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One of DCs main issues is transmission distance. Its hard to say for your case without details but its a good possibility. If you have a volt meter and know how to use it check the voltage at the start of the run and compare it to the end of the run and see how much the voltage has dropped. If your trying to push 12v over 20-30ft I would say theres a good chance of it being too little voltage over too far a length. Wire diameter is also a factor if its very small gauge wiring.
No voltage drop, but it's an easy 80', maybe 100, maybe even a little more. Hard to say as it cuts through the woods.
Picked up an old Fluke at the store, I'll try an figure out how to read amps.
Did you measure voltage drop across the pump while it's trying to start up? The voltage will only drop when there's current being drawn. So with the pump off you would see full supply volts even on a highly resistive line. You could try lugging a battery down to the pump, and jump it to get it started, then remove the battery jumpers but keep the pump running on your main run, then measure the volts across the pump.
Or you could try to measure the resistance of your wiring with your meter. You would disconnect it from your 12v source, then short the pump side, then measure the ohms across the pair at the source side. Also measure the resistance across your pump windings when it is disconnected, but with the switch on.
I'll throw out some example numbers. Say you have a pump that says it needs 6 Amps. Chances are the inrush current is more like 12A. If you measure across the terminals with the pump disconnected from power with the switch closed, you might measure around 1 Ohm. Now if you measure your very long supply line and get 2 Ohms. Then you would only get 12V/(1+2 )Ohms = 4 Amps through the whole system. With the pump drawing 4 Amps you would only see 4V at the pump terminals when it is already running.