AC OFF without ENG HOT

[zebra]

My wife says her truck's lately been randomly displaying AC OFF on the gauges, which would be expected if it's getting close to overheating... but when asking her about the temperature, she says the temperature stays normal and never displays an ENG HOT message (which the owner's book says accompanies AC OFF).

Sometimes, she'll get a CEL but not always. Today got one for P0128 (thermostat).

She also says it happens more often after a weather swing (hot to cold or vice versa).

Thoughts? I'm guessing a sensor's going bad.

 

[zebra]

i know the radiator & coolant were replaced while i was deployed summer 2021... can't remember if the thermostat got changed at that time or not.

 

[rascole]

I had a similar issue years ago, I changed the coolant temperature sensor and problem went away. Easy to replace, AC Delco 213-4233.

 

[zebra]

thanks. i might just have to drive it a few days & see what it does for me, but hopefully it's something relatively simple like that.

 

[zebra]

and looking up P0128 sounds like maybe just the thermostat's intermittently stuck open & causing it to not hit normal temp fast enough.
...don't know why that'd lock out the a/c compressor, though

 

[Alpha X]

Has the truck been in mud recently and it's still in the condenser and radiator, and/or it splashed into the airbox and it's on the air filter, causing a restriction in air flow?

 

[650Hawk]

If the sensor is going bad/intermittent; then the system will turn the AC off as a safety default, since it doesn't know what temp the coolant is at.

 

[Doc Olds]

The H3 has three levels of overheat protection. The H3 will first shut off the A/C if it was on and display "AC OFF" in the DIC; then if temp still rises and/or continues, display "ENG HOT" in the DIC and chime (ding, ding, ding); then if temp does not fall, go into Reduced Power, all before reaching an actual overheat situation at 260F +.

I think you are on the right trail. If the Tstat is not operating correctly then the PCM cannot determine true reality and it is reacting as best it can to what it thinks is going on.

How many miles on it? Is the Tstat original or already been changed out once?

Good luck.

 

[JPaul]

I would definitely start with the temp sensor. I put a new one in when I rebuilt my engine and not long after started having intermittent issues with that very same issue, primarily after starting it up after sitting overnight. I have an Ultragauge and eventually noticed that when that would happen I could see the readout was throwing -40 for the temp, which definitely is not right. Finally got around to replacing the sensor and having had any problems since.

 

[zebra]

forgot to check back in on this.

it ain't happened but a handful of times recently... but on a t-giving road trip this year, i noticed it happen a couple times while i was driving, and the engine temp showed full cold (having been on the road for hours & immediately prior showing right in the middle of the temp gauge). sounds like a failing temp sensor.

 

[Jeepwalker]

OP, Do you have a scan tool you can watch the temp readings? Or you can simply ohm out the temp sensor (back by the firewall) and compare the readings (engine cold & engine warm) to a temp-ohm table.

Also feel the pressure in the upper radiator hose when it gets hot. If it's so darn tight like an over-inflated balloon, maybe it IS the T-stat.

 

[rascole]

Sensor is $20, you can replace it in 20 minutes. AC Delco 213-4233.

 

[650Hawk]

forgot to check back in on this.

it ain't happened but a handful of times recently... but on a t-giving road trip this year, i noticed it happen a couple times while i was driving, and the engine temp showed full cold (having been on the road for hours & immediately prior showing right in the middle of the temp gauge). sounds like a failing temp sensor.

Which is pretty much what I said nearly a year and a half ago.

 

[lfootmatt]

I clipped the temp sensor plug and had to replace it. Total PITA!!!!! Instantly hits that AC OFF mode with the climate or AC controls blinking as I recall.

Good luck!

Matt

 

[zebra]

Which is pretty much what I said nearly a year and a half ago.

yep, thanks.
i was coming back here to confirm & concur with your diagnosis (for anybody else who's answering the same problem)

 

[zebra]

Sensor is $20, you can replace it in 20 minutes. AC Delco 213-4233.

what tool gets this done in 20min? farted around with this for an hour & none of my wrenches are short enough to not hit the firewall, and the wire prevents putting a socket over it.

is this one of them "gotta go buy a set of crowfoots" situations? even if i had a grinder, i sure ain't cutting up a good socket (or buying one to cut up) just for this...

 

[rascole]

Harbor Freight socket with one of the sides cut off so wires can stick out.

 

[zebra]

haha... sounds a lot like the single-purpose item i just said i was trying to avoid & don't have the other tool to make.

 

[rascole]

haha... sounds a lot like the single-purpose item i just said i was trying to avoid & don't have the other tool to make.

but a $4 socket and 2 minutes to cut is cheaper than a "specialty" socket and it can still rest in the tool box until next time.

 

[zebra]

which also involves buying an angle grinder to make a socket i don't foresee another use for.
neighbor had a 15mm crowfoot. still took a bit of finagling to get the right angle around the heater hoses & needed a cheater bar on the ratchet... but i got the sensor swapped yesterday.

started out still showing AC OFF, but after getting the truck up to temp, it seems to be working alright.