Mechanics and Corrosion

The most difficult job when finding an electrical fault is when corrosion is involved. It's hiding somewhere in a connector or poking through the wiring insulation.

So what is the best way to troubleshoot corrosion in the electrical system? One way is to search and destroy the problem by opening the loom and exposing the green stuff but that is frustrating and  takes a lot of time. However it's necessary to make a proper repair.


Another method (when you know which circuit is faulty) would be to check for voltage drop in the circuit that is not functioning. If you can hook up a volt meter at each end of the circuit and see if there is a loss of voltage when the circuit is energized then you can be assured it's a bad wire. Checking the resistance is another check that works.

In our shop we've been using a load on the wiring circuit in question. A regular light bulb (non LED) hooked up to the one end of the wire will tell you if there is a lack of current when the light is dim after energizing with battery power. The wiring is either corroded or frayed restricting current flow.

If you have money to spend on tools you could use "diamond logic builder" software for International trucks to see the circuits working in a graphic form on a laptop. You can click on a circuit and actually see if the load is getting battery power. The pin outs on all the connectors are at your fingertips. I've used this software and it's a great tool to have compared to digging through wiring harnesses with a hope and a prayer.  

Diesel Engine Cranks Does Not Start

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.