HVAC blowing fuses.....

Crap! Panel went dead..... Probably a fuse.....FSM indicates fuse number six, sure enough it's blown!..Put another in and 30 seconds later it's done as well.....Same with the next three...
It became quickly apparent that the diagnostic paths suggested in the FSM would require actual schematics with numbered circuits, so I went with a few "old school" assumptions and floundered into my cloudy understanding of electronics by eliminating the peripherals as possibly causing this problem......
I reasoned that the HVAC module itself was probably okay because it would work normally for 30 seconds before the next fuse took it out......I also assumed, CORRECTLY, that either the blower or the compressor clutch (or both) might possibly be powered through the module from that fuse, rather than being switched in a heavier-load circuit by a relay from the control panel......I started by unplugging the compressor plug and running the panel (new fuse in) for 15 minute with the fan on high.....Obviously the fuse lived, so the compressor clutch draws too much amperage (over 30 amps against a 20 amp fuse)
While I could have just replaced the clutch solenoid ($120) or the entire compressor (about $200 to do it right with a non-oem reman)......I decided to instead give the compressor clutch it's own power source with a 30 amp resettable breaker and some 12g wire......In the FSM schematic of the Clutch power path, a light green wire goes out to the cycling switch....A light blue wire then goes to the compressor relay (From the cycling switch) but splits into both the relay control wire AND THE RELAY-SWITCHED POWER WIRE! I clipped this power wire away and put power to it from the battery cable power post through the resettable breaker..... I chose 30 amps because it's ten more that it had, but is probably below the threshold of melting the wiring or plugs before tripping......The compressor relay NOW just switches the higher-amp compressor clutch circuit that NOW has an isolated, independent, heavier-duty power supply path....... The original fuse no longer powers the clutch, but powers the panel which switches the clutch power with the relay.......System now works with no more blown fuses!
Oh, and my only cash outlay was $4 for the relay and about $10 for some wire and mic wiring supplies that I needed anyway!..... And a box of fuses!

well done,. now as time goes on and the coil in the clutch gets worse,. have you priced 240V switching box and 00 guage wire?,...

Welll.... I think the voltage will ALWAYS be 12, but your point is relevant, as it is the amperage that melts stuff...... I think I'm already as far as I can go, amperage wise, without risking melting stuff..... EVEN IF I have to next replace the compressor clutch, giving it it's own high-amp source of power is STILL AN IMPROVEMENT! I'm having a difficult time understanding why GM's engineers thought it was a good idea to power a high-draw accessory through an electronic control panel (?)

AzDon wrote:

I'm having a difficult time understanding why GM's engineers thought it was a good idea to power a high-draw accessory through an electronic control panel (?)

So that you could shut off the compressor by selecting "Vent".

The panel "controls" the compressor the same as it always did, It just does not any longer "power" it.....

is that like Viagra for Roady A/C?

AzDon wrote:

The panel "controls" the compressor the same as it always did, It just does not any longer "power" it.....

The design likely powers the compressor via a mosfet transistor or solid state relay incorporated into the panel. Nothing wrong with that; in fact, there are several advantages:

- Infinite lifespan (barring electrical overload). No moving parts, pitted contacts or carbon build-up.
- Constant output resistance over time.
- Bounceless operation
- Not sensitive to mechanical shock, vibration or humidity.
- Fewer wires and connectors than a separate relay would require.

what's kinda lame is that there IS a compressor relay, but it was not wired to be a remote power station for the clutch.....hard to say what purpose it was there to accomplish......The original schematic powered the clutch for 24 years without losing the panel, so I guess they knew what they were doing.....A need arose that provided an opportunity to take some of the load off the panel and I believe it is an improvement that will help the panel live longer......

I also found that the ground path for the compressor relay is back to an ecm port that was dead, so clipped that wire and grounded it.....A few days later, the ground path for the initial AC relay also stopped working....Unfortunately, that ground path functions as the switch and is inside the HVAC control panel., solo... I pulled that relay and jumpered the switched pins with a toggle switch so that the panel no longer switches the compressor.....The panel seems to function normally in every other way and hopefully isn't dying....Chances of finding replacement HVAC control heads are probably not good...

GM has energized most of their electrical circuits by completing the ground, for as far back as I can remember, and that’s awhile! While it works, and there must be some sort of reasoning behind this, it has never impressed me. Keeping the live DC powering voltage on so many different circuits seems like “asking for trouble”, I’d think.

But anyway, the description of your issues raises the possibility of one or more bad grounds to the chassis. Have you gone through the car and checked (and maybe cleaned) the major grounding points in the car? These cars are getting a lot of years on them, and a little corrosion here and there can create big headaches.

The theory for the switches grounding the circuits is to allow them to use lighter duty switches, because the load drops the voltage before going through the switch.

If I remember correctly, the poles of the battery were originally labeled backwards, and it has not been corrected, therefore the electrons go from the - pole to the + pole. So it seems that it may not make any difference where the switch is located.

That’s one of those “circuit theories” that’s been confusing people for 100 years.
Current flow is from + to -, but electron flow is from – to +. (Don’t ask!)   There can be circuits that would allow a lighter contact current or voltage on a “ground-side” DC switch, but in most cases there would be no difference.
Having a device “hot “ at all times isn’t permitted where safety is an issue, i.e. AC home and commercial appliances, as an example.
Let’s just hope that batteries remain labeled as they have been since their invention! At one time there were vehicles manufactured with “positive” ground (Chrysler-?). That was bad enough!

A number of British cars had positive ground electrical systems as well. I believe the Triumph and BSA motorcycles were positive ground.

paart wrote:

At one time there were vehicles manufactured with “positive” ground (Chrysler-?). That was bad enough!

Most vehicles were positive ground (and only 6v) back when wires were insulated with cloth. Moisture would penetrate the cloth and cause galvanic corrosion of any positively charged copper conductors. Making the conductors negative made the wiring more reliable.

IIRC, the switch to negative ground (after the introduction of modern moisture-resistant plastic wire insulation) improved spark plug performance. The majority of erosion occurs at the negatively charged electrode. Since the centre electrode is negative in a positive ground vehicle, the centre electrode would erode away, changing the effective heat range as it wears. Switching to negative ground meant primarily the outer electrode eroded instead. The effective heat range remained unchanged, and one could easily re-gap the plugs by bending the outer electrode.

Of course, metallurgy has improved considerably and modern plugs with precious metal electrodes are far more resistant to wear. So we have wasted spark systems (one plug of each pair is positive and the other is negative, relative to each other), Coil-On-Plug systems (both plugs are often negative) and Ion-sensing Ignition systems (plug is negative during spark and positive during sense), without any issues.

So now the choice of negative ground is more to do with convention than anything else. If you tried to plug an electronic accessory into the lighter plug of a positive ground vehicle it either wouldn't work or it would release the magic smoke.

I had a "White Compact" diesel semi with a 220 Cummins that has a negative ground system AND it had a 12 volt system with 24 volt starting using a series/parallel switching setup.......
My 68 Suburban has wipers that are switched to ground and virtually every stock hornbutton I've ever seen grounds to the steering column.....