I know this a very straight forward procedure of connecting a direct power source to the pumps and if they go on they work and if they don't they're broke.

But...is it possible for there to be a gray area?

My case in point, I picked up a pair of used pumps that should work but when I connect direct power to them they trigger but don't run, almost like a broke solenoid in a way. They also do the same when installed in the car using the key the start.

Could this be a result of the direct power source I'm using not being powerful enough or the connection being weak? I used a jumpstarter to test with.

I was able to test the older pumps of the same model the same way and one did run the other didn't.

It's looking like the pumps I picked up are bad...

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500