We have an '89 F-250 460 shop truck that indicates the alternator is barely charging. Slow turn signals, BIG power drop when the lights or A/C are turned on that sort of thing. So we replaced the alternator, same symptoms. We had both new and old alternators tested and it turns out the old unit was fine, new one ok. We get 14vdc at the output of the alternator but around 9 - 11 volts indicated. The really weird thing is the truck can sit for weeks and it fires right up (?) if it weren't for the sluggish signals and the dim headlamps I'd write it off as a bad gauge; this one has us scratching our heads. Any suggestions? Internal voltage regulator on the alternators by the way. No dead short and as I said above the battery is charging, just slowly.

 

How is the cable from the alt to the battery?

These years had issues with the connectors(?) or wiring and pretty sure it was a recall back then because they can catch fire.

 

Sounds like you have corrosion at your connections or you have a bad ground(s) in the system. The slow blinkers usually always indicate that there is a restriction of your electrical flow. Bad, corroded, pinched wires. Connectors really dirty, Bad corroded, rusted, loose ground wires or cables.

 

The terminals are all in pretty good shape at the battery, we'll check further down stream to see if something is loose or corroded. We just found out the battery is about 15 years old, we're going to replace that. Perhaps enough crap has settled into the bottom of the battery case to cause trouble; Strange one. We'll get after the wiring next...

 

I've seen batteries be the cause of lots of things but not slow blinkers, but you never know. At 15 years, I'm surprised the battery could hold any juice at all.

When rechecking wires and connections, don't forget to check under the wire shield as well as possible. Just because they look ok from the outside doesn't mean they aren't bad on the inside.