M1009 starting problem. Batterries, relay switch?

I have a 1985 M1009 Blazer with 32504 miles on it with the military spec batteries which I figure they are the original because of the low mileage. The problem I'm experiencing is the batteries are fully charged but when the ignition switch is turned on the amp gauge moves up to just below the yellow and the glow plugs go through the cycle and the wait light shuts off but when you turn the key to start the starter barely cranks. Turn the headlights on and they are bright, horn works, I have working transceivers and when you turn the switches on they operate good. Any suggestions and knowledge to resolve this problem would be much appreciated.

There are lots of problems that can cause this. In my case, one of the ring terminals on the buss bar on the firewall was corroded and creating high resistance.

Before beginning any troubleshooting of an electrical issue, make sure your batteries are fully charged and load tested. Test them. Do not assume.

I am working on a truck with a similar issue. The front 12v system battery is good. The rear battery may be bad. It only had 7.5 volts. I am hoping I can recover it.

I hooked up both batteries on trickle charge for a day and a half. Would not take charge. I then removed batteries and took them to the local Excide Battery Warehouse to have them load tested. Both failed.....less than 5% in both batteries. Found out you cannot purchase authentic government issue 12V batteries anymore like in the past. Had to purchase civilian brand. Oh well, at least I got $40.00 for both batteries for recycling them. Harry

Glad you found the issue.