Heat & AC Control Module schematic

[GMTGMC]

To make it short. My AC Clutch relay will not work because it has inadequate voltage. This appears to be ground switched through the controller. There is 12v present on the other side of the circuit and adding good ground to the relay will cause it to work properly. So, this appears to be a bad connection or corrosion. The blue/white wire from the relay to the controller is good. I am trying to determine which wire is used by the controller for ground, so I can check that for voltage drop....although the blower relays work fine, so unless there are multiple grounds to the controller then perhaps it is bad. However, when the AC switch is pushed in, I get a signal on the blue/white wire.

I hate to replace the controller but may have to do so. I can find nothing that shows all the wires in the harness connector to the controller. An the best schematic I have found shows a different color wire going from the relay to the controller, making me no trust any of the other info.

A schematic, any thoughts, or experiences, would be appreciated.

 

[AK49BWL]

You are correct in that all the relays are ground-switched. I would have to assume corrosion or a loose connection over a bad controller in this case tho... The ground switching chips in the controllers don't usually fail. Try pulling the connector off the controller and off the relay, make sure all the pins are still where they should be and relatively clean, plug it back in again. Sometimes all it takes is a wiggle to fix the issue.

This here is part of the schematic I made for my entire truck, should give you an idea of what's what in the HVAC harness itself (not so much the I/O connector and relay/recirc disabler switches as that's entirely custom to my truck) - tho mine's based on a 4-speed 94 controller, not sure what you've got but it should be similar.

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[GMTGMC]

You are correct in that all the relays are ground-switched. I would have to assume corrosion or a loose connection over a bad controller in this case tho... The ground switching chips in the controllers don't usually fail. Try pulling the connector off the controller and off the relay, make sure all the pins are still where they should be and relatively clean, plug it back in again. Sometimes all it takes is a wiggle to fix the issue.

This here is part of the schematic I made for my entire truck, should give you an idea of what's what in the HVAC harness itself (not so much the I/O connector and relay/recirc disabler switches as that's entirely custom to my truck) - tho mine's based on a 4-speed 94 controller, not sure what you've got but it should be similar.

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Thank you so very much for this and your kind consideration. It will be a great help.

I have not given it a proper look yet, but let me prey upon you knowledge.

I do not understand my problem. The clutch will engage and all the pressure switches work as they should. The problem is that when the AC switch is pressed the relay does not engage the clutch. If I substitute or add in a good independent ground, on the control side, the relay will engage the clutch (but then, obviously, there is no control). The control module works perfectly otherwise (blowers, recirc door etc). The control wire from the module to the relay is good (less than 1 ohm resistance). I have taken the control mostly apart and found no obvious solder issues or blown capacitors etc. I did not get to the back side of the AC switch, but it appears to operate crisply and normally. There are several wires in the connector that ohm to ground. Some have high resistance. I dont know which is the path for the AC switch (perhaps your chart will help me better determine that), but I have back probed one of the high resistance grounds (56ohms) and jumped it directly to the control on the relay and the relay operates as it should. all the other relays (blowers) work fine with the controller. It would seem the issue is either the ground the switch uses is bad (which is doubtful since even the high resistance ground I tried would operate the relay), or the switch on the controller is somehow faulty. Still, that switch seems good and I can tell it has at least some effect on things when engaged.

So, if you have further insight, it would be most appreciated. Finding a different controller is a bit of a pig in the poke. They are all used, they only test to see if they light up...so one can't know if they are going to solve the issue.....plus they are quite expensive now.

Thanks again for your help

Gary

 

[AK49BWL]

We are talking about one of the 88-94 digital controller modules with the VF Display right? I'd have to assume so given the later models don't use relays for the blower (except high) and A/C... If you press the button, the A/C text should appear on the display, if that works then the switch is fine and the problem is likely internal (IC or other circuit board component) given the things you've already tested. Given the age of the system, component failure is likely, especially electrolytic capacitors which don't always show their failures visually.

 

[GMTGMC]

You are correct...it is a 'o4. As you say, the AC lights up when the switch is pressed, which is proof the switch is doing at least something. All that is required for things to work properly, is for the switch to send a good enough ground signal to activate the coil on the relay. I can't imagine there are any electeonic components inline with that process.

I am going to use your chart and basically try to find which wire, in the harness, the switch is using for ground. As I said, I have found several that have varying degrees of continuity to an independent ground. Maybe I can find that circuit and see how good a ground it is, or if there is resistance somewhere n the IC. I have it in my mind that the controller is okay and that it is a bad ground connection somewhere, maybe in a splice, since there seem to be no other ground issues.

I seldom use the truck and I may wind up wiring another ground to the relay, with a push button switch inline. It'll never see a long trip and maybe I wont forget I have over-ridden all the safety switchs and wind up blowing out the system.

At any rate, I want you to know I appreciate and value your responses. I'll be sure to let you inow what it all winds up being.....when I have finally given up!

 

[GMTGMC]

Brandon

Just a FYI. I could never find anything wrong with the module. However, despite the fact that everything else works fine, and, even the AC light goes on and off with the switch, I could never make any of the pins ohm to ground with the AC switch (at the internal pin connector or at the harness connector). So, I am adding a separate switch to send ground to the clutch relay. That keeps the circuit in series with the pressure switches, for protection. Jad to add a relay for the switch so I could have an LED indicator. Never could find another module that ai had confidence in, given the peculiarity of mine, at the price point they are asking.

Just wanted you to know that your schematic was a Godsend on getting everything tested and worked out. Thank you so very much.

Gary