Voltage levels dropping when AC turned on in my 67 GTX?

[AR67GTX]

Well, no joy in Mudville. After struggling through a bulkhead connector replacement I still seem to have the same problem. At least I did get some peace of mind from that work as I found this.

After I got it back together and started it up I did notice that my running voltage levels are basically dependent upon AC fan speed. With air on the voltage loss on low fan speed is negligable. Put it on medium speed and the voltage drops 1 - 1-1/2 volt. Put it on high speed and it drops 2 - 3 volts. I can’t test on max right now as my linkage is out of adjustment and I haven’t got around to it yet. The alternator gauge is in sync with the voltage gauge and starts showing a discharge as the voltage sinks to 12 or even lower.

I tried the heater setting and found the fan didn’t work. That turned out to be a blown fuse and replacing it brought the fan back in operation on heat.

Took the mechanical voltage regulator off and put the old 15 volt electronic one on but had the same results. At idle with air on I’m down to about 12 volts. Turn on the lights and lose another little bit.

With AC off the lights drop the voltage slightly at idle - pretty normal. I don’t have any relay modifications. Pulled the AC compressor connection apart in the engine bay and cleaned it up but that seems to make no difference. I suspected the clutch before but not now as at low fan speed the voltage drop with AC on is little. I don’t think compressor clutch pull would change with fan setting.

Although the fan motor works I’m now beginning to suspect it. But it does run and responds to speed settings. Can it create that much resistance to pull voltage down and still run?

Any suggestions appreciated.

Thanks

 

[AR67GTX]

I do have a low voltage current tester so this afternoon i’ll put it around my blower motor power lead and see what kind of amps the motor is pulling on the various fan speeds

 

[oldbee]

With last couple posts, I’m guessing a bad diode in alternator.

 

[RJRENTON]

IM

I do have a low voltage current tester so this afternoon i’ll put it around my blower motor power lead and see what kind of amps the motor is pulling on the various fan speeds

IMO....most, if not all, heater fan/AC fan motors are simple series wound devices....that is, the field windings are in series with each other and in series with the armature and brushes. Different speeds are determined by resistance added to the power supply via the lo, medium and high speed switch......the more resistance is added, the slower the motor will run. At High speed, full voltage is supplied to the motor or no resistance applied and the motor will draw max amperage, up to 20 amps at high speed. These speed control resistors get very hot and are usually located in the supply duct where they are cooled by air passing over them. The resistors can corrode and develop poor connections resulting in erratic operation of the motor. The motor brushes will wear and cause erratic or no operation. .......just a few thoughts and talking points.......the FSM should list the various amps at different speeds.
BOB RENTON

 

[AR67GTX]

With last couple posts, I’m guessing a bad diode in alternator.

Someone else mentioned that earlier. It’s a newly rebuilt alternator but I have another one so if testing the fan motor amp pull doesn’t bring anything to light I guess I’ll swap back to the other one and see.

Thanks

 

[AR67GTX]

IM

IMO....most, if not all, heater fan/AC fan motors are simple series wound devices....that is, the field windings are in series with each other and in series with the armature and brushes. Different speeds are determined by resistance added to the power supply via the lo, medium and high speed switch......the more resistance is added, the slower the motor will run. At High speed, full voltage is supplied to the motor or no resistance applied and the motor will draw max amperage, up to 20 amps at high speed. These speed control resistors get very hot and are usually located in the supply duct where they are cooled by air passing over them. The resistors can corrode and develop poor connections resulting in erratic operation of the motor. The motor brushes will wear and cause erratic or no operation. .......just a few thoughts and talking points.......the FSM should list the various amps at different speeds.
BOB RENTON

Yes, the fan on heating cycle is on a 20 amp fuse and it was blown. But the AC fan operation does not run through that heater fuse. It may be drawing power through a fused AC circuit connected by separate AC harness on the back of the fuse block. I’ll have to research that.

