Information overload

Sometimes too much is never enough, but according to Neil, you can never have enough information on a repair

Published:  01 December, 2021

To carry out electrical and diagnostic repairs on vehicles, technical information is a must. With modern vehicles now having so much wiring and sensors, trying to fix for example a wiring fault without knowing what each wire is, what it does and where it goes in my own personal opinion is ludicrous. In this article I intend to show the importance of having not only technical information to hand but the correct information.

Recently, a customer telephoned looking for some advice regarding her vehicle and whether it was safe to drive. The customer explained that she had a 2010 Ford Mondeo and that the engine management light had came on while driving along the road. On the plus side, the car drove fine. We explained how without seeing the car it was hard to comment and recommended the vehicle be booked in.

The next day the customer phoned to say the vehicle had broken down and had now become a non-start. She wondered what her options were so I explained in my opinion, the best route to take would be to get the vehicle recovered to allow me to carry out a diagnostic assessment of her vehicle including a thorough test plan which included referring to wiring diagrams and technical information. This would allow me to come to the correct conclusion of what was wrong with the vehicle. We agreed to an initial assessment and I would then notify the customer what I had found with potentially even a fixed vehicle.
A few days later the vehicle was recovered to our workshop and another call was made to the customer to notify her that the vehicle had arrived and to ask some more questions about the faults. The customer complaint was that while driving along the road the engine management light had illuminated but the vehicle drove fine with no loss of power or any other notable symptoms other than a warning light on the dashboard, however the following day while driving along the road and hitting a bump the car cut out and now when turning the key to the crank position nothing happened.
After pushing the vehicle into the workshop, I firstly confirmed the customer complaint. As stated, there was nothing when the key was turned to the start position, however with the ignition on all lights illuminated as they should except the engine management light, if you read my last article (which I am hoping you have) you will know this is a big clue. After attempting to do a global scan with Ford IDS, it reported back many fault codes for no communication with the powertrain control unit (PCM), Ford’s name for the ECU, checking all other fitted modules they responded ok. I then accessed Autodata as I use it daily and it is my first check for a wiring diagram and usually easier to understand than the manufacturers diagrams However, for this engine there were four different diagrams listed depending on specification of the vehicle. After some checks I was unable to correctly identify which diagram was correct and checking each one against my vehicles wiring none matched correctly.

Manufacturer information
At this point I decided to go for manufacturer information, accessing FordEtis. I printed off the relevant wiring information for the PCM. While logged in I checked for any technical bulletins referring to the fault in case it was a known issue. None were found. After studying the diagram, it was noted there were several power feeds and grounds to the control module and also can bus communication lines which all required testing before condemning the control unit itself.
A visual inspection was carried out but no issues could be seen. A test plan was formed to test the relevant power feed fuses which were located in the engine bay fuse box and if all was ok then access the PCM and test the power supply wiring, grounds and communication lines for correct operation.
The voltage was measured at relevant fuses while under load with a digital multimeter and were all found to be good, so access to the PCM was required. Following technical information, it was noted the location of the control unit was behind the splash guard on the nearside front. Removing the wheel, plastic guard and necessary covers allowed access to carry out testing. Following the wiring diagram and test plan, each power and earth wire was load tested which immediately found an issue with one of the power supplies which came from fuse 38 in the engine bay fuse box. This identified an issue with this particular wire. The wire was located at either end, isolated and checked for resistance with a multimeter which showed excessively high resistance which was causing a large voltage drop to the control unit. A temporary wire was overlaid to bypass test the faulty wire and check for correct operation before proceeding any further with the repair. The vehicle now started and communication to the engine control unit was now possible which also allowed the clearing of the many stored fault codes, however the EML was illuminated as per the customer’s initial complaint so the fault codes were checked again which revealed a fault code – P2033 “Exhaust gas temperature sensor 2 Circuit high.”  Being wary of time, I decided to carry out some basic wiring checks as I had the control unit exposed and a diagnosis could also be made for the fault. Testing the wiring at the sensor found no voltage supply but a ground which indicated either an open or short in the supply wire from the control unit.

Temperature versus resistance
You may be wondering how this sensor works and why I was looking for voltage at the sensor? Bearing in mind I am no teacher and to try it keep it short and sweet. There are two types found in automotive applications; An NTC type and a PTC type. Either can be a thermistor type sensor or thermocouple type, which work very differently. On my vehicle the sensor was an NTC thermistor which is a resistor that changes with temperature and stands for negative temperature coefficient. This means as temperature increases, resistance decreases whereas on positive temperature coefficient (PTC) when temperature increases, resistance increases. The resistor in the sensor together with a fixed value resistor in the ECU forms a voltage divider circuit. The sensor is fed a 5-volt power supply and therefore if the temperature changes, the resistance change of the sensor causes the signal voltage to change. The ECU is able to then determine the temperature from the voltage and convert it into degrees Celsius. As the ECU was not seeing a change in exhaust temperature from that sensor but was from other sensors, and with a fixed high voltage, the fault had been stored.

Repair procedure
Carrying out a check from control unit to sensor with my multimeter found the wire to be open circuit, so now both complaints had been diagnosed and a repair procedure could now be undertaken. The wiring loom from the PCM had been further exposed with the removal of the cover and splash guard so a further visual inspection was carried out, while doing this the wiring loom was seen to be close to a metal support frame for the radiator assembly. Gently pulling this back exposed a spot where the conduit had been rubbing against the frame and had worn away the plastic. Opening this up revealed a corroded wire matching the colour of my power supply to the control unit and a broken wire matching the exhaust gas temperature sensor wiring. A continuity test on both to the relevant pins at the PCM connector confirmed this.
With my initial assessment time nearly up I phoned the customer and explained my findings and requested a further hour to repair and reposition the wiring loom and to reassemble and road test to confirm correct operation of the vehicle. The customer was delighted to hear both issues had been diagnosed and approved a further hour’s labour to repair the vehicle and road test the vehicle. Using approved manufacturer methods both wires were repaired, sealed and refitted into the conduit protection which was also sealed. The loom was then rerouted to prevent further contact with the radiator support frame and the vehicle put back together. A full fault clear was carried out and extended road-testing monitoring exhaust gas temperatures and correct diesel particulate filter operation. On return, a global scan was once again carried out which revealed no stored faults confirming both of the customer’s complaints had been rectified.
Without the correct information I would not have been able to fix this vehicle. Imagine trying to determine which wire was which and even where the PCM was housed, bearing in mind on average most vehicles now can have up to three connectors on the engine control unit with all having upwards of 50 wires on each connector. This goes to show how using technical information along with a good understanding of electrical circuits and sensors makes light work of fixing vehicle faults, saving the customer both time and money.

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