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Old 02-09-2018, 02:16 PM   #1
rgunlock
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Adding my own electrical issue to the mix

I am wrapping up restoration of my '78 K15 and trying to work out the kinks. After a weekend of sitting, the truck didn't have enough power to crank but still had lights. Next morning, not even the lights. I'd driven it that Friday and all was fine. This truck has a brand new battery and has been completely rewired with an AAW harness. I have added a few aftermarket goodies as well. Best I can tell, everything electrical on the truck is working fine other than the lighter.

So I read a bunch of threads here concerning various electrical issues and last weekend went out and pulled the positive battery cable off. Used a voltmeter to measure from the end of removed + cable to the + post on the battery. Got 12.6V. Proceeded to pull every fuse from the fuseblock one at a time (and not replacing). Also all the connectors from various aftermarket stuff until nothing but flasher relay is still on fuseblock. Pulled all inline fuses as well. Still 12.6V from +post to cable. Read that it might be alternator so pulled both the 2 wire connector as well as the heavy red wire. Still 12.6V.

Trying to get armed with some suggestions for tomorrow. What's left to check? Pretty much anything with constant power (key off) that might be grounding out? Starter maybe? Thanks in advance for any suggestions.
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Old 02-09-2018, 03:34 PM   #2
75C10
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Re: Adding my own electrical issue to the mix

I am not surprised you are seeing voltage. Whats more important is the current draw. I use a small light bulb (like a dash light or side marker ie 194) and connect it in the circuit (using a plug in socket). If there is enough current the lite will illuminate and this is enough to run down a battery. Connect a small 12 v lite and unplug things until the lite goes out. Disconnect the positive wire at the battery and connect the test lite between the pos post of the battery and the pos battery cable which is hanging free. I think an ammeter will also work but I don't know the minimal current draw. Maybe somebody can help with the current. The 12 v light bulb (small) works every time and costs very little - maybe free if you have the socket already . Sometimes a small spark can be seen when disconnecting the battery. This could be a radio or clock or other. Regardless, the lite test will work.

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Old 02-09-2018, 04:40 PM   #3
rgunlock
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Re: Adding my own electrical issue to the mix

Thanks 75C10. Now that you mention it, most of the threads mention using a test light rather than voltmeter, but don't actually explain why the test light would be a better choice. I saved all the bulb sockets from my old harness so it should be no problem for me to fix one up.
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Old 02-09-2018, 07:16 PM   #4
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Re: Adding my own electrical issue to the mix

Voltage is pressure, amp is flow.

Think of an air tank, just because it's at 100psi doesn't necessarily mean there is air flowing out. It could be sealed up tight, and it will stay 100psi forever. Crack open a bleeder valve, and now you have flow, and now that air pressure will drop.

Measuring voltage won't tell you the drain. You need a multimeter that will measure current draw. Repeat your same test, same configuration, with a meter on an amp setting, and start pulling your fuses. The one that causes the meter to drop the most is your culprit.
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Old 02-10-2018, 11:59 AM   #5
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Re: Adding my own electrical issue to the mix

Yeah, I was afraid my method was off - really still a rookie at this electrical. I did wire this truck myself but only because I can follow instructions. But I am getting plenty of opportunity to learn because I've had to troubleshoot one problem after another. Thanks for the help guys, I knew I could count on forum members to set me straight!
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Old 02-10-2018, 12:59 PM   #6
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Re: Adding my own electrical issue to the mix

Haven't figured out the ammeter on my multimeter, or it just doesn't work so I made a test light. I still have zero fuses installed and the alternator still has all connections removed. So I think I'm back to where I started this thread, looking for suggestions where to look next.
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Old 02-10-2018, 01:24 PM   #7
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Re: Adding my own electrical issue to the mix

You have a fairly big leak.
Did you leave the drivers' door open?
Are the cab interior lights (dome and courtesy) on?

What isn't fused?

Stock off the top of my head...
-The alternator BAT wire.
-The alternator Sense wire... SI alternator plug terminal #2.
-The headlamp switch.

The alternator could leak if the voltage regulator is not happy about life. Easy enough to unplug it and see.

Are any of your non-stock wiring, on the Junction stud blocks that I can see on the RH side of the firewall, powered outside of the fuse panel through the fusible link off the stock J-Studs or direct from unswitched battery power?
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1976 Chevy K20, 6.5L, NV4500/NP208
1986 M1008 CUCV
2000 GMC C2500, TD6.5L, NV4500
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RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.
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Old 02-10-2018, 01:56 PM   #8
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Re: Adding my own electrical issue to the mix

Doors are closed, and the dome and underdash courtesy lights are out if I reconnect the positive battery cable.

All cables/connections are currently removed from my alternator, but hadn't thought about the headlight switch.

