Tracing an open circuit

Andy Crook attempts to untangle a Doblo's wiring loom

Published:  17 September, 2012

fiat_crook1.jpgA Fiat Doblo 1.9td – the type that is a van with windows – came my way recently. The vehicle had been taken in by a local garage with the MIL light on but with no obvious symptoms and the customer came with the old request: “Can you turn this light off for me?”<

Clearly, I wanted to diagnose and repair this one as efficiently as I could. From the information I had, the error code read ‘P0480 cooling fan relay 1 circuit malfunction’. The garage had replaced the relay with another known good relay but the code would not clear and the light remained illuminated. They had informed the customer of the need for further investigation and they had agreed that a maximum of two hours labour would be sufficient to fix the fault, plus any parts required.

This is where the diagnostic approach is vital – you could waste time testing wires, motors and pulling connectors off. Always follow a proven test routine. In this case I used a diagnosis path as follows: I used a scan tool to check that it still read the same error that the garage had reported and I performed a functional test of relay 1 and relay 2.

Bridge

Next, I bridged relay 1 with a fused link to observe output. Research the control function of the relay and draw a wiring diagram of that circuit and the expected readings. I then test that circuit and carry out any repairs.This approach will result in successful diagnosis every time, it requires a multi resource approach and test preparations, which look to use valuable time but always save time. A favourite quote of mine comes from Australian aviation pioneer Sir ‘Mac’ Robertson and goes: ”Time spent in reconnaissance is rarely time wasted”.

The functional tests proved the circuit fault and the fan operated on high speed so the fan motor is OK – this has reduced the number of tests that need to be performed. The relay was bridged and the fan did not operate as expected. This will require further investigation, how does the fan operate on two speeds and why does it need two relays?

More info needed

So far we have used a scan tool and a fused link. Now we need information. There are a number of information sources available and most will provide wiring diagrams and ECU pin numbers. With this information it is possible to draw your own diagram. This should contain only the wires and components you want to test.

From the functional tests we have proved the fan motor (‘M’ on the diagram) and relay 2 work. We know how the fan works on two speeds – there is a resistor (R1) in the circuit between relay 1 and the fan motor.This is the next test, with the relay bridged the volt drop can be measured across the resistor R1. This test proved there was an open circuit at the connector of R1. Removing and cleaning the contacts restored the fan operation with the relay bridged.

This did not clear the fault code, the reason for this is in the diagram. The ECU has no way of checking the fan output only the relay solenoid circuit. The ECU is monitoring pin 20 for relay 1. With the ignition on there should be 12V at pin 20. If no voltage is present there is an open circuit. A quick check at pin 28 of the fuse box showed 12V with the ignition on. The test is then repeated at pin 20 of the ECU. This showed 0V so the fault is an open circuit between the fuse box and the ECU. A quick repair of the wiring cleared the codes and the light was extinguished.

It might sound complex but the whole process, including preparation time, only took me about one hour – well within the time that the garage had estimated the fault would take to be fixed.

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