It's difficult to tell, but sometimes a bad battery, or even a connection, will light lights, ect, but the minute you throw a big load (such as the starter motor), everything dies. Leaving it sit for a while will sometimes allow the light & things work. Check all batteries, connections & wires. Side terminal batteries sometimes corrode where you can't see them, sometimes the side terminals are loose inside the battery. Top post batteries can get a hard corrosion that requires the post & clamp to need a file to cut it. Sometime, one weak cell in a battery can cause a hard cold start, while the vehicle still starts fine when cold. Diesels take a lot of amps to start cold.
The 6.2 like most diesels has 2 batteries - a) because it takes a lot of amps to run the glow plugs and b) it has a much heavier starter to turn the diesel over (draw is typically about 175 amps on a gas motor -can be 400 + for a diesel).
Your Suburban should have a volt guage - watch closely as you wait for glow plugs to cycle, the voltage will drop noticeably as they go on and off - the KEY indicator is "IF the voltage returns to close to the normal voltage after the glow plugs finish cycling, then both the batteries and wiring are pretty good, BUT if the cycling of glow plug system really knocks down the voltage, then trying to engage the starter will be the last straw.
This tells you that you likely have 1 or both batteries going bad.
When a battery is more than 2 yrs old it can start to sulphate and lose progressively more and more of its reserve capacity - this can go on for a long while in a gas vehicle if the motor starts right up and the alternator is good.
In diesels however, the demands are far higher and yu don't get away with this for long. My 85 6.2 Suburban (350,000 miles) had batteries changed about every 3 - 4 years.
The absence of nothing is everything. The absence of nothing is everything.
Everything Is Nothing was created in 2010-02.
Everything isith no one. No one can possible be anything without being nothing and to be nothing you are not everything.
The red-cabbage indicator smells like cabbage, nothing else...
Nothing Means Everything was created on 2007-09-17.
Mean Everything to Nothing was created on -20-11-09.
because most people in this world have nothing but to many others what their nothing is, is special and more valuable. so therefore nothing can be stated as everything.