Okay, so I posted before about my truck not starting ,slow crank, etc.

7.3 4x4 Ex w 136,000 (just bought it and before me it had low miles for the last 3-4 years and sat in a hot climate)

Took batteries in to get tested and one is bad, so I replaced them both.Starts fine, but I am always leery of unintended consequences.

(what had me even start doing this was after replacing the batteries the hood lamp flickered, got steady, flickered, etc.)

Just to be sure running a bad battery for some period of time didn't work the alternator, I decided to volt test the batteries.

All #'s tested at terminals (actually pos to batt, neg to alternator housing shows the same)

Battery #1 engine off - 12.97
Battery #2 engine off - 12.94

Battery #1 engine on no acc's - 13.8
Battery #2 engine on no acc's - 13.85

Battery #1 engine on, high beams, fogs, heater on high - 12.77
Battery #2 engine on, high beams, fogs, heater on high - 12.76

My concern is if the alt is keeping up with the elec system at draw.

This is based on my completely limited/elementary knowledge of charging systems.

What does everyone think, anything else I should test, are these numbers normal. Could a belt and /or tensioner have anything to do with the low load #'s?

Honestly, if my wife takes this vehicle out and it dies, it is not something I want to deal with.

Thanks.

 

I would check right at the output from the alt..it sounds like it's going out though, or the batteries/poor connection are causing the voltage to drop..when my alt died it of course stuck my wife in somebody elses driveway so I can relate.
My before #'s had me at about 13.5 volts with the ac on and low beams..at the batteries, I upgraded the alt/battery cables, now I read 14.3 with the high beams/driving lights and heat/ac on(at the battery)
hope that helps

 

Bcrew, thanks for the post, I had thought about the battery connections, because they are stamped steel, not the solid lead connectors I am used to.

Anyhow, did like you suggested, alt output (as tested from neg/housing - pos/back of output lug) is pretty close (12.98'ish) as compared to 12.85'ish at the battery terminals.

So not too much loss from cabling as I can tell.

Hate to invest in an alternator if it isn't the issue.

 

pull the cable off the alt, test it with no load on it at all...if you have less than 14 off the back of the alt with no load...you have a paperweight

 

You may have the volts, but low on amps. My old alternator was bad, made the battery light on the dash flicker from time to time, but when I tested the volts, it was good.

I changed the alternator with a rebuilt one for $100 and all's good now.

 

Here are issues:

A) batteries cannot be tested together - they average each other out and cover up bad problems.

B) alternators are real bad --- but often it is the alternator wiring.

When replacing alternators, always replace (if needed or suspected) the pigtail.

C) corrosion is a serious issue -- spotless clean is the word.


See below for alternator / battery / etc. links