Before we go any further I think some clarification is in order.
I assume that we are talking about a 1986 bus with an International chassis with a DT466 engine.
If it is a Type 'C' bus with the engine under a tilt hood out front finding an answer will be easier than if it is a Type 'D' bus with the engine completely under a dog house up front or in the rear of the bus.
Knowing which body is on the chassis would be helpful. Every body OEM connected the body electrical system to the chassis electrical system in different ways.
I assume you have already made sure the transmission selector is in neutral and it hasn't accidently been moved into reverse or drive. If you have been messing around under the dash it is easy to do. I have been there and done that more than a couple of times so I know of what I speak!
Since right now the main concern is getting the bus started you need to figure out what is the chassis harness and what is the body harness. Depending upon who made the body will determine how the body harness is powered. Regardless of who made the body the chassis harness will be pretty much the same.
Even in 1986 buses came with safety interlocks. Any bus with vandal locks had an ignition interlock that would prevent the engine from starting as long as the vandal locks were locked.
A test light is your best friend in order to find out where juice is getting lost.
If when you turn on the key nothing happens you will need to determine if you have juice going to the key switch. When I say nothing happens, if when you turn on the key no lights or buzzers come on then you know you have no juice coming into the key switch. If there is no juice to the key switch you need to find where the power in wire has gotten disconnected from the main power feed at the fuse box. It should be somewhere close to the fuse box which is to the right of the key switch and to the left of the glove box on S-series chassis buses.
If you have lights and buzzers but the starter does not engage one of two things is happening. First, there are some interlocks like a vandal lock or neutral safety switch that will keep the starter from engaging. But you will have juice on the key switch going out to the starter in the start position. Second, if you do not have juice leaving the key switch in the start position that could be your problem--the key switch.
If you have juice leaving the key switch in the start position you are going to have to determine what is preventing that juice going down the line to the starter solenoid. With the key in the run position try jumping from the hot lead on the starter to the small terminal on the starter solenoid. That should energize the starter and start the bus and let you know that the problem is still between the key switch and the starter solenoid. If nothing happens that could mean the starter solenoid is bad and it is time to replace the starter.
If by jumping the terminals on the starter solenoid starts the bus you have two choices. First, wire in a momentary push button that is hot whenever the key is in the run position and run a new wire down to the starter solenoid--it is a bit of old school/shade tree/red neck engineering. But it could save hours of tracing problems through a maze of wires. Second, if you don't take the first option your only option is then to determine where between the key switch and the starter solenoid the juice is getting locked out. It will require a lot of time under the dash board, crawling around inside the engine compartment, and crawling around under the bus.
In most cases in a bus the vintage of your bus there isn't anything in the body harness that is directly connected to the bus starting circuit unless you have some sort of vandal lock. If your bus does not have vandal locks nothing in the side control panel will be controlled by the key switch unless it has a solenoid operated by the key switch to power up the side control panel.
Good luck and keep us posted as to your progress.
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