strange problem, 86 chev. PU

86 chev. pu, 305 engine, Engine cranks over slowly sometimes, battery voltage is where it should be, 12.4-12.6, voltage at starter is the same, sometimes it cranks over slowly....when the dash volt guage shows 8-10 volts
it doesnt wanna crank, when voltage is up close to 13 on volt gauge the pickup cranks over just fine
when the dash gauge shows its low reading and doesnt wanna crank ........the battery voltage is still where it should be, so i am losing power somewhere from the starter to wherever the power goes to,
battery voltage is always up, i even went and changed starters with a known good starter from my other pickup and still the same thing, still doesnt wanna crank sometimes
what is going on here???
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battery for free to see if it has the amps necessary to operate everything properly. It takes just a couple of minutes.
Loyd
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Could be loose connection or corrosion. I've seen brand new batteries lose enough current through corroded cables to not be able to power headlights.....remember, the current (electrons) travels on the outside of the wire, and that's where it corrodes (electrons can't move through the corrosion)...
HTH
Jeremy
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I would have the battery checked before you do a bunch more work, sounds like a weak battery to me, a good battery should have 13 to 13.8 volts with no load on it, might have one cell thats bad in it. Have you tried putting jumper cables on it from another vehicle? that will tell you if you cable is good to the starter, however you could still have a corrision problem at the battery.
Tim

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Another thing I've seen on eBay are relay fixes for this problem. Something about the wiring system getting "worn out". It diverts full power (basically) to the starter... I don't know if I like that, sounds like a bandaid not a fix. I'd have the battery tested with a load tester if possible. If not, disconnect the battery when fully charged (good long ride - 4+ hours, or put it on a charger) and get the voltage. Then put it in your truck, and turn it to run, but don't start. Turn on every possible electric thing you can, and then test the voltage. Less than say 1volt in change is good.
~KJ~

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