Aux fuel pump lower voltage supply problem

With 200 hours on my aircraft, my main fuel pump started running a bit slower and less confident sounding than the aux pump, just opposite of the problem you have. I traced the problem back to a high resistance connection on Socket 11 of the X-1 (Lane A) connector on the bottom of the fuse box assembley.  The socket in the #11 position feeds the main pump and it was discolored and oxidized like it had been warm.  I believe the heat broke down the spring tension in the femal socket causing a poor connection. In your case for the aux pump, you would look at socket 11 on the X-2 connector   

As you troubleshoot this, be aware that it is the negative side of the circuit that is switched, and the positive lead is not  The positive lead runs straight to the fusebox and is hot anytime start power is applied, or of course when the engine is running.  You could test the positive lead of the aux pump to the alternator A ground plate to make sure it’s not a ground wire problem.  Doing this will also bypass the switch, so if the switch is bad this will point you in the right direction.   

Full wire diagrams will be included in the newest installation manuals in the very near future from Rotax.

Cheers

A report on the FIX. First it was easy to determine the fuel pump switch was working simply through continuity checks with ground and the ground fuel pump probe. Same exact ohms on the main/working and aux/not working.

Back to the low voltage at the always hot pump connection. With the guidance of Dean Vogul Lockwood Aviation, pulled the middle plug off the fusebox and noticed one of the pins was gray and not shiny like the others. Turns out male gray plug 11 as Dean looked up was the fuel pump hot, so sanded off the gray with 400 grit sandpaper carefully, must have been some type of corrosion, cleaned out the female 11 with a drill bit by hand to clean the tiny hole, and POOF, it worked. Measured same voltage and fuel pump worked. Why the one probe corroded and the others did not is a mystery.  

I really appreciate comments so I want to provide solutions to all. Thanks Jeff and Eric.

Paul,

It’s great you found this, it was exactly as I suspected. You could also do a couple of preventative tasks to help stop this from happening again.  First, clean the X1 and X2 connectors with Deoxit.  This will actually remove any oxidation and leave an enhanced conductive surface.  After that dries, you could flood the X1 and X2 connectors (female sockets) with dielectric grease.  I know it seems odd to use a non-conductive grease to make a connection better, but the idea is that the contacting surfaces will scrape off the grease as it’s inserted, leaving the grease to fill any voids, thus inhibiting arcing and oxidation.  I flooded my X1 and X2 connectors with dielectric grease about 160 hours ago and all is working well.  The rotax MMH allows for dielectric grease on all connections except the ECU connectors. For the ECU they only specify Deoxit.