Hello Christer:

It sounds like you have the wrong voltage DPU or a wiring problem or voltage regulator problem in the DPU.

Have you verified that the DPU label indicates it is a 14 volt unit and the voltage at the DPU is correct? Use a volt meter to measure between J4-14 and J4-15 and verify it matches the bus voltage. It may be that there is resistance in the ground line such that  the positive is at +13 volts and the ground is at +4 and the DPU is running off of +9.  You also want to confirm that the power to the VM probes is not being excessively loaded. 

The DPU input power supply is very basic. There are a couple of capacitors, a linear regulator and input (reverse current) protection diode (see pictures).  The linear regulator is an LT323AT, data sheet available here: http://www.linear.com/pc/productDetail.jsp?navId=H0,C3,P2045 . Remove the bottom cover of the DPU and verify the voltage regulator is correct (the 28 volt units use different parts) and it looks like what is shown in the picture.

For the definitive test, wire the DPU power to a variable power supply and disconnect all the powered probes (Oil Pressure, Fuel Pressure etc.). Vary the supply voltage and confirm the drop out voltage (between 13 and 9 volt). If the unit performs normally then connect the power supply ground to the airframe ground and repeat the test. If If the problem returns then it is likely in the probe wiring or the connection to the display. If it still performs normally the reconnect the powered probes one at a time until you isolate the culprit.

If you confirm that the problem is in the DPU then let me know. Some time back  I had a problem with an oil pressure sensor where the l pressure would intermittently drop to zero, triggering a cautionary landing. I purchased a replacement form VMS but the problem returned. I replaced the wiring, but the problem returned. I acquired another DPU from a fellow Lancair builder (Fred Egli) and swapped out the Analog Board, but the problem returned.  This brought me back to the pressure sensor which, despite being brand new, was also defective.  I ended up buying an industrial pressure sensor and machining a connector adapter for it. Problem solved.

The DPU's are no longer available from VMS. The one I got from Fred (thanks Fred!) is 14 volt but my airplane is 28 volt so I have no use for the processor / power supply board as I only needed the Analog board. If you are sure your  DPU is defective I will send you the DPU board I have to try/use.  If it works then you are welcome to keep it and send me your old board. If I ever need one I can likely fix your defective one.

If you want me to send you the board then email me your shipping address.

All inanimate objects, sooner or later, must respond to reason.

Please let us know what you find.

Regards
Brent Regan