12 v Power Take-off

[Warren Milberg]

My 1986 H28.5 had a cigarette lighter receptacle -- which recently failed an needs to be replaced -- for the cockpit that took power by using the hot stud on the back end of the ignition switch and a ground off the negative post of the tachometer, an arrangement I don't understand. Not sure why a PO who wired the old system up decided to take a ground off the tach stud rather than simply use the lower negative stud on the ignition switch. The fact that he did not do what appears to be the obvious is what concerns me.... On my boat, the ignition studs are "hot" as soon as the main battery switch is turned on. No switches on the main panel need to be thrown. It also makes no difference if the ignition key has been turned or not. So the only concern would seem to be whether I would be taking power away from the ignition process... or is there something else I'm missing here? Even if that were the case, I don't have the GPS on (the primary use for this receptacle) when starting the engine. Once the engine is started and the GPS is on, the generator takes care of power, not the batteries. Any thoughts?_________________PS: I plan to put an inline fuse on the hot wire when I connect the replacement up.

 

[Benny]

There is no ignition circuit

in a diesel engine. The key switch operates a solenoid in the starter circuit which activates the high amp circuit to the starter to turn the engine. The batteries are a component of the charging circuit, the alternator supplies charge to the batteries which in turn supply power to meet load demand. If your 12V socket in the cockpit does not have a load it will not affect the circuit. A GPS chartplotter is a low power demand item so the load is insignificant. I guess the PO chose a ground at the tach post to have a ground on a circuit not interrupted by the key switch so that he could use the 12V outlet while the engine was off. Lighter receptacles and marine 12V sockets although similar in form and function differ in contact surface and corrosion resistance. I would suggest you replace yours with with a locking plug and receptacle. These are my thoughts, now you make try to make sense of them.

 

[Warren Milberg]

For Benny

Thanks for your comments, very helpful. As suggested, I do plan to use a locking plug and receptacle. On my boat, the studs on the back of the key switch are "hot" whenever the main battery switch in on, so there is power there when the engine off as well as on.

 

[Baysailor2]

Pain...

Warren, I know it may be a pain, but consider running a dedicated wire run (pos & neg) from your main panel to power your GPS at the helm. Give it it's own breaker or double up on your other instruments' breaker. Wiring shortcuts always seem to come back to haunt you at the worst moments. Electronics such as the GPS like steady voltage, not subject to dips and surges.

 

[Baysailor2]

Also...

consider another wire run from the breaker panel to a replacement cigarette lighter outlet, with a 10 amp breaker. Plugging in something like a hand-held spot light could "smoke" the wiring off the engine panel if the fuse is too big to protect the wiring.