Installing shower drain pump - voltage goes to 0v

[Nor]

I'm replacing the bilge pump style shower drain pump. I measure 12.5v on the cables before I connect them to the pump, and then it goes to 0v as soon as I connect them. Is the pump faulty, or is there something else I haven't thought of?
I don't understand why I measure 0v. If there is a short circuit in the pump then a fuse should blow and I'll see a high current going through the dc panel?

[Tin Tin]

With your multi-meter check for continuity (Ohm-meter) on the pump terminals/input wires. There should be none. How many wires, 2 or 3?

How is the pump triggered, by a float switch on an electronic sensor? Post a photo.

[Nor]

Originally Posted by Tin Tin

With your multi-meter check for continuity (Ohm-meter) on the pump terminals/input wires. There should be none. How many wires, 2 or 3?

How is the pump triggered, by a float switch on an electronic sensor? Post a photo.

Thanks! The pump has 3 wires, one brown for manual operation and another brown white for using the internal float switch.
I measure 0 (short circuit) between negative and brown, and also 0 between black and brown white when float switch is open, and then no continuity when the float switch is closed.

[HankOnthewater]

I would say one of the contacts or connections has a very high resistance. When you measure with a multimeter switched to Ohm, the current used (to measure voltage) is extremely small and even a resistance of a few Ohm will not drop the voltage. But as soon as the pump is switched on, and a larger current is attempted to be drawn, this resistance will drop the voltage to close to zero.
That is assuming your pump is working when connecting directly to the battery terminals.
Anyway that is my take , and if I was a betting man, I would say the odds are about 8:1

[Nor]

Originally Posted by HankOnthewater

I would say one of the contacts or connections has a very high resistance. When you measure with a multimeter switched to Ohm, the current used (to measure voltage) is extremely small and even a resistance of a few Ohm will not drop the voltage. But as soon as the pump is switched on, and a larger current is attempted to be drawn, this resistance will drop the voltage to close to zero.
That is assuming your pump is working when connecting directly to the battery terminals.
Anyway that is my take , and if I was a betting man, I would say the odds are about 8:1

The pump works off the battery!
I don't see how/why the connections would have a high resistance.
The red cables in the photo is the previous float switch that I removed.


Could it be something on the cabling on the way to the pump then maybe..?

[dougweibel]

Originally Posted by Nor

The pump works off the battery!
I don't see how/why the connections would have a high resistance.
The red cables in the photo is the previous float switch that I removed.


Could it be something on the cabling on the way to the pump then maybe..?

Yes, it could be anywhere in the circuit, but from your description it definitely sounds like you have poor (high resistance) connection somewhere. You could also have a break in the wire somewhere with strands barely making contact and providing only a high resistance pathway.

[Senojev]

Whatever the solution is, make sure you do not connect the wiring as shown in the photo. Use good quality heat shrink adhesive lined butt connectors of the correct size. The photo shows yellow connectors (10/12 AWG). The cables do not look that big.

[rwidman]

Most likely you have a high resistance connection in the wiring to (or from) the pump which is causing the voltage drop. A shorted pump would blow a fuse, trip a circuit braker or make a very large spark when you connect it.

[Nor]

Just reporting back that this was as you guys said. It was solved by replacing the cable from the connection point not far from the pump. Works great now!

Thanks!