Assuming the standard Whale domed switch, it is simply a single-pole affair which switches either the 12V or GND (can't remember which, but it should be the 12V +ve). But thinking about it, if the pump is OK and all wiring / fuses are intact, you should still be able to measure 12V across the switch - but only if the pump is actually connected to the socket outside. Many things could stop this working though. So, things I would check - in this order: Look at the copper connectors on either side of the plug on the end of the pump hose, clean with wire wool or green scratchy, the plug back in. Look at the back of the pump connector under the sink, check both spade connectors are connected. If they are, pull them both off and look at the spades on connector body, if they look corroded, clean with some fine sand-paper/emery cloth and reconnect. Check for 12V across the connectors whilst pressing the switch down. Check for 12V between each switch terminal and battery -ve and again to battery +ve. I went through this at the beginning of the season this year, my pump had siezed and had to be replaced :( My setup does not have a fuse for the pump (it probably should), but yours may. If both of the last two tests are negative, I'd check for that. If you are getting voltage to the pump but it's not pumping, I don't believe they are serviceable, it will need replacing. If you have a current range on your meter, switch to it (high range) and put the probes across the switch (one to each terminal). The meter will complete the circuit and would ordinarily make the pump run. You can see how much current it is taking, mine was pulling 7A which is way too much - it should be 2-3A, I think. Can you trace the wiring back from the switch to the battery? It's not far, and should just be under the floor and up into the cupboard on the left and then to the battery. Anyway, if you do need to replace the pump, it is possible to replace just the pump rather than the whole pump/hose/plug assemly. I bought a GP1352 Premium pump off eBay for £20 rather than £40 for the assembly. You will need a soldering iron, although when I replaced mine, the previous person to do it had just twisted the wires together and wrapped with tape! I hope this is of some use. Simon
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