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A problem halved – By Andy Crook


Imagine the scene – a good customer is on the phone informing you that his son’s car is on the back of a recovery truck heading to your workshop. You have never seen the car before and he wants you to take a ‘quick look’ at it. 

This is the sort of scenario that we face time and time again. The customer expectation is that the technician can diagnose the fault quickly and quote for the repair work without, or for little cost, to the customer. This may be true for mechanical work, but for electrical diagnosis on today’s modern computers on wheels, the diagnosis may take longer than the repair and therefore cost far more. 

However, the customer perception is that diagnosis is a low value commodity, after all, you just need to ‘plug in’ a tool and the car will tell you what is wrong! 

Changing this perception is made much more difficult when garages still offer their ‘diagnostic’ service for £25. I always wonder what they do for that £25. 

We have a robust process for diagnostic work and it always starts with a customer interview. This allows us to gather information about the customer and the problem, as well as explain what is required and the costs associated with the diagnosis. Essentially, it is a process where we educate the customer, then ask if they want us to diagnose the problem. 

Our diagnostic procedure then proceeds with a customer questionnaire. We have developed a form that enables any member of staff to gather some basic details. This is recorded on the job card and is the starting point for the detailed questions that are asked when the vehicle is dropped off. 

In this case, the process was a bit skewed as the owner/ regular driver wasn’t going to be with the vehicle. I proceeded to ask a number of questions: 

What is wrong with the car? “It keeps cutting out”.

How frequently? “Every 2-3 miles.”

Has it recently been refuelled? “No”. 

What work has been carried out recently? “A breakdown service has looked at it at the roadside and recovered it to a local garage”. 

Did they suggest what might be wrong? “No, they couldn’t communicate with the ECU, so they just towed it to a garage”. 

What has that garage done? “They recommended a Jaguar specialist as the ECU needs replacing or resetting, in their opinion. The specialist has managed to communicate with it and extract a code and has suggested it may be the fuel pump solenoid or the ECU,” he continued. “They have replaced a blown fuse, and this allowed the vehicle to start and run until it blew again. It has been replaced with a fuse of higher rating to get it running and help get it on the recovery vehicle. But it has blown again”. 

Distinguishing between diagnosis and the repair 

So, having established the recent repair history, we now needed to agree on a way forward. The customer was happy for us to spend up to an hour to find the fault and provide an estimate for the repair. 

This clear distinction between the diagnosis and the repair helps the customer understand why we charge a different rate for this service and why the fix is not included. It is a separate operation and therefore a separate charge on the invoice. 

This is a much different proposition to the ‘quick look’ they initially requested, but as we pointed out, it has had three separate repair agents look at it without a definitive conclusion. 

It is going to require specialist tools, knowledge and information to establish the cause of the fault, all of which is reflected in the diagnostic rate charged for this service. 

When the vehicle arrived we performed a global scan of all modules and noted the codes. 

The only code stored in the PCM did indeed relate to the Fuel Metering Valve Circuit. 

Next, we used an information system to obtain the wiring diagram relating to the fuel pressure solenoid/metering valve. Identifying the various elements of the circuit allows us to carry out the required tests more efficiently. I drew my own diagram identifying: 

  • Supply
  • Protection
  • Load
  • Control
  • Ground Path

Figure 1

I inserted expected values at the logical test points and formulated a test plan with the results evaluated at every measurement step. See Figure 1. 

It was clear from the customer interview the problem was an intermittent short circuit. Where the short was located is another matter entirely, we needed to come up with a test plan to prove where the short was in the circuit. 


Halving is a process used to reduce the time to solve problems; it can be used to great effect during circuit diagnosis. In theory, you should be able to narrow the fault by a process of hypothesis, test and evaluation, repeat. Within five steps you should have proved the fault. 

Picking a point halfway through a circuit and applying tests, or logic, will eliminate 50% of the possible culprits if the test is robust enough. Halving the remaining 50% leaves you with 25%, halving that leaves 12.5%, halving that leaves 7.25%, and so on. 

It isn’t always possible to be so precise, but I’m sure you get the idea. If you know what result to expect anywhere in the circuit, you can measure anywhere and if the result is positive you can eliminate the circuit up to that point as the cause. 

In this case, we could rule out the ground path including the ECU from our enquiries. Shorting this side of the circuit would result in a reduction in rail pressure with the maximum current flow possible in the circuit. That’s half the components ruled out without even taking a measurement. So, we focused on the remaining half of the circuit, the wiring to the metering valve, and the valve itself. We needed to prove if the wiring or the valve was at fault, so we needed to come up with a test that would do just that. 

Figure 3

We removed the fuse and replaced the wiring to the valve with a fused link of the correct rating, connected the oscilloscope amp clamp to the fused link and grounded the control side of the valve. The oscilloscope trace shows the result, 2.2 Amps drawn by the metering valve. See Figure 3. 

We removed the ground from the metering valve and connected the plug, cleared the codes and drove it for 10 miles without any issues. This was to prove that the valve was not failing due to heat build-up. This proves the fault is in the wiring after the fuse. One test and the fault was isolated to a length of wire. We then gave the customer two options; strip the loom to locate the fault or a ‘loom overlay’. A loom overlay is simply running another wire. The customer was given estimates for both options including some advice on which way to proceed. 

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