Ok bone headed move I know, installed battery backwards, tried to start, wouldnt lol, disconnected battery and connected right way, replaced 1 of the mega fuse's, checked fuses in power distro box, and under dash, still wont crank. any help much appreciated.
A little more info 4.2 xlt, checked all the lights work, headlights, turn signals, dome, instrument cluster, connected a scan tool to obd connector no codes set, mil light is off, looking at electrical diagram power from switch goes through transmission safety switch then to starter motor relay, before energizing starter. any fuses to check i could have missed, when I said it wouldnt start, in doesn't even turn over. any thoughts? what about starter motor relay?
Does the start relay on the firewall pull up by sound?
If yes the starter or it's cables are open.
If no, the start circuit from the ignition switch through the trans range selector to the start relay winding is open. Only fuse in this line is #21.
What can happen with the battery backwards is if the start circuit pulls up, the starter would try to run in reverse under a lockup conditon. It is a DC motor and is polarity sensitive.
The huge current could open an already poor cable.
There is not much to the circuit that a voltmeter check would not uncover aside from the starter cables being an issue.
The mega fuse has no part in the starting except as a convient connection point on the route between the alternator and the battery for charging.
Only possible reason the mega fuse would blow is from the reversed power from the battery shorting to ground through the alternator diodes.
Then you would need to replace the alternator if it does not charge after the motor cranks and runs.
From what you report, the start relay is still good.
This leaves the wire harness between the fuse block and the trans range switch and to the start relay with a short to ground.
I would remove the (usually tan/red wire) off the start relay and with fuse 21 out, check it to ground for the short.
If you see it grounded, look at the harness for overheating somewhere along it's length.
There may be other wires that overheated and caused the ground to occurr.
What has happened is the huge currrent that flowed from the battery did instant damge to wiring and the alternator.
The Mega fuse limited the current to it's distruction value. That is 12 volts times 175 amps = Two Thousand One Hundred watts of power and there is well three times that much in the battery.
If it were not for fuse 21 and the mega fuse the truck might have went up in flames.
That's why they were engineered into the circuits.
DEM INGEN EARS are pretty schmart!
Yesterday I couldn't spel ingineer, today I are one!
Kidding aside, there is a lot of power in that heavey black box with those big termnals (dats why there big).
I don't think you have one.
It would not be damaged by reverse polarity in any event.
The fuse 21 that is blowig is due to a short to ground.
The start interrupt relay is only it's winding being fed from that fuse and is not polarity sensitive.
If the relay were stuck in the operated position, it 'opens' (interrupts) the start chain only and not cause the fuse to blow.
In a Haynes manuel see page 12-17 for the start circuit to see all this.