We just aquired a 71 521pl and i don't know much about it, been workin on my schoolbus too long and it is the only motor i seem to be familiar with anymore.

shouldn't the alt. keep the truck running with batt. diconnected like any other vehicle? I just put in a new hitachi alt. (was missing) and not sure if it is connected right. T plug is plugged in, but should the wire from the A post on alt. run to the starter solinoid or to the batt.? it appears to have been "ghetto rigged" at some point or another.

let me see if i can't find a manual of some sort for the 521... i'm not all that sure where it should go on your truck

as far as running with the battery disconnected... i have no clue... i've never tried it lol

yes your truck should keep running when you pull the positive or negative battery cable off the batterythis is not exactly how to do it but it still answers the question. your alternator, regulator or wiring is the problem one easy way is remove alternator. take alternator to a parts store that will bench test it for free if it passes then look at wiring or the regulator

i looked at an old wiring schematic copy and the white wire that comes off the A post on alternator does go to battery post on the starter solenoid(large post)which goes directly to the battery which you could do as wellgoing to the battery is not recomended because of corosion from battery

ok thx the alt wire is on the wrong side of the solinoid then will switch it and try it. but have to fix something else now, my wife put the battery in backwards(pos. 2 neg/neg 2 pos), she didn't look to see where the cables went, just looked at the colors(not origanal cables and wrong colors lol). so now something else is fried. one of the relays or something(little rectangle boxes mounted on inside passenger side fender), now it clicks when u attach the batt. cable and it didnt used to. so i'm guessing that it needs replaced now. and hopefully it the only problem that was caused.