Ok, the title may be a bit misleading. My better half has a 2007 Cobalt that has the a/c working intermittently. When the air doesn't work, the temp read out is just dashes, and the radiator fan is on. I changed the coolant temp sensor, and that had no effect on the problem. I'm thinking either the wiring, or the BCM. Anyone seen this issue before?

Thanks,
Rob

 

Does it heat full time or intermittently?

Is the A/C intermittent in any pattern?

The fact that the BCM turns on the underhood fan leads me to believe it is not the root cause of the problem, since the AC Request is handled properly by the BCM, enabling the Cooling Fan and AC Relays.
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To me it sounds like there is a loose connection between the AC head unit and the temperature sensing device that feeds it. Gonna guess it is in the central ductwork in the front and senses the air temp when it enters the cabin. There is probably one for measuring the recirc air temp being pulled back in so that the system can use the two temperatures to reach an equilibrium (auto climate control)

So to me the issue is with one or both of those sensors or their connections to the AC head unit.

GUESS

 

Derf,

Thanks for the input. I may have misled you a bit. When I said temp display, I was referring to the coolant temperature read out on the information center. I'm thinking that since the computer doesn't know the engine temp, it defaults to running the radiator fan and killing the a/c.

 

When she gets home, I'll hook my scan tool to it and see if I can monitor engine temp. If it is zero, the the cause of that problem is probably the answer.

 

Ummmmm,

That was a TINY bit misleading......
But I stand by my analysis of the non-existent issue.

Ok moving on, didn't know those cars had a digi temp display.


EDIT: Read the whole thing in one sitting -- I'm not so sure it is the wiring anymore.....

Since it was intermittent, and changing the ECTS had seemingly no effect, positive or negative, I'd go for wiring -- more specifically the 5V wire supplying the sensor being chafed and intermittently shorting on the frame, which is a damn low resistance, which corresponds to way hot, which , as we learned from Keith, triggers the fail-safe operation of the cooling fan. Where as an open circuit makes the car act like -40C, the shorted source voltage/shorted ECTS would be expected to trigger the failsafe fan operation.

I've not heard of many actual ECTS units failing this way (shorted). but there have been a handful. But we know yours is not one of them since you replaced it w no effect.

Take a really good look at the connector and wiggle it while someone watches the gauge.

After that, check for 5V to frame ground from the power lead to the sensor.
In theory, if my hunch his correct, you could then set your meter to continuity mode while tracing the hot lead back to the BCM; if it is chafed and bumps ground, the continuity "EEEEAAAAEAEAEAEAAAHHH" should go off....maybe....depending on what max resistance your meter considers continuous.

or I suppose you could watch the cooling fan.

Wait, no you can't because that should always be on when the AC is on, so the fan is on with or without AC actually cooling but IS on when AC is requested (AC is turned on), so again, per my earlier erroneous analysis, the BCM's logic functionality w r t the fan circuit seems intact.

It sounds like there is an additional step in the Cobalt BCM logic that requires the coolant temp to be below a certain temp so that engaging the AC from the panel does not overheat the car.

What temp is the coolant temp sensor reading at op temp at idle? And standing at idle for 10 min?
Might you be running really hot without realizing it?

Will the behavior cycle in the driveway AC on/Fan on --> AC off/Fan on (cools engine below threshold) ==>AC on / fan on

This may very well be dependent on ambient temperature -- how hot is it down there? Coolant flush lately?
___________________
I HAVE heard of GM vehicles to this day (the Cruize) shutting of the AC when they sense a need for addl acceleration

 

I was cruising the cobalt forums last night and it seems that this may be the thermostat. Her car has a low coolant temp code that according to the forums, will kill the ac and make the temp display all dashes. Everyone claims that the thermostats have been redesigned and there is a gasket on the old one that fails and gets stuck between the valve and the seat. They said it is crappy programming that is the fault. I'll check the two wires from the coolant sensor before I change the thermostat.

 

Dtruck man,

You're hiding codes from me now????
Give a guy a chance!!!! (just messin with ya)

Well I guess I was barking up the right tree but the wrong branch.

I guessed too hot, programming is for too low coolant temp (dropping it out of operating range and running rough). Might be for both -- hope you never have to find out.

Low coolant temp light could be wiring and is worth checking but BCM logic + thermostat + known problem.....

 

I wasn't for sure about a mil lamp code. So, scanned the car tonight and sure enough, it has a P0128 which is engine coolant temp below thermostat regulated range. According to the cobalt forums, the programming that deals with this code will blank the temp readout and cut the A/C. Looks like I'm doing a thermostat change along with an oil change this weekend.

Along with putting up 1500 bales of straw, weather permitting.

 

Glad you found your way to it---not the summer to have finicky AC...

I was born n raised a suburban city slicker. Is that a lot of bales?
1500 of anything sounds like alot.....

 

I don't miss the days of hauling hay. What size bales are they, Rob? 40's or 50's? 1500 is still a LOT of hay in one day.