97 GMC K1500 XC A/C not working - electrical

[Scrat]

Howdy all,

So last fall when i bought this truck the AC was working fine. Last weekend I went a a trip and the outside temp was hot so i turned on the AC and nothing... fan blowing hot air. I figured it needed to be recharged... no biggie.

Well today i went to charge the system and again nothing. I started checking fuses and relays... all good. I got out my test light and did some checking and stumbled across this...when i touch the test light probe to the 86 prong on the AC relay, the AC clutch engages and the AC system works. I swapped relays with a similar one and same thing happened, so the relay is good.

I keep the system running this way long enough to verify that the coolant charge is fine. This leads me to believe it is an electrical problem (duh). Trouble is the schematics i have available to me suck. It seems like its either a broken ground connection somewhere or the AC control module is going out?

Thought I'd look to the board for wisdom before i go and start tearing into things blindly. So my question is, has any one had this experience before and how was it fixed?

Its getting hot here in AZ and I'd like to get this fixed...

Thanks in advance...
Scrat

[ChevLoRay]

to the forum!

My only experience with a non-working A/C system in my '96 was due to low freon level which kept the system from running when the low-pressure switch kept the compressor from engaging. I had to R&R my compressor to resolve my issues.

If you have charged the system, I would have to assume that all is good in that respect.

[bwood]

You say you ran it long enough to verify the charge was good, what were your pressures and what was the outdoor temp when you were testing? Is the light on in the AC button on the dash? It sounds like one of the pressure switches are not engaging. I believe there a three different switches on your truck:
(1) low pressure switch located on the dryer accumulator (I believe it opens around 24 psi)
(1) high pressure swithc located on the high side port of the compressor manifold
(1) more high pressure switch located in the back of the compressor as a failsafe to the other

My guess is the low pressure switch is the problem as you charge may below requirement. When you made the relay engage, it ran on what is left in the system. This really isn't good on the compressor as oil flow isn't very good when the refrigerant is low. You can test the switches by removing the connector to each one and jump it to see if the compressor starts. The compressors in these trucks have been called "belly leakers" as the seam in the compressor will leak and it will show up as oily residue on the belly of the compressor and on the top of the compressor bracket.

[Scrat]

Thanks (again) ChevLoRay ;-)

Thanks for the reply bwood.

I did try to jump across the lower pressure switch thinking that was the case as well..and nothing happened. The lights in the dash switches are both illuminated.

The pressures were:
28 psi on the low side
168 psi on the high side
about 68 - 70 degrees air temp

The "system" seems to have been serviced prior to me purchasing the truck (reciepts tend to confirm this)... new acumulator, expansion valve, PAG oil and a new compressor (as the clutch started squeaking and was going out)... thats why i was surprised when it wasn't working. But then again, with the quality of work at some places, maybe i shouldnt be so surprised. lol.

Again, thanks for your advice.

[sparkydog]

Maybe this will be a better schematic???

[sparkydog]

After staring at that schematic for a minute it looks like you have 2 or 3 diodes that have to be working - maybe one of them has failed.

[Scrat]

Yeah, I see what you are saying about the 3 diodes sparkydog. But this truck has a mechanical fan... no electrical fan for me. So that circuit is eliminated. That looks like the schematic I have. Thanks for posting it however.

One other thing, when i "jump" the AC relay, the compressor runs continuously, regardless of whether the AC is even "on" on the dash controls.

I'm still scratching my head on this but am starting to lean towards the AC control module, whatever, whereever that is lol.

[bwood]

Originally Posted by Scrat

One other thing, when i "jump" the AC relay, the compressor runs continuously, regardless of whether the AC is even "on" on the dash controls.

Forcing the AC relay to engage would make the compressor run continusly as you are basically bypassing the control head. If the compressor runs with the relay forced on, I wonder if your control head is actually working. Using a voltmeter, see if terminal #85 in the socket (should be the lower right hand corner if standing on the drivers side looking down on the relay/fuse center), changes voltage when the AC button is pressed. You should do this with the truck running. The control head basically is "switching" terminal #85 to complete the circuit for the coil in the relay. I went and tested mine, with the truck running and AC button OFF, I read approx 12.5 VDC. With the button ON, I read the current battery/charging voltage (should be 13.5 VDC+). Here is a bitmap of the relay socket (not the relay) to show terminal locations.



