Dead short with IGN off.

[Kim Burke]

Hi All,
79 GMC C1500, twin tanks, BBC, A/C, Rebuilt motor w/700R4.
With Ign off, every fuse and lead pulled from the fuse box under the driver's side of the dash, I have a pulsating current with a test light between neg batt cable and neg batt post. Any thoughts would be appreciated. Pulled the alternator plug and the A/C clutch plug, still pulsating test light. This short will run the battery down in two days. The 700R4 has lead connected to power. If the lockup wire was hot with the key off, could it pulsate?
I can't think of anything that pulsates. Electrical crap makes my butt chew gum!
Thanks in advance,
Kim

 

[DanMcG]

I have a pulsating current with a test light between neg batt cable and neg batt post.

Do ya mean neg cable to pos post?
I'm not sure what would cause a pulsating draw except a flasher.

 

[Kim Burke]

I disconnected the neg batt cable and jumped from cable to batt w/test light. I don't know how to post the vid I took showing the pulsation, but it's weird.

 

[Kim Burke]

try this:

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[77 K20]

Using either the negative post or the positive post should result in the same thing.

What do you have for aftermarket stuff on your truck? Any stereo stuff? Amps? Car alarm? Remote start? Maybe someone wired it in direct instead of going thru your fuse block... Often it is non-factory items that can cause drain.

But the pulsating light is very odd.

 

[chengny]

Hi All,
79 GMC C1500, twin tanks, BBC, A/C, Rebuilt motor w/700R4.
With Ign off, every fuse and lead pulled from the fuse box under the driver's side of the dash, I have a pulsating current with a test light between neg batt cable and neg batt post. Any thoughts would be appreciated. Pulled the alternator plug and the A/C clutch plug, still pulsating test light. This short will run the battery down in two days. The 700R4 has lead connected to power. If the lockup wire was hot with the key off, could it pulsate?
I can't think of anything that pulsates. Electrical crap makes my butt chew gum!
Thanks in advance,
Kim

Sounds like you have cut power to everything - by pulling all the fuses and having the ignition switch in the OFF position (provided the ignition switch is good) - except the headlight switch. The power supply to the contacts used to switch the headlights on/off is fed directly from the battery (through one of the fusible links down at the starter solenoid). That refers to just the main headlights, and not to any of the other exterior or interior lighting circuits controlled with the H/L switch - they are all fused somewhere in the fuse block.

Try unplugging the harness connector from the H/L switch and see if the pulsating current flow stops. The wiring to the headlights is protected by a 16 amp auto reset circuit breaker, which is located in the switch itself.

This is pure speculation, but if there is an internal short within the switch, it could cause the breaker to continuously open and close - which would result in a pulsating current flow.

Also I see that you pulled the two pin connector to the alternator, but you didn't mention whether you disconnected the big red lead - the one that is connected by a stud/nut. If it wasn't disconnected, you might want to do that.

If the power lead for the 700R4's TCC circuit is plugged into one of the IGN sockets - left side of the fuse block - it should be ignition switched and also protected by the GAUGES fuse.

 

[JoeR Jr]

Any chance you have a multimeter capable of measuring amps? The pulsing test light is common on newer vehicles. For example, say a cmputer on the vehicle is trying to power up through the test light. The test light provides enough power to the circuit to power it up only briefly. Then it shuts it self off due to low voltage. Then it powers on again after the voltage to it recovers. Wash, rinse, repeat.
Maybe you've got a stereo amp, radio, dash cam....whatever. I bet something is power cycling.
An ammeter is a better tool to use if you can get your hands on one.
Joe

 

[roundhouse]

You using a LED test light
Or an incandescent light ?

 

[BikerBob]

Does your truck have a factory clock? Have seen them cause a pulse draw like you are describing.

 

[Matt69olds]

You need a meter that will read amps. Most newer cars have several modules and computers that take several minutes to “ go to sleep” I know your trick is old enough not to have any of that crap but a test light is best used to find a short to ground. Put a proper meter on it, if you see more than about 50 milliamps start pulling fuses.

 

[Kim Burke]

Okay, been a little busy and the truck is in the tranny shop, however, I will attempt to answer some and try the other suggestions as well as I can.

Chengny-
1. I did not pull the harness of the headlight switch, but that sounds encouraging. (no change)
2. I indeed pulled the lugged red alt wire as well as the 2 pronged plug. (no change)
3. The tranny wire is connected by fuse in the fuse box. (one of those funky clips that covers one leg of a spade fuse) I don't remember which slot the fuse was in. I'll move to 'gauges" if not already there. Thank you so much.

JoeR jr-
1. Yes. But the surging gave me readings on the DMM I was too dumb to use. So I went to the $2 test light.
2. There is an aftermarket stereo installed(hump mounted) and the OE is still in the dash. The new unit is keyed but not sure which circuit. I'm assuming if the stereo comes on and off w/key, it's unable to surge in "off". Thank you very much as well.

roundhouse-
Literally a $2 test light old school incandecent. Thanks

BikerBob-
It does, I'll make sure power is killed to it and try it. (interesting) Thanks

Matt69olds-
I'll try the DMM again. Thanks

 

[Kim Burke]

BikerBob- whoops, no clock. dang it!

Joe & Matt hooked the DMM up again and the draw ranges from 109-198 mA

Going to unhook the aftermarket radio. <fingers crossed>

 

[Kim Burke]

FOUND IT!!
The "audio pro" that installed the aftermarket stereo hooked into a non keyed hot wire. He wasn't a complete idiot, it was fused. But pulled the fuse and .

Thanks for the support, everyone.

On to the speedometer, gas guage for the main tank, and wet sanding!