To fuse or not to fuse?

[firecpt12c]

I have been rebuilding a 1987 bass buggy with a 70hp Merc. I have transferred all equip to the new console and the wiring has been going great "I think". One question, does the grey sensing wire from the egnition to the tach need to be fused?
Also, I see no power to the instrument cluster does the tach sense wire supply 12v to the tach and then the purple jumper to my volt meter?

Thanks Monte

 

[Don S]

Re: To fuse or not to fuse?

One question, does the grey sensing wire from the egnition to the tach need to be fused?

No..

Also, I see no power to the instrument cluster does the tach sense wire supply 12v to the tach and then the purple jumper to my volt meter?

Again no. The gray wire is the sense wire for the tach, not a 12 V supply for anything.

 

[sschefer]

Re: To fuse or not to fuse?

Yeah but Don, would it be more noticible at 750hz or 5800hz.. LOL... Just kidding.

 

[firecpt12c]

Re: To fuse or not to fuse?

Thanks Don, so I can run a 12v hot from the ignition to the "I" post on the tach with a jumpper to the volt gauge with common gound and I should be set right? I have the instrument lights "Daisy chained" and conected to my Nav light switch so thats not an issue.

Thanks Monte

 

[Silvertip]

Re: To fuse or not to fuse?

Gauges are powered from the "A" (accessory) terminal on the ignition switch. Ground for each gauge obviously goes to ground. A fuse is not needed in any "sense" wire from (tach, oil pressure, temperature, fuel, or trim sender). Here is how gauges get wired. Since power comes from the ignition switch via the engine electrical system, those circuits are protected by the fuse under the engine cowl. Allother accessories are powered via the "boat harness" and the fuse panel.

 

[bruceb58]

Re: To fuse or not to fuse?

Thanks Don, so I can run a 12v hot from the ignition to the "I" post on the tach with a jumpper to the volt gauge with common gound and I should be set right? I have the instrument lights "Daisy chained" and conected to my Nav light switch so thats not an issue.

Thanks Monte

Your ignition circuit already has a fuse protecting it so unless you are adding a wire after that which has a lower current rating than the fuse that already protects that circuit, you are fine.

 

[Don S]

Re: To fuse or not to fuse?

Gauges are powered from the "A" (accessory) terminal on the ignition switch.

Just to be clear.
The purple wire from the ignition switch is what powers the instruments. That purple wire may be on the "A" terminal on the switch,
but the A does not stand for Accessory, it's the ignition terminal on outboards. Outboard ignition switches normally do not have accessory positions.

EDIT: Small text above is wrong information :redface: :redface: :redface:

 

[firecpt12c]

Re: To fuse or not to fuse?

Thank you. very helpful.

Monte

 

[Silvertip]

Re: To fuse or not to fuse?

Just to be clear.
The purple wire from the ignition switch is what powers the instruments. That purple wire may be on the "A" terminal on the switch, but the A does not stand for Accessory, it's the ignition terminal on outboards. Outboard ignition switches normally do not have accessory positions.

Picking nits here - but more clarification. While I do agree that there is no "accessory" position on the ignition switch for a two stroke outboard which is what we are dealing with. I disagree that the "A" terminal is the "ignition" terminal for an outboard. The fact that the "A" terminal has 12 volts on it when the key is iin the RUN position does not make it the ignition terminal. The two "M" terminals are the ignition terninals (OFF position) kills the ignition on a magneto system by closing the two "M" terminals thus gronding the ignition. They enable the magneto system in the ON/RUN position which is an open circuit between the two terminals. "A" indeed is the accessory terminal as it provides 12 volts to accessories (instruments to be precise). Although some folks (rightly or wrongly) do wire other stuff to that circuit. The term accessory in this case applies to the circuit, not the positions on the switch. If the "A" doesn't stand for accessory (in the same manner that "C" stands for "choke" or "primer", "B" for battery, "S" for starter/solenoid, then what does "A" stand for? To prove all of this, disconnect the control harness plug at the engine. Jump the solenoid and the engine will start. So much for needing an "ignitiion" terminal on a magneto system.

 

[Don S]

Re: To fuse or not to fuse?

Well, I have to admit, I screwed that up. My mind thought "Accessory" position on the switch when the A terminal actually goes to the Tachometer/Accessories Harness Connector (as Mercury calls it it the service manual). :redface: :redface:

Here is a highlighted page from the manual showing MY mistake.

View attachment 2d 15.pdf