Applying power to terminal #30 will engage the compressor clutch provided all the safety pressure switches are satisified. Check for power at #86 and #87, they both should have power in run. #86 is power for the relay coil, and #87 is the power for the clutch circuit, this is what is switched to #30 to power the clutch circuit.

[Scrat]

Thanks bwood.

The results are in...
With the engine on and AC on MAX (relay installed),

#86 = 13.6
#87 = 13.9
#30 = 0
#85 = 13.6

With the engine on and AC on MAX (relay NOT installed),

#86 = 13.7
#87 = 13.8
#30 = 0
#85 = 0

With the engine off and key OFF (relay NOT installed),

#86 = 0
#87 = 12.2
#30 = 0
#85 = 0

With the engine off and key ON (AC ON or OFF didnt matter, AC relay not installed),

#86 = 11.4
#87 = 11.7
#30 = 0
#85 = 0

I also swapped relays again just for kicks and still nothing works. I'm hoping a connecter came loose behind the dash during the recent earthquake we felt here... (sorry, but I'm trying to shed some humor here).

These results lead me to believe that I still don't know what the heck is going on.lol. I've ordered the GMC service manuals and hope they have a kick butt trouble shooting section.

I think I'm still leaning towards a control module or something.

Thanks again for your help

[sparkydog]

#30 seems to be your problem? Looks to me like you have to find out why that's not coming on.

[bwood]

On terminal #85, did you read this from 85 to ground or a positive source. I saw a change in voltage here reading from #85 to a +12 VDC source. Terminal #85 is basically a ground for the coil inside the relay, the resistance to ground changes with pressing the AC button.

With the truck running and AC button pressed, the resistance to ground was .555 mili ohms (basically closed or minimal resistance)

With the truck running and AC off, the resistance to ground was 8.55 mili ohms

I duplicated your tests, my results are in red:

The results are in...
With the engine on and AC on MAX (relay installed),

#86 = 13.6-----13.75
#87 = 13.9-----13.76
#30 = 0--------13.76
#85 = 13.6-----0

With the engine on and AC on MAX (relay NOT installed),

#86 = 13.7-----13.74
#87 = 13.8-----13.75
#30 = 0--------0
#85 = 0--------0

With the engine off and key OFF (relay NOT installed),

#86 = 0--------0
#87 = 12.2-----12.37
#30 = 0--------0
#85 = 0--------0

With the engine off and key ON (AC ON or OFF didnt matter, AC relay not installed),

#86 = 11.4-----11.74
#87 = 11.7-----11.86
#30 = 0--------0
#85 = 0--------0


As you can see our tests were indentical except for the first one. I did notice something, while testing my compressor was cycling (low evap temp and probably a little low on refrig), but when the low pressure switch cut out, so did the relay. So I checked all the switches for this and the low pressure switch and the high pressure switch located on the rear of the compressor both did this. I never saw a change with the switch located in the compressor line manifold. I would recommend testing the switches to make sure they are both closed (no resistance). An open switch will keep the compressor off. Good charge or not, the low pressure switch can cycle a lot when the temp in the cab gets low (low evap temp) or when the system is low on refrig, my point is they do wear out. Something else to note, remove the AC relay and push the AC button in, do you see a slight increase in RPM? When the computer sees the AC button on, it will idle the engine up slightly to compensate for the compressors drag. If the engine RPM isn't changing, you problem may be the control head.

It is not uncommon for these to fail, I'm on my 3rd one and need another one as the fan switch isn't shutting off all the way and will bring the fan on and off intermittantly (this also cycles the compressor on and off if the button is pressed)

[Scrat]

bwood, first I really appriciate your help and effort here...THANKS!!!

I got home late tonight so tomorrow I will continue my trouble shooting efforts.

