ECM fuse blows when vehicle hits a bump

[FarmerDan]

We have a problem with a 2002 Td5 110 Defender SW. It will start and run normally (engine management warning light not on) until the vehicle hits a bump in the road, at which point the fuse that powers the ECM blows and the engine stops dead. The obvious cause is a cable with damaged insulation somewhere which is shorting to earth (chassis, engine, body) when it gets shaken around. Looking at the circuit diagrams, it seems all engine sensors, instruments, injectors, etc. are powered from the ECM and so the fault could be in the cabling for any of these.

So far we have looked at all accessible cabling in the engine bay, behind dash, around gearbox area and cannot find any sign of damage. We have also rocked, shaken, thumped the car and all accessible wiring, connectors etc while it is parked with the engine running and cannot make the damn fuse blow! But as soon as we get 20 metres from the gate we hit a small bump and it stops dead.

Anyone got any ideas on likely places to look for a fault?

Or a more systematic approach we should be using to trace it?

[errol209]

Applying some reverse logic also suggests that the short could be anywhere after the fuse, including in the back of the fuse box. The only sure-fire way of finding such an intermittent fault is to replace each wiring run in turn until either you find a bad bit visually or the problem goes away

I'd start at the fusebox and follow the power wiring route looking for the pointy ends of screws, melted bits or loom passing over edges and corners. The other easy check is inside the ECM housing to make sure there isn't a washer or loose blob of something floating about inside.

[carlosbeldia]

I have exactly this problem. I track it back and found it's the CKP sensor short circuit caused by dirt and wire cover damage. When you take a bump, the engine moves over his mountings and make the cable move. Don't look anywhere else, take the sensor out and look at his wiring loom and loom. Temporary solution for me has been taking the section of the intake tube that goes to the snorkel in order to release tension from the wiring set. Tomorrow I'm planning to rewire all the loom as final solution.

Looking with my electrician, we found this damage on the wiring loom is caused by heat and bad connections in the blue blind connectors behind passenger (remember I'm in Colombia, passenger side is right). Solution for this was to take that connectors out and solder all them. This is a common issue in TD5 living in tropical weather, due to wet and temperature. It's a good fix if you are thinking in travel around the world or something....

[carlosbeldia]

These are the connector, the broken part in it, and all the fuses this %$#$$& took from my pocket in just one day (a very bumpy day, jejejje). As I said, this is the real cause for wire damage, but you have to fix the damaged wire and loom that comes out from the CKP sensor.