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Five Top Tips For Fault-Finding - Tip 2

By Damien Coleman

Although voltage testing is valid for circuit and wiring test, monitoring a current flow trace is the most accurate way of testing a component.

Current flow testing is suitable for high current consumers such as fuel pumps, injectors, ignition coils, etc. The current flow through (low current consumers) sensors can also be monitored but the equipment to measure such small current flow is expensive.

A fault with a Volkswagen Caddy required use of the current clamp. The vehicle was fitted with the 1.6L common rail diesel engine (code CAYC). The engine was misfiring and a fault code was stored:

  • P0202: Cylinder two injector control malfunction

This engine is fitted with Piezo injectors, so the current clamp is the ideal tool to monitor the operation of these injectors.

The cylinder 1 and cylinder 2 injectors were tested to compare a functioning cylinder and the faulty cylinder. The difference in current flow on both cylinders is evident, but it should be noted that the voltage in both traces is similar.

Cylinder 1

Cylinder 2

Another example is testing a fuel pump. Below are two waveforms from a 1.6L petrol Ford Focus fuel pump. The first image is pre-repair and the second is post-repair. It is again clear from both that the fuel pump motor is faulty and once it was replaced, the vehicle operated as expected.

Fuel pump pre-replacement

Fuel pump post-replacement

Click on the doors below to read our other top tips.