Best I can recall, all the non-stock stuff either gets power from the fuse block using special connectors that plug into the face of it (all currently removed) or off the red junction block you see in the picture.

The red junction block has power directly from the battery. Everything coming off it has an inline fuse, all currently removed. I no longer have the stock J-studs. Nothing in the AAW wiring instructions called for it as I recall. I've removed the wire from the junction block coming from the battery but the test light still lights.
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Old 02-10-2018, 03:11 PM   #9
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Re: Adding my own electrical issue to the mix

Well, I think I found the culprit. I'd installed a battery disconnect from AAW on the positive battery cable. If I bypassed it, I had nothing on the test light. Reconnect it and remove the switch for the disconnect - test light on. Unmount it from my fender but leave the positive battery cables connected and nothing on the test light. Apparently, this disconnect was grounding through where it mounted on the fender. I'd have assumed it would have been isolated internally from doing that.

So, I've removed the disconnect and temporarily connected both sections of the positive battery cable. I'll add fuses, etc. back one at a time checking with the test light since I've gone this far. Thanks for taking the time to make suggestions.
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Old 02-10-2018, 05:12 PM   #10
hatzie
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Re: Adding my own electrical issue to the mix

You'd think an outfit like AAW would provide a quality switch or at least instructions for installing it on an HDPE isolation plate or some such.
They could always charge extra for a spacer/isolation plate.
Guess not.
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1959 M35A2 LDT465-1D
1967 Dodge W200 B383, NP420/NP201
1969 Dodge Polara 500 B383, A833
1972 Ford F250 FE390, NP435/NP205
1976 Chevy K20, 6.5L, NV4500/NP208
1986 M1008 CUCV
2000 GMC C2500, TD6.5L, NV4500
2005.5 VW Jetta BRM PD-TDI, 02J 5 Speed
2009 Impala SS LS4 V8


RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.
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Old 02-10-2018, 09:11 PM   #11
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Re: Adding my own electrical issue to the mix

Good job on the diagnostics. It would have been a long time before I checked that particular item...
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Old 02-11-2018, 08:33 AM   #12
rgunlock
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Re: Adding my own electrical issue to the mix

One of the reasons I built this truck was to get back to a simpler vehicle, the more gadgets a vehicle has the more that can break. Figures it was an unnecessary gadget that broke

I really appreciate the service you guys do for this forum. You're asked to help because "its broke". You know how it should work, but you didn't install it, no telling what's been done that's not stock, no direct access to poke around and yet I've read many threads you've posted in where the OP got it figured out. A lot of times your questions and suggestions at least get the OP thinking and looking in the right places, as it did in my case. Thanks!
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Old 02-11-2018, 05:10 PM   #13
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Re: Adding my own electrical issue to the mix

Glad you figured out the problem.
Just for everbodys info industry standard is to mount disconnect switches in the Negative side of the battery. I'm sure you can see now that if yours had been on the negative side then you may have never had a problem.
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Old 02-11-2018, 05:51 PM   #14
hatzie
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Re: Adding my own electrical issue to the mix

The disconnects on Transit Coaches are on the hot side of the 24v and sometimes, depending on the manufacturer, an additional disconnect on the hot side of the 12v output from the battery tray. I can't think of any that I've worked on in the last 28 years that had the disconnect on the negative side...
That's as close to industry standard as one could ask for since automobiles and pickups generally don't have OEM disconnects on the battery.

It would still be an issue on the negative side of the battery since the short to sheet metal defeats the purpose of the switch.

I used a switch duty 60A surface mount circuit breaker on the power feeds to the cab on my 76, since it's not a daily driver, and a the engine bay is full of retrofit 1998 & 1993 GM diesel engine tech. The remote starter solenoid next to the RH battery cuts off the hot wire along the oil pan, when the key isn't calling for engine crank, and the low current circuits are fed by an 8ga wire off the side terminal on the LH battery to a junction stud strip on the inner fender. It de-clutters the firewall and the starter BAT lug. Most everything is in the top of the engine bay. The fusible links are in easy reach without climbing under the truck. The alternator still has direct connections to the battery pair LH + has the charge wire RH + has the Sense wire.
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1959 M35A2 LDT465-1D
1967 Dodge W200 B383, NP420/NP201
1969 Dodge Polara 500 B383, A833
1972 Ford F250 FE390, NP435/NP205
1976 Chevy K20, 6.5L, NV4500/NP208
1986 M1008 CUCV
2000 GMC C2500, TD6.5L, NV4500
2005.5 VW Jetta BRM PD-TDI, 02J 5 Speed
2009 Impala SS LS4 V8


RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.

Last edited by hatzie; 02-11-2018 at 06:09 PM.
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