Strange battery drain

[marriedmanw]

The other day while getting firewood I noticed my battery needle dropped down to 8V and the battery light was on the dash. I went to the auto parts store and they checked battery then said the alternator was bad.

I drove truck home and took battery out and measured at 9V, so I put on slow charge for 2 days then tested at 12.9V. I put the battery back in the truck and it started up, then I measured voltage coming back into battery and it was 14.34V which tells me the alternator is working. I checked my serp belt for tension and it was tight so I know it is not slipping. I did unplug the electrical connections at the alternator, cleaned and reconnected before putting battery back in.

I will be driving it tomorrow for 50 mile round trip so I can monitor. What else could I check for? The starter is new and not drawing much juice at startup. A older guy I know told me to start truck and while idling take the positive battery cable off. He said if it stalls then the alternator is bad. Is that safe to try?

Truck is 92 F150 Custom with 4.9L I6 with 233K miles.

 

[4x4junkie]

A older guy I know told me to start truck and while idling take the positive battery cable off. He said if it stalls then the alternator is bad. Is that safe to try?

Truck is 92 F150 Custom with 4.9L I6 with 233K miles.

That was fine to do on carbureted cars back in the days of this older gentleman, however trying that on today's cars (anything with EFI made in the last 25 years or so) is inviting trouble.

The way I test an alternator without a bunch of equipment is to simply turn the lights on (hi beams) and also turn the heater/blower fan on Hi, and then check the voltage at the alternator (between the output terminal and the alt housing) with the engine at 2000 or so RPM. You should have 14 volts or maybe a little above (a little less voltage at idle is not always cause for concern).

 

[adsm08]

14.3V sounds good to me.

It almost sounds like you have an intermittent issue.

 

[kimcrwbr1]

If you disconnect the battery when the engine is running the alternator goes wild and the surge can damage the computer. One thing to try is fully charge the battery and let it sit overnight disconnected and check to see if it is holding the charge. I had a bad starter one time give me grief doing alot of short trips it was pulling to many amps for the alternator to fully charge the battery and would quickly drain the battery. Bad cables or corroded cable ends also draw alot of power especially when it gets cold upgrading to larger cables and adding a 10guage ground wire from the alternator body to battery neg. Sand all the grounds metal to metal you can find.