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I am in the process of installing Fire Boy systems in my engine spaces. The instructions say to run the negative feed for the blowers through the pressure switch. Short of cutting the wires and testing, how can I tell which one is negative?

Pschuass,
Depending on the year of the boat or if the blowers have been replaced, the blower wiring coming out of the blower motor, the colors of the wiring are black and white or black and red if replaced with newer blowers. The original white and yellow stripped Bertram wiring is usually the positive side of the wiring, the original wires are twisted together coming down from above (usually exiting the fiberglass below the gunwale) they are yellow and white with a solid white wire, depends who did the wiring sometimes the blower wire is white with a black stripe and a solid white wire. Sometimes you can get the sharp end of a multi meter into the butt connector near the blower motor. Red probe is usually for positive 12 volt black probe is for !2 volt negative. Don't be afraid to cut the wiring and always use epoxy filled butt connectors (usually blue in color not the cheap automotive yellow butts). If you don't feel comfortable cutting wiring hire a qualified marine electrician. Where is the boat located? If in Mass I can come down and help you I was a certified marine electronics technician for 15 years and an SAMS Nationally Accredited Marine Surveyor. for 17 years before I had to retire because of bad knees.
Pete Fallon 561-310-7179

Pete,

Thanks for the offer to help. I am on Long Island, so it's probably too far for you.

All of the connectors that I use are the heat-shrink type as you have suggested and, after nearly two seasons with my boat, I am reasonably comfortable using them.

At least on the starboard side I can see the white wires with the yellow stripe feeding the blower. Now that I have some clues as to the color coding system I will know what to look for. Do the ground wires on these blowers go directly back to the main panel or do they connect via the terminal strip on the forward bulkhead of the engine space?

Peter

Do you have a digital volt meter? If not you can one of the $3 cheapies from Harbor Freight for this job. Just hook the digital volt meter to the wiring. If it reads 12 volts correctly then you know the red lead from the volt meter is touching the positive wire. If you get a negative volt reading then you know the red wire from the volt meter is touching the negative wire.

To feel confident about it, just test it first on your batteries.