My truck is a 99 5.2 4x4. It starts fine but after I drive a couple of minutes the battery charge starts to drop and then holds about 11-12. Sometimes it will start to drop a little more, sometimes it doesn't. If it drops too low it dies. Battery has been tested and it's fine, alternator has been tested and it's fine too. Went through and found a couple of bad grounds so I replaced them. Also replaced a couple of wires. Thought it might be the voltage regulator so I replaced the PCM, but it's still doing it. I'm out of ideas and could use some help.

 

Do you have a code reader? It should throw a code related to the alternator.

Battery temp sensor?

How many miles on the truck?

Since it seems to happen after you've driven it for a few minutes I'd still consider the alternator suspect. As it warms up parts expand. You may have a bad internal connection to a diode, etc.

The brushes also do not last forever.

 

I don't have a code reader. I just tested the battery temp sensor and it read 14.6 (I'm not very good with a voltmeter). Book says it should read between 9 and 11, so that could be it. And my truck has 205,000+ miles.

 

It may be the temp sensor, it does control charging based on temp.

Is it the original alternator? If you don't want to replace it I'd at least do the brushes.

 

For a 99, if your cell phone is Android based, there's nice ELM327/Bluetooth modules and a free program (Torque Free) that can read the codes for you. (There's also the paid version, which last time I paid for it, over 5 years ago, had just gone to $5 ... )

Since ELM327s can be had for under $50 easy (seen them down to under $10, but I have to wonder what was left out ... ), I'd seriously consider grabbing one for your truck so you can read codes.

With Torque paid, you can also monitor almost all the accessable data feeds to the ECU (not simultaneously, though, there's only so much bandwidth available!)

RwP

 

Replaced the battery temp sensor today. Thought it was fixed, but then my battery level dropped down again. I don't know if it's the original alternator or not.

 

The alternator would be the next choice. They don't last forever. You could pull it and have it tested again but it may not show up until it's been run for a while.

 

Check out the wear on these parts best is replace it just replacing the brushes is just a temp fix.


Your shaft bearings are probably a bit worn also buy new rebuilt are always an inferior product.


https://www.google.com/search?q=alte...FYajiAodht4DaA

 

One exception to 98Dakaz's note: There's still starter/generator shops around that some have been around since the 50's or 60's (I use one that the son of the guy who started it is about to retire - and HIS son is getting ready to take it over!) They can rebuild it for you.

Otherwise, yah, try to get a new one. Check carefully - you may find a larger (higher output) alternative that you can drop in.

RwP

 

ACDELCO new 136 AMP high output $140 + shipping rockauto.com no core as its new.