I can add some experience to this issue:  any connection problem will get worse with altitude.  Issues such as burned or corroded contacts, worn or dirty brushes, bad connections or cold solder joints increase in resistance with altitude.  In studying my previous Cessna autopilot trim system problems I eventually discovered the same symptom of a motor running perfectly on the ground but failing at altitude. Starting voltage is a good indicator of condition. Starting voltage on my 10v motor went from 5v down (dirty) to 1.5v with servicing.   Brushes and rotors, if dirty or worn, can fail intermittently at altitude or stall (original thread symptom).    Relays contacts in the case can be inspected with an ohmmeter for resistance and values that are inconsistent or that are higher than "zero".  I had relay contacts that were more than an ohm on the ground but varied to 50 ohms when moved.  In my particular situation, the trim relay and board is in the tailcone of the Cessna 400 series which is exposed to water.  Over time, the circuit board was flooded and had corroded solder joints.  Proper conformal coatings can protect circuit boards and connections from moisture.  The relay socket, exposed to moisture, was lightly corroded adding resistance but still worked on the ground with 28v.  The relay points internally had varying resistance when tested but they looked good to the naked eye.   The overall effect was to have perfect trim ops on the ground and failing performance at altitude.   These problems enrich a lot of avionics shops who can never seem to duplicate the squawk on the ground.

These are things to keep in mind with intermittent electrical issues and when designing systems that need to work well at altitude.

Paul Miller
Legacy N357V

Jeffrey Liegner, MD wrote:

I recently ordered a new pump and placed it in parallel with the old pump during some ground tests.  Sure enough, the new one came on and stayed on at 9V when the old one did not, when the sensor/switch was calling for pump, even up into the 12V range.