I have 2 engines, so 2 ignition switches and one wiper timer circuit.
I want to power the circuit board with either ignition on switch.
I want the one timer board to power up 3 wiper motors, each motor draws 1.5 amps.
I was thinking to do this with diodes.

I tested with 5 amp diodes and they power the wiper motor fine.
So it will be three 5 amp diodes on the relay going to each wiper motor. The on pulse from the timer circuit is short duration from 3 to 30 seconds, so diodes are not continuously powering the wiper motor.
The timer circuit simply sends a momentary power pulse to the wiper motor.
The motor has internal switch run off the gear, so the momentary power pulse bumps the the motor past the cam on the gear and the motor is powered by a second wire, this is part of its normal typical self parking design.

So for powering the circuit board, have a diode in each 12vdc wire coming from each switch into the circuit board.
This way turn on either ignition switch will send power to the board, and block power from turning on the other engines ignition.

How much power will this circuit consume? Think a small 1 amp diode will work?

 

1A diodes will be fine for getting power to the timer circuit.
I am not clear what you are doing with the 5A diodes but I don't think they are necessary. Can you give us a diagram of where you plan using these diodes?

 

The 5 amp diodes need to be in the output from this circuit's boards relay .
As OEM boat design, each of the 3 wiper motors have 3 individual independent carling switches to turn them on.

I need to isolate the timer board's relay from back feeding power turning on all 3 wiper motors if I manually turn on any of the 3 carling switches for each wiper motor, so the 5 amp diodes are like one way valves.

So the one timer circuit board will activate three wiper motors
And each wiper motor will retain their individual on switch.

 

If you have separate switches for each motor then the switches would do the necessary isolation, no?

 

D2 is the wrong way round.

 

D2 is the wrong way round.

Indeed it is.

 

If you have separate switches for each motor then the switches would do the necessary isolation, no?

Until you turn on one switch, it would then power on all 3 wiper motors since all the outputs are ganged together as on at the relay on the timer circuit board.

 

Is the diode reversed? Maybe the diagram has an error then.
I am buying a board already made off ebay since I can not build it cheaper myself.
here it is
http://www.ebay.com/itm/140478292585?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

Does D2 protect the transistor from the relay coil emf?

 

Yes that's what D2 is for, the pcb has it the correct way.

 

I am buying a board already made off ebay since I can not build it cheaper myself.

Yes, there is a lot of it about