turtlehead wrote:
I know so little (NOTHING) about electrical - what would be the problem I would need to solve here and how would I test for it?
I am not a motor expert but I am struggling with how a DC motor could be wired so that it can be
reversed using positive at one or the other terminals and ground for the negative. The electrical flow goes through the same path. The only way I can see this working is if the motor itself had a switch that changed where the ground connects in the circuit. If it only has to rotate in a single direction then a single positive terminal and body ground would work.
For Turtle:
A bit of background on how a DC (direct current) motor operates. But first a bit about DC. In a DC system the power flows from the positive side to the negative side. The negative side is often referred to as 'ground' in our Jeeps. For the most part, almost all cars have a negative ground but there are cars that have a positive ground. So the negative connection from your battery is attached to the body and engine. If you took a test lamp and connected one side to the positive of the battery and the other side to a metal part of the body or engine the test lamp would light. You would get the same result if you simply connected the lamp to the positive and negative posts on the battery. (You said you know NOTHING). If your car was made out of plastic or some other non-conductive material then it would need to have both a positive and negative wire run for every circuit. By using the metal body as ground there is no need to run a wire all the way back to the battery for the negative side of the circuit.
DC motors can operate in either direction. Normally this is rotation. The direction they rotate will depend on the flow of the electricity.
Most DC motors use the two terminals to 'reverse' the polarity. To rotate one direction the positive flow goes in one terminal, through the motor and out the terminal that is negative. To run in the opposite direction the positive goes into the other terminal, through the motor and out the terminal that is now negative. Because the positive/negative paths are switched the negative side is not connected directly to ground, there is always a switching mechanism in place. This is what Serehill's relays do, they switch which terminal on the motor is getting positive and negative when in operation.
Some DC motors only need to run in one direction, an electric fuel pump for example, is made to operate in one direction. Positive to one terminal and negative to the other (or ground). An attempt to reverse the positive and negative on such a motor will get mixed results. In some cases it might not work if it is protected with a diode. In other cases the gearing the motor is attached to might not rotate in the other direction.