If a diesel engine cranks but does not start the first thing you want to look at is the power feed to the engine controls. In this case it's a Mercedes mbe 900 diesel engine in a 2007 conventional school bus. The first check is the fuse block since this powers up the ignition relays that operate the engine control modules.
Here is the Engine Power Distribution Module (photo below) with the relay that sends a signal to the engine control module.
The wavy line on circuit here has a ground to one side. With no ground the coil does not energize which will not allow the circuit to close.
Mercedes diesel engines have 2 modules, one on the engine and the other inside the cab. The ignition switch activates the relays that gives us the start operation. We found voltage to the cab module but when the engine was cranked over there was a voltage drop down to '0' Volts.
Tracing the wires and checking for continuity was the first step since a voltage drop means a resistance problem in one of the circuits. With the help of the Thomas Bus Tech Line we found the circuits to both relays then found the ground to be non-existent for both ignition relays.
These 2 ground wires (photo below) energize the coils within the relays. No grounds of course caused these relays to be become inactive.
After running a temporary ground the engine started. So obviously we had to check the ground connections on the firewall and the frame. After some digging there was a broken connector to a frame ground below the cab. Once repaired, the engine was back to normal once again.
The photo below shows the broken ground cable found just below the cab firewall on the outside of the frame rail.
This is Mechanical information that points out that a bad ground can cause a lot of electrical problems and should always be checked first starting at the batteries. A digital volt meter will tell you a lot with voltage and resistance readings. The higher the resistance the lower the amperage.