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Rookie Electrical Question


BobRob

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Maybe I'm overthinking this, but it's been a while since I've done extensive wiring on a 50 year old car, so bear with me...

 

I just finished wiring in my Dakota Dash and while double checking everything prior to hooking up the battery and testing, I was checking for grounds and found the BAT terminal on the fuse block has a dead short to ground. Poking around, most of the other terminals did as well. I read between the positive and negative cables and there was continuity there. Disconnected the bulhead connector and pos to neg was now open, still dead short inside the car at the fuse block. Disconnected the D/D from the fuse block and those are now open, the grounds to the D/D are good. 

The only other modification was to remove the AC wiring since I put in a Vintage Air system. The wiring is straight forward with one wire going to the battery and another to the IGN terminal on the fuse block, a gouple of grounds and a heater control valve, which is under the hood.

I disconnected one thing at a time (lighter, courtesy lamps, glove box lamp and anything else I could think of on a constant power circuit and no changes.

Any suggestions? This is giving me a headache. 

 

Update: Tried the old test light trick (disconnected battery then put a light between the + battery cable and + battery terminal). Test light was bright as all get out, then realized the headlight switch was on(oops). Test light dimmed significantly after shoving it in. Pulled the fuses one at a time and it made no difference whatsoever. Seems something is pulling current in the car associated with that BATT circuit, but not anything fused. 

The question remains though, should there be continuity, at all, beween the BATT terminal on the fuse block and ground? I'm hesitant to connect the battery until I get this sorted. Any advice would be greatly appreciated. 

 

 

 

 

 

Edited by BobRob
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Ok, I'll do some thinking out loud here: It sounds as if you have a mystery battery drain. About the only thing I can think of that is supposed to draw from the battery is the clock, and that guy has a fuse.

I'm going to make a wild guess then and suspect it's a regulator or alternator issue. I have seen them work great when the engine is on, but drain the battery when the engine is off. Try disconnecting wires one at a time at the alternator, then the regulator and check the results. Your test light between battery & cable is a good method.

Testing continuity can be tricky, as the test instrument often only gives a yes or no response. Remember that the test instrument can "see" through light bulbs and such and give a positive result because the other side of the light bulb is grounded. For example, if you put one test probe on the parking lamp circuit, and the other probe to a ground, and have the tester set to indicate continuity, the tester might give a YES answer. This result only verifies that the wiring eventually finds a ground...it could be a short in the wiring, or just the normal path of electricity to a ground through the light bulbs. Rather than test for continuity, try testing for resistance (ohms). This will give a clue as to how much effort the current has to expend to get to the other end. If it reads zero ohms, it is as good as a dead short, and if the ohms are higher it's probably just showing the resistance in a bulb or something. Ohm testing should be done with all power off.

Also, at the fuse box, with a fuse removed, it would be normal for one side to show hot and the other to seem dead, or grounded. (if you have your test light grounded to the chassis). You may be getting a false positive with your meter checking continuity between the BAT terminal and ground, as you are essentially creating a path through the meter you are using. You have a terminal marked BAT which is electrically the same as the positive battery post, and you have a chassis ground which is the same as the negative battery post...the meter might show 12 volts, and also show continuity.

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That's the rub, since I haven't re-installed the battery, I'm not sure if I have a problem at all. 

I disconnected the bulkhead connector and was able to isolate the issue I'm seeing to something under the dash. The odd thing is, even with all the fuses removed, and no battery in the car, I am still seeing continuity between the BATT teminal on the fuse block and ground. 

My old Fluke gave up the ghost a couple of months ago so I'll have to run out and grab a new meter and see what the resistance is.

 

 

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I would start with the last thing you did... wiring the Dakota digital dash. If you can, why not unplug the dash connector and see if you still have continuity between BATT and ground. That will at least tell you whether or not your new dash is the culprit. With that being the most likely thing to suffer if you do have an electrical issue I would remove it from the circuit to prevent shorting it out and ruining it, and then perhaps hook the battery up to see what happens next. That is after you have checked everything else visually and have no obvious bare wires contacting metal, checked the regulator by isolating it, checked the alternator by isolating it, etc. Pay careful attention to the new wiring you did, how it is routed, etc. during the physical inspection. 

 

While you are doing that, I will be walking out to the garage to read my BATT terminal to see if it is shorted to ground. Could be that is what is supposed to happen as whatever you hook up to the BATT terminal should be something that requires constant voltage, like your clock or radio constant power, cigarette lighter, etc. 

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My results are as follows:

 

I backed the car out and disconnected the battery to simulate your conditions. Using a multimeter I read a dead short between the BATT fuse block terminal and ground. With the battery connected and a test light hooked between the BATT terminal and ground I get a steady light. 

Hope that helps

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Somehow I volunteered to get involved in this. LOL

So it would seem normal to show a connection from hot to ground at the fuse box when the battery is disconnected. Hmmm

I wonder why that is? You would think the two sides would be separated. Maybe battery power opens a relay and disconnects the two? 

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Easy enough to go and check your car Wallaby. My car is super easy to check because the fuse block is relatively new and all the graphics are easy to read. I know with old cars sometimes the lettering can be missing or really faded which is why i figured it would be easy enough just to go check mine and see what's what. 

 

I thought it sounded right when i read the initial post but wanted to verify before opening my big old mouth. I learned a long time ago not to shoot from the hip when it comes to electrical stuff. Years of working as an electronics tech and I still like to read circuits and signal trace stuff before giving internet advice. 

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1 hour ago, wallaby said:

 

So it would seem normal to show a connection from hot to ground at the fuse box when the battery is disconnected. Hmmm

I wonder why that is? You would think the two sides would be separated. Maybe battery power opens a relay and disconnects the two? 

Just thinking out loud here, even a light bulb in the circuit would make the system seem like a dead short. Perhaps the dome light circuit, because maybe the door is open, completing the circuit while checking things with a voltmeter? It would also maybe be part of the 'BATT' circuit since it would always be powered up regardless of the ignition switch. Just a thought...

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Well, it's all sorted now. 

I picked up a cheapie multimeter at Advance Auto along with a bunch of new fuses.  Blew out the fuse panel, popped in the new fuses and everything is as I thought it should be. Reconnected everything, one at a time and it remained open between BATT and ground the whole time. Checked the draw between the positive cable and the positive terminal and got nothing. Hooked the battery back up and all is right with the world. Maybe there was some crud in the fuse panel?

Or maybe it was just a gremlin.. 🤔

Thanks for responding and checking your cars. This was driving me nuts and may again in the future, but at least it's working now. 

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