1987 Ford Ranger Power Issue

Ok, so I am completely stumped and boy oh boy could I use some help. I have an 87 Ford Ranger, 2.9 engine. Was going down the road the other day and turned on the AC and noticed that my truck suddenly started losing power. So I turned off the AC and just ran the fan...same issue, losing power.

The next night, I was going down the road and turned on my headlights...same issue, losing power. It now loses power whenever I run the fan, ac or headlights.

Now....here is the puzzling thing, when the truck is at idol or I am driving at low speeds, everything (lights, ac, fans) work great, no power drainage. But as soon as I pick up speed and use these accessories, the loss of power almost kills the truck.

WHAT could this be?? I have check 90 percent of the connectors and grounds.

I would start with the charging system and maybe the belt tension, you should be charging from 13.8V-14.2V with a load (lights or some other power robbing source) increase the engine RPM and moniter voltage, it should remain fairly steady with no drop off.

Spoiler 429 is right. Check the condition of your battery. Check the output of your alternator. And of course make sure your belt is not slipping. You said you don't have this problem when idling. While stopped and in neutral or park, rev up your engine and see if you have the same problem.