I've been scouring the threads and collecting information to track this down. The situation is I turn the key and faint lights on the dash, but no start, no clicking, nothing. Headlights, wipers barely turn on/ just click and nothing. Batteries are new and fully charged and alts are working. WHEN I HOOK UP A JUMPER BOX TO THE MAIN BATTERY, THE TRUCK STARTS UP JUST FINE AND RUNS...LIGHTS WORK, WIPERS WORK, RADIO, EVERYTHING. AFTER RUNNING IT FOR 15MINS+ I TURN IT OFF AND SAME THING, NO START, NO CRANK, NOTHING AND LIGHTS DON'T WORK.

HERE ARE THE FACTS:

• 22R MOTOR WITH CARB 3000 MILES ON BOTH, PROFESSIONALLY INSTALLED AND TUNED A YEAR AGO.
• STARTER A FEW YEARS OLD
• DUAL BATTERY, DUAL ALTERNATOR INSTALLED BY PREVIOUS OWNER
• I TESTED BOTH BATTERIES AND ALTERNATORS AND THEY'RE WORKING AND CHARGED.
• I HAVE NOT TESTED IGNITION SWITCH YET
• I HAVE NOT LOOKED AT ALL FUSES YET

Also, I don't have a wiring diagram for '85 pickup with CARB. All I can see are diagrams with efi.

Any suggestion of focus areas would be helpful. My next plan is to test power to ignition switch, check fuses, check batt wires/grounds.

 

what did you test on the batteries? load test or voltage? tell us what you have for voltage at each battery both running and when shut off!
check your battery cable ends, clean and tighten them and keep in mind that even with good voltage readings a battery can fail a load test.

 

• I tested both batteries' voltage with vehicle off. They both read approx. 12.75V
• With the vehicle running I tested voltage on both batteries and they both read approx 14.5V
  • Same as voltage I tested at each alternator.
• With the vehicle running, I turned on lights, radio, wipers and saw only a small drop at battery voltage.

I need to check/clean the battery connections definitely.

I've started to map the wiring of this dual battery system to see what the PO did, but I don't have any completed visuals yet.

 

you still need to load test your batteries if cleaning the cable ends doesn't help

 

UPDATE:

I worked on the truck this weekend and found the problem. There was a split in the ground wire from the main batter close to the terminal that overtime had filled up with some corrosion and seems at though is was limiting the voltage to the cabin electronics. Replaced the cable and strong voltage everywhere! Thanks for the help all!

 

You mean the cable that goes from batt negative to fender ground near fuse block (like this)? Yep, that supplies ground to most, of not all, of cabin electronics. The thicker cable that goes to the engine block is for starter cranking, engine sensors, etc. That needs to be solid and clean as well.

 

I got a similar issue, brake and battery light.
Changed Alternator and belt 2 months ago by mechanic, just changed the battery since it was flat (I dont drive alot) and the lights come back when engine is above 2300 rpm.
I suspect there is an issue with the connector but why only above X RPM ?

 

For me it was vibration. The higher the RMPs the more the vehicle shakes and weak connections show themselves.