electrical help

[jmeadows]

I have a 1978 sierra grade 1500 2wd with a Chevy 350 in it. It was not starting yesterday. Here is the situation:
-I just bought a new battery 2 hours ago
-When the truck is running the battery reads 14v as if the alternator is charging it
-When I connect my voltmeter from the positive terminal of the battery (connected) to ground (negative terminal un-connected) I get no reading
-When I connect my volt meter from the negative battery terminal (unconnected) to ground I get a reading of 12 volts and the postove terminal is connected
-Nothing is switched on in the truck, EXCEPT the fan, the fan runs all the time when the truck is started, and thus is switched on. However it is normal for the fan to be on all the time it says so in the owners manual


Please help me, I have a suspicion that something is a rye here even though I haven’t had ample time to see if my NEW battery does in fact drain.

-Jordan

[curtis73]

Originally Posted by jmeadows

-I just bought a new battery 2 hours ago
-When the truck is running the battery reads 14v as if the alternator is charging it
-When I connect my voltmeter from the positive terminal of the battery (connected) to ground (negative terminal un-connected) I get no reading

You shouldn't get a reading because the circuit is not complete and you have 14v of deadheaded voltage coming into the system. Never run a vehicle with the battery disconnected. You can fry the alternator's diodes and void its warranty or make it unusable as a core return.

-When I connect my volt meter from the negative battery terminal (unconnected) to ground I get a reading of 12 volts and the postove terminal is connected

That makes sense. When you disconnect the negative terminal and then test between the ground and battery, the only way electricity has to get back to the battery is through the voltmeter.

nothing is switched on in the truck, EXCEPT the fan, the fan runs all the time when the truck is started, and thus is switched on. However it is normal for the fan to be on all the time it says so in the owners manual

That is correct. The ventilation fan on that truck is on low by default

Nothing sounds out of whack to me. Here are some tests you can do. With everything connected, test the voltage at the battery with the key off. You should get 12-12.5v. Start the truck and test the same thing. You should get 13.5-14.5 if the alternator is working right. You may have to rev it a little to get things started charging, but at no time should you run the truck with the battery cables disconnected.

[jmeadows]

oh, you must have mistunderstood i never did run the truck with the battery disconnected

but thank you for all the help

[curtis73]

Sure thing