The turbo on our Sprinter has been an on-again-off-again thing for quite some time, and I've been struggling to diagnose the problem. We have access to a DAS computer, and I decided to crack the shits and buy a vacuum gauge because I didn't know anyone close by who had one.
I started out confirming I didn't have a vacuum leak somewhere else - no, definitely no vacuum leak, the thing will hold vacuum all day if you let it. I'd already replaced the vacuum lines to the turbo controller, so I knew it wasn't leaking there - and the computer/gauge confirmed it. Even setting the turbo to 100% duty cycle, we had nothing but atmospheric pressure. This left me with, I thought, two options: the boost controller or the wiring. Lucky for me, the Sprinter's computer is clever enough to eliminate or confirm the second option.
A dig through the DAS documentation told me that the ECU has a couple of failure codes for the wiring to the boost controller. It reports if the wiring is shorted or if it's open - and I got neither of these codes. Clearing fault codes and starting the motor with the boost controller unhooked yielded the latter code, and I decided that was good enough for me - I dared not short the wires and test that.
I called MB in Melbourne, and after a very long wait they finally found what I was talking about (unfortunately I couldn't figure out how to pull a part number out of DAS to give them) and quoted about $425. That's approximately a 20% jump over the price I was quoted ages ago before I confirmed the fault (if only I'd pulled the trigger and bought the thing then).
We also leave for Roxby downs tomorrow morning and I'd have to race in there, pick up the part, and throw it in before driving ~1000KM all day. That didn't sound like fun, so I hoped I could live in limp-home-mode for another ~1500KM.