Yet another air conditioning thread!

[cjmspartans]

With the temperatures hitting mid to upper 90s almost daily now, I threw in the towel and decided to start poking around with my 97 Suburbans (7.4L, no rear A/C) air conditioning system. It hasn't worked at all in the 4 years that I've owned it.

To start, I tested the clutch on the compressor and it spins freely. No problems there... so, just to see if it'd work I threw in four cans of r134a (4lbs, no stop leak garbage, sticker on the receiver drier calls for 4.4lb charge) while jumping the compressor where the relay would normally be installed. Pressure on the low side with the compressor running was roughly 50psi with the outdoor ambient temperature of 90 degrees.

Now, with the system appearing to be charged up, it would blow cool air (not super cold, system has a known leak and I didn't fully evacuate it when it was at atmospheric pressure) when I had bypassed the relay. When I reinstalled the relay, the A/C clutch would not engage at all.

Things I have done:

• Checked the high pressure switch. The switch is open; there is no continuity on the switch its self, and when I jump the connector on the wiring harness with a multimeter, I get 0.003A of current and the electric cooling fans turn on.
• Checked the low pressure switch. When I jump the wiring harness connector with a piece of wire, there was no change. The compressor clutch still wouldn't engage.
• Attempted to backprobe the low pressure switch with my multimeter to detect current. Couldn't detect any current, but I also don't think there was a good connection.
  
• Measured current at the 10A A/C fuse in the fuse box under the hood. When I bypass the relay to engage the compressor clutch, this fuse reads 0 amps which seems... unusual. This fuse always reads 0A, clutch engaged or not, HVAC controller in the dash set to max A/C or no A/C.
  
• Current of the compressor is ~3.5A when I used my multimeter in place of the relay. Seems on par with my Ford Bronco's R12 compressor, and sounds healthy.
  
• Swapped relay with a known working relay.
• Measured ~13V across the two pins on the relay that energizes the electromagnetic coil to close the high current side of the relay. It is always reading ~13 volts, with the HVAC controller in the truck set to max A/C, A/C and having A/C turned off as well as the blower motor turned off. This seems unusual to me as I'd expect no voltage when the HVAC controller is not calling for A/C,

I am flying blind as far as the wiring goes as I don't have a schematic. I did find a schematic for a 1995 C/K series truck, but I don't want to make the assumption that the 1997 is the same as the 1995 (likely is?). I'm suspecting an electrical problem, but I have never had to diagnose an A/C system before so figured I'd ask for advice.

Any ideas of what I should look for and next steps to figure out why the compressor won't engage with the relay? I would get the system evacuated, change the orifice tube and the receiver drier, then recharge with UV dye but if there's an electrical issue I'd rather tackle that instead of putting the cart before the horse.

Thanks everyone!

 

[L31MaxExpress]

With the temperatures hitting mid to upper 90s almost daily now, I threw in the towel and decided to start poking around with my 97 Suburbans (7.4L, no rear A/C) air conditioning system. It hasn't worked at all in the 4 years that I've owned it.

To start, I tested the clutch on the compressor and it spins freely. No problems there... so, just to see if it'd work I threw in four cans of r134a (4lbs, no stop leak garbage, sticker on the receiver drier calls for 4.4lb charge) while jumping the compressor where the relay would normally be installed. Pressure on the low side with the compressor running was roughly 50psi with the outdoor ambient temperature of 90 degrees.

Now, with the system appearing to be charged up, it would blow cool air (not super cold) when I had bypassed the relay. When I reinstalled the relay, the A/C clutch would not engage at all.

Things I have done:

• Checked the high pressure switch. The switch is open; there is no continuity on the switch its self, and when I jump the connector on the wiring harness with a multimeter, I get 0.003A of current and the electric cooling fans turn on.
• Checked the low pressure switch. When I jump the wiring harness connector with a piece of wire, there was no change. The compressor clutch still wouldn't engage.
• Attempted to backprobe the low pressure switch with my multimeter to detect current. Couldn't detect any current, but I also don't think there was a good connection.
  
• Measured current at the 10A A/C fuse in the fuse box under the hood. When I bypass the relay to engage the compressor clutch, this fuse reads 0 amps which seems... unusual. This fuse always reads 0A, clutch engaged or not, HVAC controller in the dash set to max A/C or no A/C.
  
• Current of the compressor is ~3.5A when I used my multimeter in place of the relay. Seems on par with my Ford Bronco's R12 compressor, and sounds healthy.
  
• Swapped relay with a known working relay.
• Measured ~13V across the two pins on the relay that energizes the electromagnetic coil to close the high current side of the relay. It is always reading ~13 volts, with the HVAC controller in the truck set to max A/C, A/C and having A/C turned off as well as the blower motor turned off. This seems unusual to me as I'd expect no voltage when the HVAC controller is not calling for A/C,

I am flying blind as far as the wiring goes as I don't have a schematic. I did find a schematic for a 1995 C/K series truck, but I don't want to make the assumption that the 1997 is the same as the 1995 (likely is?). I'm suspecting an electrical problem, but I have never had to diagnose an A/C system before so figured I'd ask for advice.

Any ideas of what I should look for and next steps to figure out why the compressor won't engage with the relay? I would get the system evacuated, change the orifice tube and the receiver drier, then recharge with UV dye but if there's an electrical issue I'd rather tackle that before putting the cart before the horse.

