Power not really off….. strange

[ramdough]

So, I have a DPST battery disconnect switch that disconnects the grounds from the 12v and 24v battery banks.

The other day, I turned the disconnect switch, jumped in, turned the main power dash switch, dash lit up, hit the starter button…. Heard the starter lurch and all lights went off. Dead.

I thought, batteries must be dead. Went and looked…. I actually had turned off the batteries, not on….

So somehow the system had stored somewhere a charge that was just enough to bump the starter.

Any thoughts? This is strange to me.


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[aw113sgte]

The capacitors in the LBCD next to the spare tire holding some charge. There's a note in the manual somewhere that when you do kill power with that switch, go in to the cab and turn the ignition on to drain the capacitors

 

[Keith Knight]

Interesting, I haven’t experienced that one.

 

[aw113sgte]

Interesting, I haven’t experienced that one.

They have been known to fail. Unfortunately not the best long term design.

 

[ramdough]

The capacitors in the LBCD next to the spare tire holding some charge. There's a note in the manual somewhere that when you do kill power with that switch, go in to the cab and turn the ignition on to drain the capacitors

That makes a lot of sense.


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[Ronmar]

The caps are there to catch the voltage spike caused when the LBCD disconnects the batteries(load) from the alternator at full output… in my experience this is usually considered a big no-no, as that spike can fry components until the regulator can get it under control, but some bright light came up with this system on the A1 to protect the alt from failing/dead batteries…

 

[GeneralDisorder]

Yep. And this is what happens when they go bad:

 

[ramdough]

The caps are there to catch the voltage spike caused when the LBCD disconnects the batteries(load) from the alternator at full output… in my experience this is usually considered a big no-no, as that spike can fry components until the regulator can get it under control, but some bright light came up with this system on the A1 to protect the alt from failing/dead batteries…

That is funny, my A1 does not have an electronic disconnect (by the NATO plug). Or are we talking about something else?

I have a manual switch I added I added.


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[GeneralDisorder]

That is funny, my A1 does not have an electronic disconnect (by the NATO plug). Or are we talking about something else?

I have a manual switch I added I added.


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LBCD is between the spare tire and the air filter housing. Early A1's may have the older polarity protection box which is just some big potted diodes to keep PVT Snuffy from connecting the batteries wrong and bursting the truck into flames. The LBCD replaced it when the 100A alternator rose to the surface as a problem child when batteries were neglected (rotate your batteries every 6 months BTW).

 

[Ronmar]

Yea the early,A1s didn‘t have them. That remote disconnect is what the LBCD would use to disconnect the batts. So if you don‘t have that, as general said, you probably have a polarity protection device. You have the circuit board power panel and the 3126? I call these an A0.5

 

[Ronmar]

Now what you experienced may have been a latent switch, IE, the switch didn’t fully open and you got power thru it until you tried to pull any significant current…

 

[Suprman]

Maybe bad ground. Try adding a big negative wire from the batts negative to a sanded spot on the main frame rail. There are relays under the power board that control 12 and 24 volts to the panel. The panels can develop bad solder joints. Relays go bad over time.

 

[ramdough]

Now what you experienced may have been a latent switch, IE, the switch didn’t fully open and you got power thru it until you tried to pull any significant current…

Just to be clear, the the battery disconnect I used is a manual rotating switch.

Mine is a 1999, rebuilt in 2008. If that helps with the timeline.


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[hike]

I haven't had a bump to the starter, though I often see the CHARGING SYSTEM dash light fade away when working on the electrical system as I turn the system on and off at the main switch near the NATO plug when I leave the dash power switch closed. Our unit is a 2003 M1078A1 with LBCD—

 

[coachgeo]

Im thinking manual switch did not turn all the way ON and when a surge of power went thru it .... it caused it to jump to OFF.

On topic... but not. on topic.... even if your switch might have not just took a spike... Might want to have a way to bypass it kept on hand..... like a bolt you can use to pull the two lines off the switch and bolt them together?

 

[Wingnut13]

Sounds like a bad connection in one of the heavy wires, ground or power. I had a similar issue.

 

[Ronmar]

Just to be clear, the the battery disconnect I used is a manual rotating switch.

Mine is a 1999, rebuilt in 2008. If that helps with the timeline.


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Yep, and most of those large rotary switches use a cam or ramp to actuate a large bussbar and push it against the contacts. If I read your initial post, you thought it was off and rotated it to on, when in actuality it was on and you rotated it to off. What I think may have happened is that when you turned it off, it may not have quite gotten to that point and the conductor bar was still making marginal contact untill you tried to pull some real current thru it.

Your description sounded exactly like a bad/high resistance connection. It will pass full voltage throughout the system untill you try and pull any serious current, then it fries and opens completely… The last one of these I had, it was a bad battery ground connection. I was standing outside the cab reaching in to push the ign and start button, and happened to be looking toward the rear of the truck when I pushed the button and got a single starter clunk then nothing. I also happened to see a puff of smoke come out of the battery box… The bolt that connected the cable end terminal lug to the battery clamp had become loose and corroded, causing the bad connection that smoked with current. A less faulty connection that didnt go up in smoke would likely cause relays to chatter/start clicking due to voltage drop across the connection. I had one of those also on my PPD, shortly after I got my truck…

If it started normally after you cycled that battery switch again, that is a possibility, or you also may have a loose/bad connection around that switch…

yours may have been overhauled in 08, but that doesn’t mean they did anything to change its configuration. In troubleshooting with folks, I have come across quite a few of these “tween” trucks with a mix of A0 and A1 electrical systems, so it does not look like any effort was made to upgrade them…

 

[ramdough]

Yep, and most of those large rotary switches use a cam or ramp to actuate a large bussbar and push it against the contacts. If I read your initial post, you thought it was off and rotated it to on, when in actuality it was on and you rotated it to off. What I think may have happened is that when you turned it off, it may not have quite gotten to that point and the conductor bar was still making marginal contact untill you tried to pull some real current thru it.

Your description sounded exactly like a bad/high resistance connection. It will pass full voltage throughout the system untill you try and pull any serious current, then it fries and opens completely… The last one of these I had, it was a bad battery ground connection. I was standing outside the cab reaching in to push the ign and start button, and happened to be looking toward the rear of the truck when I pushed the button and got a single starter clunk then nothing. I also happened to see a puff of smoke come out of the battery box… The bolt that connected the cable end terminal lug to the battery clamp had become loose and corroded, causing the bad connection that smoked with current. A less faulty connection that didnt go up in smoke would likely cause relays to chatter/start clicking due to voltage drop across the connection. I had one of those also on my PPD, shortly after I got my truck…

If it started normally after you cycled that battery switch again, that is a possibility, or you also may have a loose/bad connection around that switch…

yours may have been overhauled in 08, but that doesn’t mean they did anything to change its configuration. In troubleshooting with folks, I have come across quite a few of these “tween” trucks with a mix of A0 and A1 electrical systems, so it does not look like any effort was made to upgrade them…

The truck started up just fine once I reset the switch to on. I have not seen any sign of a failure since then.


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