Electric Draw-Arc when hooking up the battery

[Hack71]

71 C10 350, AT, AC. Key switch off and no other accessories on, when hooking up the battery there is a large electric draw enough for an visible arc, seems the like the AC clutch engages. This is not a recent event this has been like that for years, doesn't seem right though, battery connected positive first, then negative, any ideas?

[dmjlambert]

My ideas:
Use a 12v test light or multimeter to change your seems-like to a definite confirmation. Then if you find a particular item that is powered with key off that is not supposed to be powered, trace the wiring for that item to make sure it is powered from the correct place in the fuse panel. Use the 12v test light or multimeter to check the various sections of the fuse panel and the ignition switch wiring to make sure everything's OK. The clutch for A/C should be powered only when key is on, and when the A/C slide switch is all the way to the left, and when the blower fan is not off.

[dmjlambert]

I just thought of something, if you have an anti-deiseling relay in your A/C system, it is used to put a load on the engine by engaging the A/C clutch when you turn off the truck. I'm not sure how exactly those anti-deiseling relays work because that was not yet invented in 1969. It seems to me that would have to be a timer relay, but I don't know that it is. Others on the forum may have a clue.

[Steeveedee]

If it has that relay, the timer is "vacuum decay". If the engine is running, there is continuous suction (vacuum). Once the engine stops, vacuum drops and, air flows (it is always flowing in when there is vacuum) into the bleed holes until the pressure is equalized, and releases the relay. If those holes are clogged, the relay will remain engaged, and the AC compressor clutch will remain engaged. In a high dust environment, those holes can clog pretty quickly.

[davepl]

A point-based clock can do it, but only once when it winds itself.

Easiest way to trouble shoot it would be to remove the fuses (flip one end out, leave the other end in) and see if goes away. Then put the fuses back one at a time and see what circuit the load is on.

If it doesn't change with fuses then it's likely something you've got wired directly to the battery post or junction block.

[randy500]

Ammeter between cable and battery, pull fuses until amp draw goes away then trace that circuit for the draw.
If not found via fuses, unhook the voltage regulator wiring connector to see if the draw is in that circuit.

[rockyrivermark]

Originally Posted by randy500

Ammeter between cable and battery, pull fuses until amp draw goes away then trace that circuit for the draw.
If not found via fuses, unhook the voltage regulator wiring connector to see if the draw is in that circuit.

Yes. Do this. Good advice.
Chasing electrical problem is process of elimination.
When randy500 or locdoc post a reply. Ready and follow.
They know their stuff!
Mark

[Hack71]

Awesome, thanks for the ideas ill let you know what I find.

[Steeveedee]

The fact that the AC clutch engages when the battery cable is connected is a pretty clear indication of where the problem is. OP (Hack71)- is there a little black box about 1" by 1" by 4" with vacuum lines on it, and wires running to it, just back of the battery, sort of underneath the passenger's side fender? If so, unplug the wiring harness to it and see if that cures the problem.

[ubtripn]

I used to pull fuses one at a time, checking for arc each time to isolate the circuit.

[VetteVet]

Let me interject a little Vettevet logic here. If I thought the drain was the AC clutch I would unplug the AC clutch. If the drain goes away then I would find out what was engaging the AC clutch.

Here is a diagram of the AC wiring with the anti-dieseling relay.





Here is the same diagram with the wires color coded.



I haven''t had much experience with the AC wiring or the components, but I don't see any connection with a vacuum switch although I have read that there is a device on some engines that boosts the engine RPM when the AC is on to prevent engine cutoff at idle. It appears from the diagram that the anti-dieseling relay is controlled by the AC switch and the AC relay. It may be that there is a vacuum switch in the circuit but the diagram doesn't show it.

Obviously I need to do some research on how the system works.

[VetteVet]

I don't have time to read these right now so I posted them for you guys to look at.


https://www.searchencrypt.com/search...eseling+relay#.

[Steeveedee]

I doubt that he has the relay in question. I've never seen one on a truck of this vintage. As you mentioned, the control is usually a relay to power the solenoid to bump the idle speed up when the AC is on.

[Hack71]

I will be looking into all of these this weekend. I will post what I find. Thanks again!