A few weeks ago Christer Wretlind reported that his VM1000 system would shut down at low engine speeds. Through correspondence with his electronics technician, Sune Nilsson,  and investigation using a spare VM1000 DPU I had on hand, we discovered the root of the problem.

It seems the VM1000 DPU was designed with a linear regulator to supply the 5 volts that powers the processor AND the sensors. There is a 5 ohm, 5 watt series resistor that limits the current being supplied. Unfortunately, under short circuit conditions, the resistor has nearly all the 14 volts across it (the regulator is trying to supply 5 volts) so is dissipating nearly 35 watts!  This causes the resistor to adopt some of the characteristics of a fuse.

In Christer's case it is speculated that at some point the 5 volt sensor supply was shorted to ground, causing the resistor to overheat  and change resistance to ~30 ohms. The normal system loads were now required enough current to cause a high enough voltage drop across the resistor so there was not enough input voltage to the 5 volt regulator and it browned out.  The fix was simply to replace the resistor.

It is interesting to note that the DPU I had on hand also showed signs of an overheated current limiting resistor, so of all the DPUs I have seen, 66% have overheated resistors.

FYI.

Regards
Brent Regan