Thanks everyone!

4.4 lbs sounds like a rear a/c setup to me. My Tahoe is like 3.75 lbs with rear air. My Express van is 4.25 lbs and it has rear air. A normal can is 12 oz not 1 lbs. 4 cans would be 48 oz assuming you could completely empty them which you cannot. You probably have an ounce or two that you cannot get into the system. 10 x 4 is 40 oz. 40 oz is about 2.5 lbs.

 

[cjmspartans]

4.4 lbs sounds like a rear a/c setup to me. My Tahoe is like 3.75 lbs with rear air. My Express van is 4.25 lbs and it has rear air. A normal can is 12 oz not 1 lbs. 4 cans would be 48 oz assuming you could completely empty them which you cannot. You probably have an ounce or two that you cannot get into the system. 10 x 4 is 40 oz. 40 oz is about 2.5 lbs.

You're right -- I messed up on that measurement conversion. So, sounds like I need to keep going then. Any idea at what point the compressor should begin to engage on its own without me having to bypass the relay?

 

[Schurkey]

If you're bypassing the safety devices, the compressor should run with NO refrigerant charge.

 

[cjmspartans]

If you're bypassing the safety devices, the compressor should run with NO refrigerant charge.

I put a jumper wire on the harness at both the low pressure switch as well as the high pressure switch, with no success. I've done it with only the low pressure switch connector jumped, then the high, then both. Only way I can get the compressor to turn on is by jumping it at the relay.

I added two more cans of refrigerant this morning. Still have 50psi on the low side. I'm going to go rent a pressure gauge from O'Rileys so I can get an idea of whats going on with the high pressure side so I can get the full picture. This brings me up to about 60-66oz which I believe should be sufficient to at least get the compressor cycling.

 

[Schurkey]

Do you have POWER at the low-pressure switch? High pressure switch?

Have you checked the fuses?

 

[cjmspartans]

Do you have POWER at the low-pressure switch? High pressure switch?

Have you checked the fuses?

A/C fuse is good, has continuity and is rated for 10A. I have ~12V at the low pressure switch, and ~12V at the high pressure switch (on the harness connector). I tried backprobing the low pressure switch, and got no voltage.

I also checked the climate control module in the dash referencing the two schematics below. Pin 10 (light green) is 12V when the A/C button is pushed, and 0V when off. This seems normal to me.

The head scratcher is, I checked for voltage at the relay that goes to the electromagnetic coil (not the clutch). I got ~12V there no matter whether or not pin 10 was high or low.

This is seeming to be a really screwball electrical problem to me. I moved the starter relay (which works) into the spot for the A/C relay, and vice versa, and the starter works fine with both relays. Looking at the "A/C Signal Diagnosis" diagram below, the A/C control switch (pin 10 on the heater/air conditioning controller pinout; verified working) is supposed to feed power to one of the pins on the coil. But the coil appears to be constantly energized when I put my multimeter on it. But... it will not engage when the relay is installed.

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[Schurkey]

I put a jumper wire on the harness at both the low pressure switch as well as the high pressure switch, with no success. I've done it with only the low pressure switch connector jumped, then the high, then both. Only way I can get the compressor to turn on is by jumping it at the relay.

WHERE are you "jumping" the relay? WHICH two terminals? Are the low- and high-pressure switches still connected, or are they "jumpered", too?

When you "jump" the relay, the compressor clutch engages, the compressor pumps, and the A/C works? You've verified that there's power to the low-pressure switch, the wire harness downstream of the relay is working, and the clutch works. Swapping relays with the starter proves that the relays work.

If those switches are bypassed, you have not verified that the switches themselves work.

Now that you have refrigerant in the system, put an ohmmeter set to it's "Low" ohms scale across the low-pressure switch. You should have essentially "0" resistance.

Do the same with the high-pressure switch. With the compressor not turning, you should have essentially "0" resistance.

Verify proper ground at the B6 "150 BLK" wire socket that the relay plugs into.

A/C fuse is good, has continuity and is rated for 10A. I have ~12V at the low pressure switch, and ~12V at the high pressure switch (on the harness connector).

If you have "~12V" at the high-pressure switch connector, jumping that connector should engage the clutch.

 

[cjmspartans]

WHERE are you "jumping" the relay? WHICH two terminals? Are the low- and high-pressure switches still connected, or are they "jumpered", too?

Under the hood, in the electrical center box, where the relay would typically live. If you were to look at the diagram on the relay, I am putting a wire between terminals 87 and 30, which turns the compressor on or off.

When you "jump" the relay, the compressor clutch engages, the compressor pumps, and the A/C works?

Yes.

If those switches are bypassed, you have not verified that the switches themselves work.

I have tried running it with those switches bypassed, not bypassed, one bypassed and the other not bypassed. Basically, every possible scenario as far as those switches go. Neither combination results in the compressor engaging when the relay is installed.

Verify proper ground at the B6 "150 BLK" wire socket that the relay plugs into.

Appears to be working when I use continuity mode on my meter.

If you have "~12V" at the high-pressure switch connector, jumping that connector should engage the clutch.

Thats my problem. It isn't.

 

[L31MaxExpress]

The schematic you are looking at looks like a 95 TBI schematic. Vortecs usually use the PCM to control the a/c clutch. Have you jumped both switches at the same time? Its possible both have gone bad.

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