so on my '01 blazer, the a/c compressor clutch will engage for five seconds and shut off for twenty both randomly and intermittently. i do not use the auto setting ever. i am a grade a control freak. i went so far as to retrofit a manual choke on my weber aspirated cj-7.

back to the blazer. the only constant is that the blower motor has to be turned on for this to occur. if i leave the blower off, it does not happen. being december in new england, i employed the quick fix of unplugging the compressor. but this just fixes the symptoms and not the disease.

i also had a temperature control issue (and a broken mode door arm) so i replaced the whole electronic climate control box with another used one. i ohmed out the potentiometers and both units read about the same. though there seem to be identical dead spots on the pots for the temp control and i cant seem to figure that one out.

i just dont like how touchy the heat control is and how slowly the temperature actually changes compared to my memory of previous years.

so now i am looking at replacing the blend door actuator. i can feel the motor spinning and can also feel an end of motion such as a flap closing. but that of course doesnt give me a definitive answer. i will r and r it soon. i have too many more pressing items to fix first.

so now for my question:

what decides to send power to the a/c compressor?

yes i know the a/c button or the defrost mode setting activate the system. but what other controls are involved in engaging the clutch?

and can the problems all be one problem? meaning the mode door arm being broken makes the control box think the defrost is on when it isnt. is there some sort of sensor which determines mode door positioning or is it all done on the control box switch?

any input is appreciated.

 

The HVAC control module sends classII serial data to the PCM requesting A/C. The PCM ultimately controls the A/C clutch by grounding the trigger terminal on the A/C compressor clutch relay. Troubleshooting Automatic Climate Control requires a scanner capable of accessing the network, (the El Cheapos can't do it).

 

im with you as i just unplug my compressor every winter. i understand why it goes on with defrost i just dont like it cycling so much everyday using it

 

I agree^^^ a simpler, (and cheaper) solution is don't get a vehicle with ACC

Just like a convertible top, ya gotta have one to find out you don't want one

 

so it is "normal" for these models to cycle the a/c all the time? and in 60k miles, i never noticed it? if thats the case, boy do i feel stupid.

i was thinking that there was a sensor in the blend door actuator telling the control box its position and having a fault in that could cause excess a/c usage.

i do not like the idea of not running the a/c at all from october to april. i have seen too many of them seize up from lack of use.

i have a reefer van and i make sure to run that system weekly. at least for a few minutes.

 

It is not normal for the clutch to cycle all the time. The system is quite complex when it comes to determining if and when to cycle the clutch. The interior and exterior temperature sensors, sun load sensor, along with where you have the temperature set, all play a part in when the clutch cycles. Could be a problem with any of those, or their wiring, that's why you need a scanner that can access the network: to retrieve DTC's and watch live streaming data. By the way, BCM DTC's start with "B" and can not turn the SES light on. Only powertrain DTC's, (start with "P") can turn the SES light on.

 

first let me thank you for your time.

thank you. you seem to know what is going on with a system i have little experience working on.

now... does the complexity of the system remain if i never use the auto-climate feature?

i understand why all that stuff would come into play when it is in auto-climate mode.

but does not the manual functions over-ride the need for the sensors?

ultimately i agree with you that a trip to the dealer for a scan is easier and probably much cheaper than throwing parts at it until it works as it should. but i have several contacts in the local junkyards and can get nearly any small item for close to free. they usually just melt them anyways.

 

The manual function overrides where the air flow comes out, and the blower speed. On some model years, the mode switches have a seperate button to control the compressor. Does yours have the snowflake icon like this one? If so, there should be an amber pilot light in the button: if the light is off, the compressor should not come on.

 

thank you captain.

my control box part number is 15756179. i think it is identical to the one in the picture in function but not appearance. but i think the difference is just olds to chevy. the ***** are different and the face has different icons.

it does have the snowflake icon on the button.

and even when the amber light is off, the a/c compressor will still engage.

 

That's a problem. When you turn the amber snowflake pilot light off, you are manually disabling the compressor. Somewhere along the line, the trigger for the clutch relay is being activated when it shouldn't, or the compressor clutch relay contacts are stuck together. You can try swapping the relay with another one of the same design. Should be one in the underhood fuse panel you can swap. Worth a try. Something to keep an eye on during the swap: when you remove the relay, the compressor should shut off.