HEy guys,
I'm getting a new alternator for my 1985 f250 6.9 xl. Just curious as to what kind to get? I was looking at Motorcraft but I'm not sure which one I need exactly because I don't know the voltage. Looked at the one I have now but couldn't see anything. Thanks a lot

 

I think amperage is what you're looking for there. Our generation of IDIs used a fairly low amp (90-130 amps is what I've seen in them), but if you're not putting any particular load on your charging system they are all you'll need.

However if you need, or just want, to upgrade the 3G upgrade is a popular mod, instructions and info all linked in the FAQ.

 

From my years in fire service with two ambulance prep package 7.3 rescue trucks, I am not fond of big alternators unless truly needed. Wear the alternator belt and when it starts slipping, the vac pump doesn't keep up and the power brakes go away. Kind of a knee jerk reaction, but it got pretty real a couple times.

 

The alternator should only be loading down the belt with as much power as you are using. If you use 10a from a 30a alternator, it will produce about the same amount of resistance to the belt as 10a from a 300a alternator.

The voltage in an alternator is controlled by the regulator(It compensates to make the voltage stable at varying engine speeds and output loads). In our trucks, that regulator is external, which can cause issues if it is not properly grounded.
If your regulator is grounded, you'll see 14.1-14.5V once warmed up and the batteries are charged.

 

True, but that only accounts for a sustainable consistent load. If the batteries are low and then you crank the engine over the load will be pretty much whatever the alternator can put out.

For context, the HD alternators that were roasting belts were 60 amp. But that's 60 amp @ 100% duty cycle. And we would load them up badly. Sitting there idling on the side of the highway for hours with all the lights, accessories, radio etc. going.

But having said that, the max load the alt can put on the belt was about the same. It's just that we ran them at the max routinely. Many a time I wished the alternator belt wasn't buried underneath all the others.

Don't get me wrong, I don't think it's that big of a deal, I experienced this on the WAY high performance end of alternator function, and it was problematic. Pretty hard to put that kind of load on an alternator short of a LOT of winching.