I have a 1990 w250 that was wrecked and needed a cab swap, it started and and stopped by the ignition, after the accident, before the cab swap. I can get it to start through the connector on the starter relay. I can get power to the cab with the key in the on position but, when I Turn it to crank, nothing. I have replaced the lock cylinder 2 times, the ignition switch 3 times and adjusted to ensure the pushrod is engaging. I’ve replaced the neutral safety switch 2 times and the started relay 3 times, I’ve also eliminated all the fusable links and replaced them with a fuse box from blue sea systems, and replaced and cleaned every ground connection with no luck. My yellow ign. Wire runs through my grid heater module pn# R4429546 which is discontinued and I can’t find an alternative other than a delete and thats the only thing I haven’t replaced. I’m trying to avoid a delete, and a start stop switch. I’ve let the truck run for a few hours at a time and I’ve never once had the grid heaters cycle. I’m looking for advice or a next step. I’m stumped and out of local resources!

Thank you and all suggestions are greatly appreciated!

 

Have you checked that the FSS is getting power with the key in the on position?

Or, try running a hot wire right to it from the battery and try to start it then.

 

Yes, I have power to the FSS with the key in the on position. I just checked it out.

 

If you have power to the FSS and cross the starter terminals [MAKE SURE TRUCK IS OUT OF GEAR!] it should start up.
I did that with my old plow truck to finish out the winter one year....

 

You have an electrical fault somewhere. No amount of parts replacement will fix it. It will deplete your wallet. You need to do electrical testing before parts replacement. Since engine, apparently, is not cranking I would test electrical circuits starting at the starter relay. Since starter will not key activate, I would suspect a fault in the circuit from ignition key to relay or a fault from relay to ground. This is the trigger circuit to activate the starter relay. Since you can start the engine at the starter relay, the power circuit and circuit to starter appears to be good. The trigger circuit is the place to start. Easy to test.

 

I posted this a little while back on another forum. The difference being he ad some burned up fusible links.
Don't know if you have the wiring diagrams or not or what your knowledge/skill level is, so please bare with me.
There are 2 connectors on the transmission. The 3 pin is for N safety/backup lights switch. The 2 pin is for OD.
At the back of the engine there are 2 large connectors. The black one carries the neutral safety wire.
From there it goes to the starter relay coil on the left fender. This is different from the starter solenoid that is on the starter.
This completes the ground circuit for the relay coil.
From the relay coil it goes to a connector under the dashboard near the brake pedal. This connector is intended for the clutch, but for an automatic it should be jumpered.
From there it goes to fuse #3 in the fuse block.
Through the start side of the ignition switch.
Through a fusible link and back to the battery.
This completes the positive side of the relay coil circuit.
Next the starter solenoid circuit.
From the solenoid to a 2 pin black connector near the battery.
From the connector to the starter relay contacts.
From the contacts direct to the battery.