Artie,
This post is in response to your question about how to wire this with a relay. My first suggestion would be to check the rating of the dash switch. (Probably 20amps) Then check the rating on the pumps, if they are under 20 amps, then don't do anything because this circuit will be kind of complicated, with lots of connections -- said differently, lots of places for it to wiggle apart and stop working.
The drawing below assumes a few things. First, it assumes that terminals 1 and 2 are a switch that has 3 different states. the first state, is Positive to term 1 and negative to term 2. The second state, Neg on Term 1 and Positive on Term 2. Third state, nothing on either term (off state.) It also assumes that you'll use SPDT (single pole, double throw) relays - explained later. You can do this circuit with 1 SPDT and 1 DPDT - This maybe advantageous, as you'd have two less connectors and splices.
Parts List:
3 - SPDT Relays $.99: See link at bottom
2 - 12v Diodes (200milliamp or greater) : Radioshack
x - Slip connectors.
x - wire
Diagram:
Knowledge:
diodes allow current to pass through in one direction. For ease of understanding, imagine this diagram allows power to pass from either terminal 1 or 2, upto the relay. When Terminal 2 is positive and terminal 1 is negative, power passes through the diode on term 2 up into the circuit, but not back to terminal 1. You've probably heard of Light Emitting Diodes. Same thing, just these don't emit light.
Explanation:
Terminal 2 is positive, and Terminal 1 is Negative:
Power flows upto relay A, energizing the coil and turning on the relay. Power is then connected to relay B and C. Power flows to your pumps (indicated by the M symbol)
Terminal 1 is Positive and Terminal 2 is Negative:
Same as the first explanation, just this time power flows to the coil of Relay B and C which energizes those relays. They switch their position and reverse polarity on the pumps. Causing reverse pumping.
Relays:
http://www.mcminone.com/product.asp?catalog%5Fname=MCMProducts&product%5Fi d=26%2D534&info=specs The reason I suggest 3 SPDT relays instead of a SPDT and a DPDT is cost. These are $.99.
I'm quite sure this is the simplest way to accomplish this. If someone has an easier way to do it, be my guest!
(Message edited by yosquire on April 01, 2006)