Replaced alternator, but multimeter showing that battery voltage is dropping...?

Recently installed a remanufactured alternator, because the battery kept dying.

Finally got to start it with the new alternator tyoday, and a fully charged battery, but noticed the battery voltage was dropping on my multimeter, with the truck running, headlights on, fan on, other interior lights on.

Could I have a bad alternator cable?

Or could it be that the extra wiring some previous owner had installed for the aftermarket stereo, amp, and subwoofer, etc., is drawing power from the battery?

I noticed two exposed wires connected to nothing, that are under the passenger seat (I covered them with electrical tape). One of the wires is a thicker red coated wire.

There are also some extra cables and/or wires connected to the positive battery cable. Feel like I should disconnect them from the positive battery cable, to see if the problem goes away.

 

What are the voltage readings at idle with and without loads (high beams and AC full blast)?

 

@nickrick78 I will have to get back to you. Thanks for the help.

 

Sounds good, no problem!

 

Cold voltage is higher, and then drops by around .5A when things start warming up. I have a high output diode, but I fire up at 14.8 and settle to around 14.4/.3

 

@nickrick78 @alb1k I forgot to mention, I didn't record what the voltage was when I turned on the headlights (not on high beams), I didn't turn on AC just the fan, and the voltage kept dropping, to where Chris Fix on Youtube said it shouldn't drop below 12.7 volts. And it kept dropping to towards 12.7 volts, so I turned the engine off. and disconnected the negative battery cable.

I did rev up the engine a couple times, before I turned off the engine, and noticed the battery gauge needle rise, every time I revved the engine.

I am going to record what the volts were at idle, and hopefully when lights, fan, etc., are turned on (but volts were dropping, so there wasn't a voltage it was staying at).

 

If it rises with engine speed you likely got a bad reman. I've had 3 of them myself, now I just eat the cost and buy new.

At idle you should still see over 13v regardless of what accessories you're running.

 

@nickrick78 @alb1k So I started the engine again. The multimeter reading was about 14.26 volts, with the high beams on, AC on (though my AC doesn't blow cold/work), interior lights on, and cargo light on, but it started to drop into the 13's.

I turned all those things off, and the reading at idle stabilized at around 12.03 volts. I hope it's not because I have a cheap Harbor Freight multimeter tester.

 

Everything sounded fine until you said 12.03V while idling. @Festerw was right about holding above 13V. And @nickrick78 is right as well. I would have the battery load tested.

 

Just a reminder that a "bad alternator" is most commonly just worn brushes. A $15-$20 part. As a simple point of reference my 1997 T100 has the original alternator. I recently installed my 2nd set of replacement brushes at ~265,000 miles.

Now the lawyer part: YMMV.

 

Sounds like it has an issue internal to the alternator. It should be high 13s to low-mid 14s all the time

 

Cables under the seat are likely from a previous owner installing an aftermarket amplifier. You may notice a similar thickness red wire and casing attached at the battery. If so, trace it, if it goes thru the firewall, you have your culprit. Hopefully they properly fused that line in the engine bay, less than 12-18" from the battery.

I agree with the others: Bad reman or battery. Good news is, any big-box auto parts store offers two very helpful services: Free battery testing, and free CEL code pulls. I recommend to start by having your battery tested, just to remove that from the equation.

If batt tests fine, chances are you got a bad reman. I really, really don't recommend reman alternators unless it's Denso brand, but honestly, I've gotten so sick of buying reman parts and running into similar issues to you that I just buy shit OEM new nowadays if I'm doing the work. The shop I use when I'm strapped for tools or not up for the chore/task at-hand will replace defective reman parts they install, that's about the only time I'll use them anymore.