Thanks

 

[RJRENTON]

Yes, the fan on heating cycle is on a 20 amp fuse and it was blown. But the AC fan operation does not run through that heater fuse. It may be drawing power through a fused AC circuit connected by separate AC harness on the back of the fuse block. I’ll have to research that.

Thanks

Many GM heater/AC systems used a fused relay feed to the battery WHEN the fan was switched to hi. The hi speed contact on the fan control picked up the relay energizing the fan. The control switch did not handle the high amp current. A very simple and effective solution.....
BOB RENTON

 

[AR67GTX]

This testing didn’t help much. As soon as I started the engine I lost heater/defrost fan - dead. Fuse doesn’t look blown but I’ll need to pull it to make sure.

Amps on fan motor with AC running look OK. Low - 2.1 amps, med - 5.9 amps, high - 8.0 amps. Compressor clutch wire was running 3.1 amps. Voltages still taking a nose dive as fan is turned up. Took an amp reading on the big black alternator feed running to the P port on the bulkhead connector and it read 7.8 amps.

Need to get to the bottom of the heat/def fan not working. Pull the resister first I guess.

 

[AR67GTX]

Very confusing. To the control panel I see a heater power feed from the fuse box and an AC feed from the fuse box. Looks like the AC feed is just to energize the compressor clutch and circuit C4A energizes the AC/Heater resister for both heat and AC - (except the Max AC fan speed is controlled by AC damper door operation).

So if the heater fuse is not blown, it looks like the only reason the fan will not kick on in Heat/Defrost is the set of contacts in the heater/AC/Vacuum switch is not energizing. That could be trouble. I have an extra one I got out of a salvage yard 20 years ago but I’m not sure if it’s fully functional.

Looks like if the resister unit works on AC it should work on heat too. So pulling it probably is pointless. Still probably does explain where my voltage is going.

 

[AR67GTX]

No, this is not making sense: “So if the heater fuse is not blown, it looks like the only reason the fan will not kick on in Heat/Defrost is the set of contacts in the heater/AC/Vacuum switch is not energizing.”

If the feed from the heater fuse energized the resister by C4A circuit for both heat/def and AC there’s no explanation how the fan was running on AC with a blown heater fuse. The contacts in the control panel for AC/comp clutch feed (c1A) must shuttle power internally over to the C4A circuit to the resister.

 

[AR67GTX]

Well, got out the test light out and cleared enough hardware out from under dash to check the hvac control module, fan switch and resister. And on heat or def I get nothing on the terminal going to the resister. But I do when I put it on AC. So I guess the contacts for heat/def out to the resister board have failed although the ones on AC still work.

I see that OER now makes a replacement for these hvac modules. I had a spare but it’s very weathered and some of the vacuum hose connection were dry rotting and breaking. So I ordered a reproduction. Which means I have to pull the blasted instrument cluster out again.

But, I suspect this has nothing to do with the voltage drop issues. So in the meantime I’m going to put the spare alternator in and see if that has any effect on it when I get it going again.

 

[AR67GTX]

I don't think you have a wiring issue if you are getting 14 v with AC off. Not sure how much current the Chrys AC compressor draws, but they all draw a hefty amount. I think your re-built alt is the culprit. It is not producing enough amps at low rpms to charge battery with the AC on, hence the low voltage reading.

Geoff 2 wins the contest. After replacing the bulkhead connector I still had voltage issues with any applied load on the system. I put my old alternator back in, in place of the newer rebuilt one and everything is working properly again.

The rebuilt one I removed was from a rebuilder who I had also acquired one for my 66 and it worked great. But he must have missed something in this one. I see if my buddies know of a good local rebuilder who can take a look at it sometime.

The replacement hvac module from OER is pretty stiff and takes some force to change settings but hopefully it will smooth out after awhile. I think my original is OK, just had the black wire on the wrong terminal. Four wires/plugs, five terminals and vague details in the FSM sort of invites mistakes.