1993 Toyota Pickup another charging issue 4 alternators

Electrical problem
1993 Toyota Pickup 4 cyl Four Wheel Drive Manual 240k miles

My truck ran fine for years, then bam 1 battery and 3 alternators later. I have a new battery and this alt should be good. With the truck running voltage is only 12.9 and falling. The grounds seem to be good "0" resist from the alt to the block, ".03" bounces lower depends were I have the leads hooked to. I checked from the (-) terminal to the alt case reads.02 and from (+) terminal to (+) lead at the alt.06. I did disonnect the positive cable w/ truck running, it goes dead. I have since learned this is not a good idea it may cause a spike with the alt and could fry my ECM not good. Shold I check the "L" pin on the back for voltage (running or not runnin?) The80amp fuse for the alt blew once but not again. Any help would be great