Big Electrical problem
 

I am working on a 85 k5 6.2l diesel blazer. Someone "helped" me jump start the truck but reversed the the cables. Since then the truck has not started. The dashlights do not come on, so far I have checked the alternator and starter both passed. Headlights came on when I jumper from battery to junction block on the firewall otherwise all is dead. Not sure what to check next. Any help would be appreciated.

did you check the main fuse?? when the cables got put on the wrong way it could have blown it.

Is there an inline fuse between the fuse box and power source... not sure if it runs to alt or battery. If so, and that is blown, you will get no power to the fuse box. Also, where did you jump to on the fuse box? Main power in? If so all the fuse box should be fine, replace the cable or inline fuse in the supply wiring.

There is no main fuse(that I know of) and I didn't jumper to the fuse block; I jumpered to the a junction block. After doing some investigating I noticed no wire from battery to the alternator and also a fusible link that was missing by the junction block. Now I have lights and accessories but turn the key and nothing.

Fusable link at the starter ???

Had the same problem with an Olds diesel years ago. Was the wiring connections at the starter. Cleaned all connections and made sure everything was tight. That took care of it.

This is probably going to sound obvious, but check every wire and ground under the hood. Also check the fusible link(s). Similar situation happened on my 90 K5 Blazer and the results were, fried PCM, fried alternator, fried battery cables and a melted fusible link.

did you ever get this fixed???
there should be a main fuse, i think it would be a MAXI 50 amp. i did the same thing one time and it killed the whole car. swapped out the fuse and away she went.

Sorry for the late response, It is fixed. Found a fusible link about a foot long on the firewall that was missing.
And I hadnt connected the starter correctly