AT THE END of your diagnostic process, how sure are you that you’ve got it right? At one time or another, all technicians will have been sure that component X was the root of the symptom only to buy it, fit it and find that the system remains unchanged and the fault remains uncured. It’s a case of back to the drawing board with egg on face.
A good piece of advice is that once you have completed your data gathering, analysis and subsequent diagnostics, stop. You will be fairly sure at this stage that component X is definitely the problem. Consider this, imagine yourself fitting component X and then discovering that the problem remains. What test are you going to carry out now? The trick is to then carry out this test before you start buying parts. Adopting this mindset is a great way to reduce your stock of parts bought, fitted and then removed. Often, it is the cost of the replacement part which dictates how sure you need to be in your process and decision. For a £15 part you need to be sure-ish. Upping the stakes to a £200 part, you need to be quite sure. For a £1,300 part, you need to be as sure as sure can be! I guess that in a perfect world, we should be 100% sure in every case.
Symptoms
The job in hand is a ML 270Cdi Mercedes 2001 vintage which has its BAS/ESP, ETS and ABS lights on (left). There are many pattern faults which affect this model, such as brake switches, but this particular vehicle is throwing a code C1401 – High Pressure Return Pump - O/C, S/C or always on. The vehicle does seem to have a significant buzzing noise coming from the front N/S which is where the ABS modulator is located so perhaps the pump is running continuously. A quick check shows this is due to the vacuum pump and not the ABS Pump, so onto the actuator test using the Autologic. The test is shown as being carried out on the too, but there is no sign of movement in the ABS unit – it’s time to consult the wiring diagram.
The diagram shows that the ABS and traction control systems are split. The ABS pump is contained within a hydraulic modulator block which has a rather difficult-to-access 15 pin connector (see right). The Traction Control Module is linked, through wiring, to the hydraulic modulator block. The fault code is present in the Electronic Traction System (ETS) ECU, so this is where we’ll start our wiring research. As per usual, I spend an adequate amount of time examining the factory wiring diagram and form a test plan (with expected and actual results) to check the function of the pump.
A relay (K25) in the under bonnet fusebox is responsible for supplying the ABS Pump with a voltage from relay pin 5 to ECU pin 15 of the modulator multiplug. The pump is continuously grounded via pin 8 so anytime it receives a voltage via the relay, the pump should run. The relay is switched via relay terminals 1 and 2, by the ETS ECU, terminals 13 and 12 (bottom of photo). During the actuator test, neither the relay nor the pump is actuated so either the relay or wiring or pump has failed, or the Traction Control ECU has suspended the test whilst the fault code is present in the system. This is quite likely as the relay voltage supply and ground are controlled directly by the Traction Control ECU.
The test plan was to check what voltages were present at the Relay K25 Key On, Engine Off, (KOEO). Using the TESLite Pro Enhanced multimeter lead set I found that there was a good solid supply voltage on pin 85 (relay control) and terminal 30 (high current input). Relay terminal 86 was grounded by the Traction Control ECU when it wanted to drive the pump, then terminal 87 would be bridged to terminal 30 and the pump would be supplied with a current.
Curious voltage
Whilst checking the relay, I noticed that terminal 87 (pump supply) had a voltage of 5 Volts. There should not be a voltage on this wire unless there is a short to positive. Again using the TESLite Pro, I loaded the wire by pushing the button (FIGURE 6) and the voltage disappeared. Therefore, it wasn’t a ‘proper’ voltage.
The diagrams showed that the Traction Control ECU pin 11 was connected to this relay output wire, as well as the pump. Looking at the diagram, it appeared that this was the only connection between the Traction Control ECU and the modulator. I also checked that the pump circuit had a quality ground connection.
It’s safe to assume that this diagnostic voltage is how the Traction Control ECU diagnoses a faulty pump. If the pump is connected and in good condition then this diagnostic voltage will be ‘pulled’ to ground, just like when the TESLite Pro button is pushed. If the voltage remains high, as our test showed, the ECU infers that there is a circuit fault.
We can prove this assumption by placing a load, such as a motor or bulb into the circuit, in place of the ABS Pump (across terminals 8 and 15). As we understand the circuit, we can be sure that applying a load to the circuit won’t cause any damage but it will pull the diagnostic voltage down. I back probed the hydraulic modulator and connected a load (a bulb) across the pump circuit. The ECU fault code was reset and the code remained cleared after a cycle of the ignition because there was a route for the diagnostic voltage to be pulled to ground. Now, would the Traction Control ECU run the actuator test whilst my load is connected? If so, this would prove the Traction ECU, the relay and the wiring all in one go. With the bulb connected I pushed the actuator test –the relay K25 clicked and the bulb lit up. Bingo! We have undisputable evidence that the pump was definitely faulty. The ECU, wiring and relay were all fine.
Perhaps you are wondering, why go to all this bother? The fault code suggested that this could be the problem? Well, the new pump costs £1,300 plus VAT. I am sure you’ll agree that ‘sure as sure can be’ is a comfortable position to be in.
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