she gave me a heart attack - Brunnhilde had no pulse

 

I was doing a bunch of stuff, including giving more thought to the rooftop PV panel installation and the tradeoffs. I also wanted to reset the service reminder that has been plaguing me during the trip to Florida in February. Disaster struck.

I had tried to reset the service reminder to no avail. As I have the Ultragauge always on, each time I start the engine it will beep due to the service reminder past due, and I had to mute it until the next engine start cycle. I lost my access to the Auto MaxiAP ODB diagnostic dongle when I failed to note the password and upgraded my iPhone. I finally got around to recover the password and gain access without having to buy the Mercedes specific option for the Sprinter.

I thought the tool will be a lot easier to reset the service reminder. But as alway once you fire up the app and let it do the scanning, it always return with a lot of store errors.

I was able to clear all the stored error codes except one, but I deem it just a distraction. I looked for clearing service reminder - the ASSIST in the Sprinter but to my amazement there isn't one. By this time I have started Brunnhilde and shut off many cycles. So I resigned to do clear it as described in the owner's manual.

Somewhere in the middle of the clumsy process I noticed while in ignition key position 2 everything in the instrument cluster went off. I thought, great! I must just mess something up, but that makes no sense as the reset method it is intended for public consumption. I thought may be for some reason I tripped the immobilizer. I switch off the key and back on a number of time. The symptom is exactly as if something yanked the power. Even the dome light did not work. Fear set in by now. I am out in the middle of no where and if I cannot figure out what the problem is, it would be a very expensive repair.

I grabbed my freebie HF DMM and checked the battery voltage at the cable's lugs and the battery was fully charged. By now I had no idea where to start. I did a bit of search on T1N Sprinter ignition lockout but nothing meaningful came up. I removed the battery ground termite to power cycle all the Sprinter's brains. The first time the instrument displays came back as I had the key in on ignition position 2. But as I tried to start the engine all went blank again.

I power cycled the battery connection again, but all subsequent attempt did not change the dead everything. I even unplug the AC shore power as the on-board charger may attempt to charge the battery that is not there but feeding power to the computer modules.

I then remove the plastic cover to the high amperage fuses by the positive terminal of the battery. I had never open this black box and only today I discovered it houses all the high current fuses.  The voltage there was fine also. But as I probed a couple more times I notice at time I lost the 12V on the big steel plate upstream of all the fuses. I thought may be just the probe were not making good contact due to the surface oxidation and it is steel.

I stabbed on it harder and I was sure I should had made contact but the voltage was around 2 volt. I check the resistance and it was within expected ballpark. I know I need to remove the steel plate and clean it as there is a lot of current going through it. The nut was rusty but properly tighten. I could not get a good inspection of the contact surfaces due to the stiff cables, so I sand them lightly with sandpaper and then reinstalled it.

That did the trick. Everything is well after I reconnected the steel plate and restoring the ground terminal. What is odd is there are two contacting surfaces. The one on the bottom is the best current path. The surface on the top relies conduction through the rusty nut, and the rusty stud. The nut was just a little rusty but not that bad, and properly torqued down. 

Even after finding the problem I was gun-shy to reset the service reminder. I did it anyway and now all is good.