bad alternator??

[Chas 0]

1998 Sable wagon with 3.0 24v DOHC engine.  Starts fine and shows around 14 volts at battery when warming up.  Can disconnect positive cable at battery and still runs fine until reaching normal operating temperature.  As soon as temp guage reaches normal, (middle of guage) engine dies as alternator apparently loses output.  Since alternators are underneath and a pain to remove, was wondering if something may be signaling the alternator to shut down.  ECU??  Anybody have ideas?  Thaks

[Kodachrome Wolf 475]

I'm going to kindly say please don't go yanking battery cables off the battery with the car running. If you did get a voltage spike or some large flux, the battery acts a bit like a capacitor in that instance so you don't toast your electrical equipment. Plus, yanking the cable's isn't a guaranteed way to verify alternator integrity. If you yank the cable on my '87 wagon, it shuts down, but the alternator is fine. I'd hate for anything on your car to get damaged outside of the scope of what you're working on.

 

Now, all things considered, I have dealt with failing alternators that did okay when they were cold, but as they heated up, they'd fall out. Sometimes it's a bad voltage regulator, bad brushes/messed up slip ring, sometimes it's a bad winding connection that separates enough when hot to cause a charging failure. When it gets up to operating temperature, have you verified voltage output to confirm it's no longer pushing above 12v? If the electrical output is normal at operating temperature, I might suspect possibly the engine coolant temperature sending unit that communicates to the PCM and would inspect what it's reading either using a live scan tool, or measuring with resistance with a multimeter. A failed ECT could certainly cause funny driveability issues when the vehicle attempts to enter closed loop operation.

Edited August 5, 2021 by Kodachrome Wolf

[Spridget 498]

I’m confused by the diagnostic process here. Please elaborate.

So you’ve started the engine. The engine is running and voltage is normal. With engine running at normal temp, you disconnect the battery terminal and engine dies. What voltage did you see at the battery before disconnecting the terminals with the engine at normal temp? Did the voltage decline as the engine warmed up?

As Kodachrome describes above, I’ve also seen alternators that fail as they warm up.