Weird Battery/Voltage /Start Issue - SOLVED

[Syncronoid]

'90 Syncro Westy w/GW2.5. Heading into the backcountry for 4 days on Wed. Only recent changes lately were the trans and front diff rebuilt 800mi ago and then changed that oil last Wed. Alternator brand new (Bosch Remanu) 6/17/2020.

This just started happening, I noticed the van was struggling a bit to start. I even went and got a new main battery and have both the aux. batteries disconnected.....

Battery reads 12.6v unconnected. With all power off, the voltage at the main battery leads reads 12.56v. Voltage at alternator reads 12.6v. Voltage at cigarette lighter reads 12.40v., as does my dash volt gauge installed from dash power.

When I turn the key to power on, voltage at the main battery leads reads 12.5v, the cigarette lighter and dash voltage gauge both read 12.25v and the alternator drops to 12.3v.

When I join the main to the two aux. (120AH+50AH), plenty of power and van starts right up.

What in the world is happening????? Do I start pulling fuses under the dash and see what might be causing the problem? **UPDATE** Pulled all fuses individually and nothing made a different, cigarette lighter voltage remained at 12.4v.
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'90 Syncro Westy w/GW2.5L

[MarkWard]

What does the battery voltage read with the battery ground disconnected? That should be your starting point.
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[Syncronoid]

12.6v. It's brand new, off Car Quest's shelf with about 5-6 starts on it since I brought it home this morning.

[valvecovergasket]

whats the voltage reading at the starter post w/ and w/o the aux batts?

really odd situation... it wasnt doing any of this prior to new batt?
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MegaSquirt resource - SpitfireEFI.com

gone, but not forgotten: '83 tdi westy - swap thread

[Syncronoid]

I brought the old battery back because of this problem, it was only 8mos old. I did that to eliminate the Battery as a source of the prob. Problem persists.

[MarkWard]

Ok, so you should be measuring identical voltage or as close as possible between the BAT + post and the Alternator BAT + post. It sounds like you have a voltage drop either on the + side or ground side between the alternator and the battery.

You'll need to extend your test leads. On DC volts measure between the BAT + post on the battery and the stud on the alternator. Not the battery cable clamp.

That voltage should be as close to zero as possible.

Now repeat the test on the ground side. Leads go between the Battery negative post and the alternator case. Should be zero or close to zero. If you have something higher which I suspect, you'll need to isolate the drop, but at least you know which "side" to check.

I can't account for solar charge controller combiners you might have, but you should be able to isolate all that from the stock charging system anyhow. Something similar came up recently and the owner was able to follow these directions and find the drop.
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[Syncronoid]

I remeasured after the system after it settled down after earlier starting/running/stopping cycles. Voltage at battery + is 12.57v. Voltage at alternator + is 12.52V. Seems appropriate consider the longer run of cable, no?

[Syncronoid]

I was checking underneath and it appears the positive connector on the starter is quite loose. Going to try and tighten it. Very tight in there. Is it possible???
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'90 Syncro Westy w/GW2.5L

[MarkWard]

[Ahwahnee]

A loose connection there could add resistance that the battery(ies) would have to overcome. Will also get hot there if there is resistance.

Hopefully tightening it will resolve the issue.

[valvecovergasket]

[Syncronoid]

Houston, the problem is solved!!

Damn that's tight getting in there, but I spread the coolant hoses apart, pealed back the protective rubber cover and there's that little #12 bolt on the starter (+) with just enough space to get it rock solid tight. Does that sound phallic or what?! (Yes, my Syncro is a "she")

That explains a lot about the wonky electrical things I was seeing on my gauges for the last couple of months.

Thank you valvecovergasket for sending me in the right direction!! Vacation saved! TheSamba it such a great community, indispensable!!!