![]() The control head somehow knows when vehicle is in gear or park- this is extremely repeatable- always 12v in reverse-always 0 volt when in Park- I've cycle it 50 times- always repeats- and follows the shifter position.- there has got to be a relationship perhaps at park/neutral switch 2nd connector- which I haven't found a wiring schematic for. I still cannot figure out relationship between control head and the Park position. The plot thickens some more- I decide to isolate the circuit- cut the a/c request ( dk grn/wht) wire at the PCM- now monitoring the a/c signal from control head with DVM I get: In Park- 0 volts In Reverse- 12 volts in overdrive - or 1,2,3,flickers between 12 and 0 so-I also connect a power supply set at 5 volt- to PCM input dk grn/wht - with dk grn/wht still cut away from control head-and a/c works continuously in all shifter positions- no flaws- So I have learned that the a/c request signal must be 12v- (as when Isolated and in reverse) and it is being pulled down to 5.4-6.3 volts even when it works in reverse - looks like power supply could have been set at anywhere between 5 and 12v to enable a/c. 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak. Anybody out there ever have one like this- or is this just my lucky day!!!! - The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!! Just a thought. Foot on brake may be a factor as well.but not sure yet. Can anyone find a wiring diagram that relates a/c request signal to the park/neutral switch or to TCC circuit? - perhaps I'm getting a short to ground when in Park in a P/N switch. I called the owner- he said his wife's biggest bitch was that when she'd start the car to cool it down while she was loading it- that it did not cool-since it was in Park- so we've confirmed that- I told him, I was going to test drive it- because I could not get a/c to come on- but it cooled fine when I pulled it in to my shop.there is something terribly amiss here. One of my issues is that I can't seem to get the face plate (facia) removed that covers the radio and control head- there are two 7mm screws beside cup holder- the rest snaps out- but there is evidently a screw or two behind the cupholder- and the cupholder refuses to come loose to remove it-the stop tab is extremely rigid- looks like it should bow down to remove- but it doesn't do it- gee, I hate to break it loose.anyway, I tested at PCM input which should be just as good as control head output and I get the 5v signal there- just not when vehicle is in park- which took me two hours to figure out- since no one usually test a/c circuits in anything other than Park. The 1996 U-van service manual I have clearly states that the a/c request signal goes to PCM and a parallel path the TCC circuit- I will look up wiring diagram for it to compare to a 2001 U-van. Whwn a/c is on, (not in park and foot on brake), I can turn on/off the 5 volt signal just fine with the a/c button. I get 5 volts from grn/wht at PCM from control head whenever vehicle is not in Park-and mostly on when in any other gear.nothing else in control head is amiss- beating on it does nothing. Okay- plot thickens- I thought since I was having no luck parked- and that owner says a/c is intermittent- and my test it has been several hours running with no a/c request at all- that I should take it out and drive it-with volt meter attached to drk grn/wht at PCM-and tech II showing no a/c request- I drop it in reverse and a/c request is suddenly yes and volt meter show +5v-Then back in park and request is no and volt meter is zero.drop it in drive- request comes on, then drops out-RPM related? I can reve it any amount in park and a/c request and volt meter are negative.drop it in any gear, or neutral and request is now intermittent- but mostly on- just as owner states.WTF I say.older vans 1996 or so had a lead from control head that fed TCClock-up- so that torque converter would lock up with a/c on- to keep from feeling engine drag and cycling clutch I assume-but do these newer ones 2001+- the schematic for HVAC shows none. Seems I get no a/c request at all- and when I check a/c rquest signal (grn/wht wire) from control head- I get no voltage- Tech II will energize a/c relay and compressor- when reuested on Tech II relay checker.but this thing was cooling fine when I drove it into my shop. So here it comes- complaint is doesn't cool all the time- charge level pulled and weigheded out fine. Year: 2001 Make: Chevrolet Model: venture single system Engine Size: 3.4 Refrigerant Type: 134a
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