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Old July 6th, 2019, 01:16 PM
James James is offline
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Join Date: Jun 2013
Location: Greer, SC
Truck: 1964 GMC 1500 2wd
Age: 65
Posts: 251
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Default Re: '65 3/4 305V6 fleetside rebuild

So truck runs, only when jump started. Once turned off, it wont give any indication of turning over after a few minutes.
Jump start test for alternator: jumped, started then disconnected positive cable, no problems, engine continued to run, I am told this Indictates alternator is charging battery.

Any suggestions on what to replace next?
Most of the problems I have encounter over the year on many different cars and truck is bad connections. With our 50+ year truck this is common.

If you hear no clicking of the starter when the key is in start then it time to use a voltmeter to check the battery voltage and battery circuits.

The voltage on the battery posts (not at the clamps) should be over 12 volts just sitting there with nothing turn on, do this first before you turn the key or anything else on. If it is under 12 volts disconnect the battery post clamps and charge the battery on the posts. This will give you a good opportunity to check the posts/clamps for cleanliness. Also if you see anything with a green color on the bare copper wire that indicate corrosion (and a bad connection) and the wire should be cut back until there is no green on the wire. You might have to extend the wire using the same gauge wire so it can be reconnected.

With a fully charge battery, turn on your headlights and measure the voltage on the post. It should be just over 12 volts. If it is below 12 volt the battery is either bad or it need to be charge (has a very low state of charge or with a fully charge battery it indicate the battery is bad).

If it still just over 12 volts you need to check the battery circuit with the headlights on and the key off for the following tests.

With every electrical troubleshooting, use a wire diagram to know where the wire are connected at and their wire color. Also going through a multi pins connector make sure all connection are clean and not discolored (which indicate a bad connection). That might also fix other issues that you have not gotten to yet.

Test 1
Keep the voltmeter negative lead on the battery post and place the positive voltmeter lead on the engine block. There should indicate less than .5 volts, the lower the better. If more than .5 volts then you need to disassemble and clean each connections between the battery and the engine block. Also look for wire terminal that has become discolored, this indicate a bad connection within the connector (connector will need to be replaced). If the problem is not on the negative side then check the positive side.

Test 2
Place the voltmeter positive lead on the battery post and place the negative voltmeter lead on the fuse block (find a connection that is hot all the time with the key off). There should indicate less than .5 volts, the lower the better. If more than .5 volts then you need to disassemble and clean each connections between the battery and the fuse block.

The test you performed is not really that valid. It only means the alternator is putting out enough voltage and current for the ignition circuit. However using a voltmeter you should have approx. 14 VDC at the battery with the engine running at a fast idle on a fully charge battery (on a low battery it might be indicating around 13 volts).

With a fully charge battery, the battery voltage should never drop below 9 volts while cranking the engine (if it does that indicate a weak battery).

This should take care of your cranking issue. This same technique can be applied to other circuits that is not operating properly (i.e. vent blower motor, tail lights, and etc).

Last edited by James; July 6th, 2019 at 01:18 PM. Reason: Correcting spelling.
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