1994 E350 7.3 IDI (ambulance)
Have no power inside the van at all. No lights, no horn, nothing. Key in and turned, nothing. nothing nothing.

I have 12 volts at the battery, and at the starter relay (near drivers headlight). If I jump from the starter relay to the signal 'port' with a small wire, the motor turns over. So I have power in that side of the system.

Does anyone know how the power is routed to the Power Distribution box? I'm thinking that it goes to the PDB and then feeds into the inside of the van. I have no power anywhere in the Power Distribution box either. Trying to ascertain how it gets there.

Next the the starter relay I have a 'lug' that has one large yellow wire, one smaller yellow wire, and a red wire (that loops around front along drivers fender and possibly into cab. The yellow wires join the starter signal wire (blue/red) in a loom that goes down below the PDB. Seems like this should have power, but not sure where its coming from or going to....

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That would be correct on a normal van, and probably should still be on an ambulance. However, some ambulance makers do really garbage work, so nothing is certain.

I suggest you start at the PDC and find the largish wire that feeds into it, and physically trace it back to wherever it connects. You might have a fusible link that's blown somewhere along that route.

Here's a way to find out if the PDC is still in use (and thus, should have power to it). Find where that supply wire feeds the PDC. If you can, disconnect it. Now make yourself a jumper (please include an inline fuse!) and connect it to the PDC. Now check for power to the radio, interior lights, etc. If your normal functions come back, you know the PDC is connected normally and you have to find out why it's not getting power.

If you can't simply disconnect the main PDC feed line (some are crimped in place), find a fuse in the PDC that feeds some simple circuit, like the headlights, for example. If you can find that, make yourself a jumper (please include an inline fuse!) with a 1/4" male disconnect on the end. Better yet, if you can find one, a 3/16" or 1/8" male disconnect. (They do exist, but they are hard to find.) Run that from battery + terminal to the fuse. Pull the fuse, and plug your male disconnect in to one of the contacts for the fuse. Which side? If you can figure out which side is the supply and which is the load, that's great. Plug it into the load side and turn on your headlights. If you get lights, you have proven that the PDC is still being used to supply your inside voltage.

If you plug it in to the supply side, it shouldn't do any harm because that side is normally hot all the time. You just provided 12v to the PDC. So, check for power to the radio, interior lights, etc.

 

There is one large yellow wire (4 AWG if I was guessing) going to the PDC. I have the PDC loose, but not out where I can see it yet, so I don't know if this wire is supplying power to the PDC or taking it away from. Its the largest wire, so it would make sense that it was the supply, but life doesnt' always make sense.

The large yellow wire goes under the PDC and to a terminal lug mounted next to the starter relay. Along with it is another smaller gauge yellow wire that also connects to the lug and inside the same wire loom is the signal wire for the starter relay (which goes to the starter relay, of course).

On the lug that the large yellow wire connects to, there is also a slightly smaller but still large red wire (sorry for the vagueness...maybe a 6AWG). That red wire goes past the drivers headlight and along the drivers fender and appears to go inside the van near the kickpanel/emergency brake location. I havn't confirmed exactly where it goes yet...there is a mess of wires. One of my next goals is to determine exactly what happens to this red wire. I suppose it could be a supply wire, though it seems odd that it would supply power to the larger yellow wire.

I also need to uncover the large yellow wire, as it may have a fusible link in the portion that is under the PDC (That section is still wrapped).

Next I want to try and open the PDC (not sure how to do this) so that I can see what's what. Hopefully I can identify the supply side and see whats going on, following your advice, wirelessengineer.

I had to walk away from it today. Was getting a little frazzled. Its a lot of wires. And not a lot of room to work. I have identified some that I believe are feeding the ambulance add-ons (as large as the battery cables). I am trying to be patient as I go... so as not to make a silly mistake and make something worse.

At least the seller didn't cut out all the lights and sirens, so I don't have a bunch of loose wires. They did remove the Inverter, but I am hoping that was a relatively separate system. It ran through a breaker box behind the drivers seat. All the other systems appear to be intact.

Thanks for your input. It really helps to have someone else's knowledge and insight.

 

Okay, so yes the big yellow wire appears to power the PDC.
The other end of this large yellow wire was attached at a lug to a slightly less large red wire. I traced this red wire back to where it went into the cab.
And then it went under the seat to a kill switch, which was set to 'off'. I turned it 'on' and Voila! power to everything.

I am glad I didn't panic and start cutting stuff like crazy or jumping power to places *****-nilly. Although I do feel a little silly that the problem was so simple.

Now onto actually getting the van to run. Hoping this won't prove too difficult. I believe the wiring damage to the main harness (by a rodent) was why the school 'junked' this van.

Thanks for your help!

 

Great! Glad you got it 'fixed'!