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Electrical Gremlins? Dim lights & dead batteries


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I've got something odd going on with my truck. The more vexing problem is I get intermittent dimming lights that I notice when driving at night. For a tenth of a second everything will go dim, and then come back. Headlights, dash lights, interior lights if they're on, and the voltage gauge will dip from 13.x to12v and then recover.I've looked for shorts in the main battery cables and alternator charging cable, and find none; nothing is ever warm either. I've checked for loose battery connections and loose alternator connections. I'm getting close to throwing parts at it with a fresh alternator to see if that makes a difference, and an external voltage regulator if it doesn't. Considering putting an ammeter in to diagnose if draw is going up or if supply is going down, too. Any thoughts?The second and possibly related issue is a seeming ignition off draw. I can only let her sit for 24 hours if it's cool out or it doesn't have enough juice to spin fast enough to fire. That at least ought to not be too hard to track down with a DVM and pulling fuses to see what circuit it's on - assuming it isn't intermittent as well.I guess I'm looking for your anecdotes on similar issues so I can maybe have a few places to check out specifically, see if anyone has fought something similar and won. :shrug:Thanks! :thumbup2:

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Before throwing parts test it first...Pull the alternator and have it bench tested.Pull the batteries have them load tested.Check the alternator fuse for a crack. Yes these fuse can and will crack which will cause random open circuits or in your case a random dimming.Your right I would take a DVM and load check each circuit to find any draws that are excessive.

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the problem is far too intermittent and unpredictable to show up in any bench testing. Sometimes it'll be every few seconds for 4-5 cycles, sometimes it'll go hours without doing it. Road conditions don't matter, it doesn't seem to have anything to do with rough roads or anything.I like the cracked charging fuse idea. Another thought that just occurred to me is it could be a belt slipping under load - it's been through some pretty healthy oil and fuel leakage. No squeal though, and I haven't noticed power steering drop outs and I expect that puts a higher demand on the serpentine than the alternator should.

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I've got something odd going on with my truck. The more vexing problem is I get intermittent dimming lights that I notice when driving at night. For a tenth of a second everything will go dim, and then come back. Headlights, dash lights, interior lights if they're on, and the voltage gauge will dip from 13.x to12v and then recover. I've looked for shorts in the main battery cables and alternator charging cable, and find none; nothing is ever warm either. I've checked for loose battery connections and loose alternator connections. I'm getting close to throwing parts at it with a fresh alternator to see if that makes a difference, and an external voltage regulator if it doesn't. Considering putting an ammeter in to diagnose if draw is going up or if supply is going down, too. Any thoughts? The second and possibly related issue is a seeming ignition off draw. I can only let her sit for 24 hours if it's cool out or it doesn't have enough juice to spin fast enough to fire. That at least ought to not be too hard to track down with a DVM and pulling fuses to see what circuit it's on - assuming it isn't intermittent as well. I guess I'm looking for your anecdotes on similar issues so I can maybe have a few places to check out specifically, see if anyone has fought something similar and won. :shrug: Thanks! :thumbup2:

My dad's '05 3500 was doing that and it was some relay that was stuck. Don't remember which one.
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Logical approach: You are reporting dimming lights with corresponding drop in voltage showing on guage which makes total sense. So the loads are greater than the output and are drawing down the batteries. I would start at the batteries. Have you not just checked for tightness but cleaned the connections both at the batteries, inline (a few inches away where my problem was located), grounds. I would also load test the batteries. A shorted cell can be it's own load. I'd look for the key off load as it may be an all the time failure. Let us know how you make out.

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Checked my ignition off draws last night. With a door open and cab lights on, it sat at just under 3a, steady. With the door shut, I see 0.04a steady. So whatever draws me down is definitely intermittent.

Terminal connections are all clean and tight.

With as random and infrequent as it is, and as short a duration as this issue has, I doubt a bench test of batteries or alt will shed any light.

--- Update to the previous post...

Going to VA this weekend so I'll have some "testing time"

Picked up a new belt this morning, so I'll do that first. If I still see it, I'll unhook the trigger wires on the grid heater solenoids. After that I'll probably change the charging fuse just for grins, and then think about an ammeter.

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No more huge voltage drops, seems to have been the belt slipping. Still have occasional minor brownouts (.5v drop or so, according to the PIII brake controller readout) but that could be nearly anything. it's JUST enough to notice in the headlights when it's relatively dark around. Could be trailer load, relay, compressor, small short, small open... Been that way at least since the 24v swap. Doubt I'll find that one.

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  • 2 months later...

Been a while on this one, but figured I'd update.I got a new trailer that's all incandescent lighting, so a lot more draw with the lights on. The voltage issues were coming back. Disconnected the grid heaters and that wasn't it.I video recorded the voltage gauge on the dash, and found it wasn't voltage drops, but spikes I was seeing. Watching the PIII again I was seeing nominal voltage of only about 13.2 running with my lights on, spiking to 13.7.Finally pulled the alternator Weds and had it tested. It passed one or two tests and failed the rest. Testing machine was asking 14.8v from it and it was able to provide a max of 13.2 and was at 12.8 nominal. Right after startup, at idle with lights and fan on, my truck would sit at 12.0 for a little while according to the PIII. Definitely fits.I Just picked up a new alt, and it'll be installed before a trip to Richmond VA tonight - tomorrow, hopefully I'll be done with this now.Shame that an AC Delco reman only lasted 50,000 or so. I put that in the spring after I got the truck.

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Truck hasn't stayed parked for long enough to determine if the drawdown was due to charging issues or if it's unrelated. Since I disconnected the bad battery it's been better but not resolved. Hoping with a better charge while running it'll behave - starting out with a low battery is never a good place to be when sitting for a week.

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