A/C electrical issues from hell

[2jz_mickey]

hey all first post, i have searched the forum and some others multiple times and i cant seam to find a similar issue.

so i bought my truck (95, 305, 4l60e) sans a/c (it was equipped but components were missing) and installed a full system. evaporator to condensor, blower, and hvac panel (found the transistor to be faulty not providing power to the light green wire).

after about 2 days of trouble shooting i found the transister fault and fixed.

i discover that if i apply power to the dark green wire on the high pressure switch (back of compressor) the ac compressor kicks on and cycles.
i have 13.4 volts coming from the light green wire (from control panel) to the high pressure switch.
the high pressure switch if functional (i ohmed it out)
when i jumper the light green with dark green (bypass the high pressure switch), ground runs back to green wire as opposed to power going through the system.

what am i missing? anyone have this issue before?

 

[1998_K1500_Sub]

I don't have committed to memory the different wire colors and their function.

And I can't figure out what you're trying to say when you state "ground runs back to green wire as opposed to power going through the system".

But...

My advice is to download the service manual and check a wiring diagram and, if that doesn't help enough, the troubleshooting charts.

Look here:

GMT400 list

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

and here

GMT400 CK Service Manual Project

GMT400 CK Manual Project - The 1947 - Present Chevrolet & GMC Truck Message Board Network

GMT400 CK Manual Project The 1988 - 1998 GMT400 Chevy & GMC Pickups Message Board

67-72chevytrucks.com

 

[2jz_mickey]

I don't have committed to memory the different wire colors and their function.

And I can't figure out what you're trying to say when you state "ground runs back to green wire as opposed to power going through the system".

But...

My advice is to download the service manual and check a wiring diagram and, if that doesn't help enough, the troubleshooting charts.

Look here:

GMT400 list

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

and here

GMT400 CK Service Manual Project

GMT400 CK Manual Project - The 1947 - Present Chevrolet & GMC Truck Message Board Network

GMT400 CK Manual Project The 1988 - 1998 GMT400 Chevy & GMC Pickups Message Board

67-72chevytrucks.com

This is the diagram I have been working from

 

[1998_K1500_Sub]

if i apply power to the dark green wire on the high pressure switch (back of compressor) the ac compressor kicks on and cycles.

OK, that makes sense given your diagram. This tells us the rest of the AC clutch control circuitry is working as it should... or so it seems to me, anyway.

And by the way: What *exactly* is the problem you're trying to solve? Is the AC clutch not engaging when you toggle the AC switch on the Control Panel?

i have 13.4 volts coming from the light green wire (from control panel) to the high pressure switch.

Makes sense. You might want to check that this LT GRN lead from the control panel can supply a little current, e.g., enough to light a small (#193-type) light bulb.

the high pressure switch is functional (i ohmed it out)

You're saying it's "closed" as it should be, i.e., zero or thereabouts ohms... yes?

when i jumper the light green with dark green (bypass the high pressure switch), ground runs back to green wire as opposed to power going through the system.

I understand the "bypass the HP switch" part; it sounds like you're trying to check the switch to see if it's causing a problem.

I don't understand "ground runs back to green wire...". What's the voltage measurement when you do this?

Check (as I mentioned earlier here) that the LT GRN lead can supply some current, and read my next post because it may be relevant.

 

[1998_K1500_Sub]

hvac panel (found the transistor to be faulty not providing power to the light green wire).

Have you seen this circuit diagram (attached)? If that transistor (red arrow) is the one you replaced, I'm guessing it's not quite "right". Are you sure you installed it correctly... you didn't switch the "collector" and "emitter" leads when you installed it, did you?

What's the part number for that transistor?

I'm starting to think, from what you've said, that this "fix" might not be "fixed". But we'll learn more when you test that circuit with a small light bulb.


(diagram from https://www.silveradosierra.com/threads/how-to-repair-gmc-ck-series-trucks-hvac-control-unit.77522/)

You must be registered for see images attach

 

[2jz_mickey]

Alright I will check current supply, I did not replace the transistor, I changed out the entire hvac panel.

I will be looking into after work today. What would be some viable issues if there isn’t sufficient current?

 

[1998_K1500_Sub]

I will be looking into after work today. What would be some viable issues if there isn’t sufficient current?

If that LT GRN wire from the Control Panel can't supply sufficient current then, when you toggle the AC switch on the Control Panel, the voltage the LT GRN won't pull up to +12 and will remain low (possibly almost to 0V... "ground")... thus the clutch circuit won't function.

 

[2jz_mickey]

If that LT GRN can't supply sufficient current, the voltage will droop (possibly almost to 0V... "ground") and the clutch circuit won't function.

I typed issues and meant culprit , as in what would cause the lack of amperage? Besides a parasitic draw or a short? (I’ve check for a short on both ends of the system and found none)

 

[1998_K1500_Sub]

what would cause the lack of amperage?

Answer: A high-resistance connection somewhere in the circuit (poor connector, bad solder joint) or, if your Control Panel does indeed have that transistor (see my prior post), its failure or mis-installation, or the failure of related parts (the resistor and diode shown in the diagram)

 

[2jz_mickey]

Answer: A high-resistance connection somewhere in the circuit or, if your Control Panel does indeed have that transistor (see my prior post), its failure.

Understood, will check back in this afternoon