Electrical system failure

[student]

Driving the vehicle, the battery light flashes on and off for a day or two, then becomes constant. A few miles later, the "M&S" lights come on, followed by srs, abs, and basically everything else. Lights dim, engine sputters and dies. I pulled the alternator and it tests good. Battery is a month or two old because the previous one went bad, and still holds a charge just fine. 150 amp fuse-able link looks fine.

What do I check next? It has all the signs of a bad alternator, but the alternator tests fine out of the vehicle.

 

[DWB]

Couple of questions: is the problem intermittent or constant? How/where did you check the alternator? Is the battery flat after an episode of lights dimming and engine dying?

One thought- have you checked all the cable and ground connections, not just those going to the alternator?

 

[ratboy]

check the cable from the alt to the battery.

 

[Justin_Sherfy]

How/where did you check the alternator? [\QUOTE]

x2

How did you check the alternator?

Use a multimeter and measure the voltage across the battery terminals while the truck is running. How many volts does it measure?

 

[student]

Thanks for all the help so far.

I had the alternator bench tested at O'Reilly's. It's just a simple pass/fail, no actual voltage. I'll check for an output once I get it back on. Since it is not generating enough current to even keep the truck running, I believe (in my otherwise confused mind) that it is either "upstream" and not kicking on at all, or "downstream" and not getting to the battery or something draining it to fast for the alternator to keep up. Checking voltage on the truck should determine which of the two I'm dealing with. If the otherwise working alternator is not kicking on, where do I look next?

Battery was dead. I was able to jump it off another truck a few times to restart it and drive home. The battery light was initially intermittent, but the problem is now constant. The battery has since held a charge, so I don't think the problem is the battery itself.

 

[JohnB]

Idiot light only needs 3 volts to turn it off. The DII alternator is odd to me at least. I'm used to alternator that needs 12 volts to turn the field on. Older Bosch alts have three wires. Idiot, field, battery. DII only has two idiot, battery.

Put a 12 volt gauge on the battery and see how much the alt is putting out. I bet its not over 12 volts. When my alt went out. The idiot light would do what you've described.

 

[Rover_Hokie]

I just went through this. Battery light on for two days, then other lights M&S came on while driving right before is died and left me on the side of the road. Put in other fully charged good battery and got it to start, Battery light still on, ran for two days, and then same thing. Checked voltage on battery across battery post when running only had 12.7V. Replaced alternator, all is good now. So either bad alternator or bad cable or connection from alt to battery would be my guess in your case.

 

[student]

Ok, like everyone suspected, it was the alternator. Running, I only had about 11.8 volts. Napa needed 8 days to get a replacement, so they sent me to a local rebuilder. In one day, out the next. $106 out the door.

Thank you for your help.