Three days ago P0381, yesterday no A/C

[rapidoxidationman]

Related?

Three days ago I got the check engine light on the dash. A quick check with Torque showed that a P0381 code had been thrown (glow plug/heater indicator circuit). Cleared the code, it came back with the next startup and I also noted that the glow plug indicator doesn't come on with a cold start - not that it is cold in the mornings here, but I should still see it for a second or two even when it is 60 degrees. The glow plug controller was replaced on 10 March 2014, about 20,000 miles ago.

Yesterday I fired up the A/C (which had worked well the day before) and was met with air the same temperature as what was outside: 80 or so degrees. Bummer. Turned on the 260 air conditioner backup (2 windows down, 60mph) and sucked it up. Today I took a look under the hood and noted the compressor clutch is not grabbing, but the light on the A/C button on the climate control system does light up. I pulled the connector at the compressor clutch, stuck a test light into both sockets and was met with darkness - no power to the clutch. Put a test guage on the low pressure side of the A/C system, and it wound the guage all the way up tight - no need to add refrigerant.

So. What to do? I'll be taking the multi wire harness off the glow plug controller and ohming out each plug but it was my understanding that the controller will throw a plug-specific code if one fails. I'll also make sure that a solid connection exists for the main power lead into the controller.

I need ideas on the A/C. Any odd chance that the two issues are somehow related? What else should I be checking that is in the way of power getting to the compressor clutch? It's predicted to be in the 90's this week...


Edit: searched "P0381" and read both threads that came up.

Edit, 20 minutes later: I checked the glow plug controller: Circuit A (power from the battery) is fine - a solid battery voltage for that main line in. Circuit B, I checked what I could... All the glow plugs (pins 1-4 and 7-10). Plugs 1-7 are all solid at 1.5 ohms each. Plug number 8 is high at 8 ohms. Pin #11 (ground) is good. I don't know the proper method for testing pin #5 (ignition voltage), maybe someone could enlighten me... Pins # 6 and 12 are the serial data bus and I kept my cotton pickin' meter out of them cuz it seemed appropriate.

Would a single glow plug going bad (#8) cause the whole glow controller system to shut down without throwing a plug specific code?

[rapidoxidationman]

Picked up an AC Delco glow plug to replace #8. Not sure when I'll install it, cuz after the truck has been running all day it stays hot enough to not want to play with. Mornings are filled with getting to work. Might be the weekend, but I'll post up the results.

Any input on why my A/C compressor clutch isn't getting power?

[cabletech]

Maybe a bad pressure switch causing your a/c not to work. Have you checked the pressure with a gauge? Maybe low enough on refrigerant so sometimes there is enough pressure and sometimes not enough to allow the compressor to run.

Jay

[AKMark]

If your pressure is too low, it won't engage.

[rapidoxidationman]

I don't believe the pressure is low: I tried adding coolant yesterday afternoon and the guage on the cheapo walmart can adapter didn't move. I know the new can is pierced properly because carefully unscrewing the adapter results in leaking pressure.

Keep the ideas coming... This first world problem sucks in 90 degree temps.

[rapidoxidationman]

No A/C sucks after 10 hours repairing decks in direct sunlight and 90 degree temps.

Just say'n.

I stopped by the local GM dealer today to talk with Jason and Tim, the parts guys at Teton Motors in Jackson Wyoming, to get a graph showing the acceptable refrigerant pressures with respect to temperature. They did me one better by printing out a 4 page detailed description of "Air Temperature Description and Operation", specific to my truck Thanks guys!

Static pressure is good: The low pressure switch is satisfied when it sees anything above about 25 psi. It'll open at lower pressures than that, and won't close again until it sees something above about 45 psi. The refrigerant pressure sensor, whose job it is to prevent overpressurizing the system, is happy when pressures are below 429 psi, above which it'll disengage the compressor clutch. It won't re-engage the clutch until the pressure drops to less than 229 psi. The static pressure, without anything working, is about 130 psi both on the high and low pressure sides of the system, as measured by my manifold guage. Refrigerant is no longer in question.

I'm not seeing any 5 volt signals either on the low pressure switch or at the refrigerant pressure sensor. Something isn't recognizing that the request is being made for the A/C to work, even though the little yellow light does turn on when the A/C button is pressed on the climate control system.

So: the following conditions must be met for the compressor clutch to turn on:
Ambient air above 40 degrees F check
A/C low pressure switch signal circuit is grounded problem, no signal
Refrigerant pressure sensor parameter less than 429 psi again, no signal
PCM receives an A/C request from HVAC control module check, as best as I can tell but I might start poking into wires to confirm
engine coolant temperature less than 250 degrees F check
engine RPM more than 550 rpm check
throttle position less than 100% check

Where is my 5V signal?

I checked EVERY fuse both in the cab and in the engine compartment. All are good.

Anyone?

[rapidoxidationman]

Apparently mice like dark blue colored wires, as proven by the fact that both dark blue wires on the instrument panel side of the C100 I/P harness to Engine Harness plug underneath the underhood fuse box were eaten alive. One is the low pressure signal wire for the A/C, and the other is for... the wait to start indicator. Sorted the wires, spliced 'em back together, vacuumed out the debris left behind by the f~(k!ng mice and the problem is solved: the AC works and the glow light is back.

(snarkiness deleted).

[Kennedy]