Curious, where is the control head located and what does it look like? Is it a dealer item or would NAPA or Checker auto have them?

Thanks again

[Scrat]

*** News Flash ***

I found the problem

First I checked the idle speed...It didnt rise at all when the AC was turned on... so I shut off the engine and proceeded to check the switches.
They all checked good (no resistance) so I double check the relay and of course its still fine.

I was chatting with my dad (just retired mechanic of 45+ years) and he checked his all data and said there was a relay by the heater box... I wasnt too sure about this but decided to check none the less. So i open the passenger door and start peeking around and find . . . nothing.

So I'm like wth? I reach over and crank the engine from the passenger side. I push the AC button and hear a strange noise... the compressor is running.!!! Now I'm really like wth!! So i recall what i did... mess with the switches aka connectors. So i go back under the hood and start wiggling the connectors on the switches. Sure enough i wiggle the one on the "new compressor" and well ahh. I can make the compressor cycle (on and off by wiggling the wires).

So now i just need to figure out which component is the culprit. I will snug up the connector first and hope that fixes it other wise i will look at the switch and see if i can do anything with it.

At least I know where the problem is though...thats a relief.


Thanks bwood and sparkydog.

[bwood]

Glad to hear you found it. The control head is where your AC/heater controls are under the stereo. That is one unit with all three knobs/switches, GM calls it a control head.

[asbill71]

did you ever find your problem? Mine is doing the exact same thing...

[Scrat]

Originally Posted by asbill71

did you ever find your problem? Mine is doing the exact same thing...

Yes... see post #13 above. the wire on the back of the compressor.

[teehee65]

First post/comment/question on this site. I realize this is an old post, but hopefully someone will see this and respond. I have two almost identical Chevy Z-71s....a 96 and a 97 with both having the 5.7and automatic transmission. The 96 is old to me and the 97 is to be it's replacement. Now to the problem....

The air conditioning works properly on the 96, but the 97 is experiencing the same problem that prompted Scrat to start this thread. Scrat listed several voltage readings and then Bwood followed up with similar readings from his vehicle. I have repeated those voltage readings on both of my Zs with identical results. My 96 with working ac mirrors Bwood's readings, but the 97 readings mirror Scrat's readings. I swapped relays in the two trucks and nothing changed, so the relay isn't the problem. I also swapped the switch mechanisms (power heads) with no change. So much for the 96. We won't talk about it now. On the 97 all pressure switches check good. Jumping the low pressure switch on the 97 does nothing.The compressor will kick in when I remove the relay and use a jumper to apply power to the clutch terminal.I verified that the system was extremely low of freon with gauges and topped the system off with 2 cans of r-134. As long as the relay jumper is in place the compressor runs and the system works ok. Remove the jumper and all is dead. I have double/triple checked all connections.....there is no power going to the compressor. SUGGESTIONS?

[speedygonzales]

Start another thread.

Use a scanner to see if the PCM is sending an AC request.

[Roughneck4r]

Originally Posted by speedygonzales

Start another thread.

Use a scanner to see if the PCM is sending an AC request.

Hey speedy my pcm is sending a request but won't let relay kick the compressor on! I had a scanner on it today and it said a request wad sent! I have a brand new compressor accumulator and orphace! I have power at low psi switch but not at plug that goes into compressor???? Please help!!!

[Danielroper918]

I am having similair issues with my 1997 Chevrolet C1500. The blower motor only works on the two speeds in the middle (low and high dont work) also my recirc and AC buttons do not illuminate.i have replaced the control head twice both were used however the first one came out of my dads old truck the AC worked in AC worked for 2 weeks then it wouldnt engage clutch or switch from floor to vent to defrost nor switch temperature. I bought another one from a salvage this past weekend and the recirc light lit up for a split second then nothing. If i jump 87 to 30 my AC clutch will engage. My question is what voltage do i need to verify for the head unit as im sure my head unit is fried. I just dont want to buy a new head unit if there is something else shorting it out. I might also mention that my "new" (salvage) head unit will operate all modes except fan speed and AC/Recirc.