22 May 2024
The voice of the independent garage sector

Face the Pro Ace

Neil finds that the most dangerous game is indeed man, when a mistake made by a mechanic in the past proves to be a real head-scratcher

By Neil Currie

In this month’s article I decided to write about a recent vehicle I had in the workshop which turned out to be rather interesting when the cause was found. The fault was man-made, and these can be the trickiest to solve, especially when it is an electrical fault. This is because software engineers do not write software to anticipate someone manipulating circuits or changing it from how it was originally designed.

The vehicle in question was a Toyota Pro Ace van, more commonly recognised as a Peugeot Expert, the only difference being Toyota badges. The van was originally brought in with a blown turbocharger which had failed mechanically. Once this was replaced by my colleague, the van was road tested. However, it had no boost and quickly went into limp home mode storing an under boost code. Returning to the workshop, it was checked over and it was found that there was no vacuum to the turbo actuator. A few checks were done to make sure everything was where it should be, which they were and at this point the vehicle was handed over to me to have a look.

The 2.0 diesel engine fitted to this van uses a vacuum controlled waste gate actuator. The actuator itself also has a position sensor fitted, and starting the vehicle showed the engine control module command to demand a position of 90%. Despite this, the actual position stayed at 0% confirming the complaint. I decided to first test at the control solenoid which was placed awkwardly down the back of the engine, unbolting the solenoid and gaining access to the pipework allowed me to carry out some checks, on this solenoid we have 3 pipes, the first a vacuum supply in, the second which is the vacuum output which goes to the turbo to control the waste gate and the third to atmosphere to allow the solenoid to release excess vacuum when required, as with the solenoid being constantly supplied vacuum it cannot release vacuum via the turbo actuator as this would cause the turbo actuator to over boost which is not what we want.

For my first check I applied vacuum to the outlet pipe to the turbo watching live data on the scan tool to make sure the pipe was ok and the turbo actuator was capable of moving. This test showed the position to move to 90% like the command from the engine ECU was asking for so my problem lay elsewhere. Next, I started the engine and checked vacuum supply to the solenoid. My gauge showed a healthy vacuum of 30inHg (inches of mercury) and testing the last pipe showed a clear route to atmosphere. This meant my problem lay either in the solenoid itself or the control from the ECU. The solenoid is controlled using Pulse Width Modulation (PWM). Please refer to Fig.1. This is where on this particular component, the ECU sends a 12v supply to one side of the solenoid and as the solenoid is a effectively a coil of wire, the voltage flows through it and back down the other wire to the control unit. When it wants the solenoid to operate, it quickly pulses the return wire to ground over and over continuously while maintaining battery voltage on the supply side, this voltage difference causes the solenoid to work and it does this at such a rate of change that it can accurately control the speed and movement of the solenoid which we refer to as the duty cycle. By doing this the solenoid can be operated at any position from 0 to effectively 100% allowing much finer control compared to how it was previously done by feeding a power and ground to a component and it was either on or off, also by using PWM the current consumption of the component is much lower so lighter wiring can be using saving on production costs.

Testing the solenoid
Upon testing the solenoid for control it was noted there was 12v on both wires as expected. If we only had one 12v supply this can quickly identify we have an open or high resistance internally within the solenoid. As I had no fault code for the valve it showed the return wire to be intact to the control unit. This is how the ECU itself does a circuit check to set a fault code. It looks to see the supply voltage that is sent out return on the other wire to show circuit integrity. However, upon starting the engine the control unit did not attempt to control the solenoid by applying a PWM signal. Testing the solenoid itself by applying power and ground to it showed that it clicked and operated so it was in working order. Why then was the control unit not attempting to control the solenoid on start-up as expected?
It was at this point I decided to step back and re-evaluate things and go through what could possibly cause the issue. We could have an input issue stopping the ECU from commanding the solenoid, or an internal fault in the ECU. But what about the possibility of the connector for the turbo actuator solenoid being connected to the wrong component? Using wiring diagrams, I could check to see if the wiring colours match on my vehicle which would prove it. Carrying out a visual inspection, I noticed a solenoid close by which was identical to the one I had been testing, there was now a high chance we had a mis-match and the wrong connector was on the wrong solenoid. This would explain the lack of control and also why no other fault codes were stored as with both solenoids working and connected there was no reason for a fault code to be stored.

After consulting wiring diagrams and wiring colours, I did indeed have an issue. The other solenoid was for the EGR bypass exchanger and the wiring colours matched the plug on the solenoid I had been testing. Disconnecting my connector on the turbo actuator solenoid and reading fault codes I confirmed it was indeed the EGR circuit. Please refer to Fig.2. I then disconnected the EGR solenoid and re-read codes which now logged an open circuit turbo actuator fault. Please refer to Fig.3. I then reconnected the wiring connectors onto the correct solenoids and retested the system. Now on engine start up the solenoid for the turbo control applied vacuum to the turbo itself which could be seen monitoring live data for the demanded and actual position of the turbocharger waste gate. Please refer to Fig.4

Road testing showed the vehicle could now create boost and have the get-up-and-go you would expect from a turbocharged engine. Returning to the workshop and rescanning for fault codes showed no fault codes present so the issue had been rectified. Why the plugs were swapped over is anyone’s guess. As the connectors were identical, they fit either way. I can only presume someone previously doing work mixed them up by accident.

On this vehicle as I knew how the solenoid operated I did not choose to refer to technical information early on in the process. In hindsight this may have led me to the issue quicker. If I had seen what the wiring colours were for each wire on the solenoid, I would have spotted the issue. Even so, in total the job took less time than it took for me to write this article. Like I always say, following a solid process and thinking logically and methodically will always lead you to the cause of the fault.

Fig 1
Fig 2
Fig 3
Fig 4