Electrical Problem with Main Fuel Pump

EDITED!

You obviously have a wiring problem.

Turn ON the Pump and measure the voltage from the Negative (Ground) side of the Pump to the Battery Negative post.
It should read very close to ZERO.  If Not, you have a grounding Problem.

With the pump still ON, measure from the Positive (Hot) side of the pump to the battery Negative post.
It should read very close to 12 volts.  If Not, you have a Powering Problem.

Keep moving the Pos Meter lead back up the wiring to each item in line; Relay, Fuse, Buss Bar, Etc. until you find 12v.
If you find, for example, 12v on one side of the Relay and 5 volts on the other, the problem is at the Relay.
Suspect connectors first.  A poor connection will be HOT to the touch!

The same principal applies to the Ground side of the circuit except it will be all wires and connectors.
Where the voltage changes, lies the problem.

"the engine would not get enough fuel without the auxiliary pump"

What do you mean by this, exactly?  You said it was your main pump having problems, right?  But won't it fly on just one pump if the other fails in flight?

You say that the main pump comes on when you turn on the igntion? Are you saying the current path for the main pump goes through the igntion switch? That doesn't sound like a very good idea to me. You might want to check that the switch is rated for the current of the pump. You might also want to consider re-wiring and giving each fuel pump its own dedicated circuit with its own dedicated switch, especially considering the 914 won't run without an operating electric fuel pump.

Thank you, guys, for your great help, especially to you Bill. I will measure the voltage the way you explained it and let you know.

The main pump has no separate switch and activates when turning on the ignition, that’s just the way it’s build in the factory. I agree, I would prefer to have a separate switch for each pump. 

Correct, the engine would run with only one of each pump going, main or axillary doesn’t matter. However, the secondary should not become the main pump when the main is not working. Flying with only one (back-up) pump is not a good idea. 

Of course you would not fly without a backup pump; I was just wondering what made you say your engine would not get enough fuel without the aux pump.