Charging system issues

[jacob dehart]

I have a 94 K1500 5.7 tbi. I was working on the trucks brakes and had started it to let it get warm as it has sat sone time while working on it. It suddenly revved higher and then went back to idle quickly and in doing so my battery light came on and voltage dropped from 14v to 9v. I immediately checked the battery and alternator output post and they were reading 12.03v at idle under minimal load. (no lights on or ac or nothing like that, just the door was open)

As for some back story I bought the truck and the alternator wire to the pos. battery terminal was not connected. When I tightened it down and made a connection I noticed my alternator was getting a bit hotter than what I considered normal after running at idle with minimal load. That being said I had replaced 2 alternators and the same problem reoccured. Within 20-30 minutes running at idle, I read alternator temps all the way up to 135-140 degrees which is quite hot to the touch.

The truck also has the “Big 3” upgrade to the charging system, which I had just added 2 bigger ground wires from neg. terminal to motor frame and body. It has new battery cables and alternator wire, as well as a new battery. Also I have no aftermarket options as far as electrical goes.

Anyone have any ideas why the sudden voltage drop on the dash but not to the battery and alternator? And any ideas on why my alternator is “overheating?”

 

[GoToGuy]

What temp do you think is normal operating temperature for an alternator working as designed? What do you think makes heat? Are the bearings failing? Any noise that's unusual?

 

[jacob dehart]

@GoToGuy from other replies I have heard alternators should be good for operating all the way up to 250-300 degrees farenheit.

assuming that is okay, i still do not understand the sudden voltage drop.

 

[GoToGuy]

I would like to see a higher voltage at the alternator post. Ground at back of RT head to body ?
What size wire from alt' positive to battery?
Under full load ( everything on / powered ) the voltage is good at 1500 rpm?
Brakes, at wheel positions or under hood, the spike in RPM , " that's a concerning mystery " , the voltage drop observed at instrument panel? Then confirmed under hood?
Did you check or had a fully charged battery before swapping in new alternator?

 

[Erik the Awful]

And any ideas on why my alternator is “overheating?”

What temp do you think is normal operating temperature for an alternator working as designed?

An alternator isn't overheating until the windings melt out the backside. My race team has done that on our E36 BMW race car a couple times.