be 0 v and the compressor will still not be commanded on)


You are assuming that he has a mulitimeter and knows how to use it. Plus you have to realize that he may not be a Pro like you.

And once again I did not give him advice on troubleshooting the circuit just information concerning the wiring. How he goes about troubleshooting his car is his business.

BTW - Going out to Radio Shack and getting a resistor is a very easy task especially if the person is intent on DIY their way.

I've never heard of resistance consuming all the available voltage....that's a first. You'll have to speak on a level of electronics that I understand. A new meaning to Ohm's Law.




http://i5.photobucket.com/albums/y186/lizzywiz/gtp.jpg
Thrasher CAI, DHP v1.0 PCM, SLP Headers,
3.25 SC pulley, CAIT GMAF, 2.5 DP/Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 72mm TB,
TB spacer, MSD 8.5mm wires, Autolite 103,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials
1/4 ET: 13.210 sec. @ 104.490 mph
0 - 60 ft: 1.945 sec.

Great information, I will give it a shot. THANKS EVERYONE!