74 Merc 850- Dash Gauge Wiring & Grounding

[Bradmph]

Wiring Dash Gauges and Items to Power and Ground.

* I am wiring up a dash on an older Glastron and have replaced all gauges except the MPH gauge. The gauges list I have are...Tach,Fuel,Volts,Amps & MPH.
Instead of running the dash cluster off of the main pos cable from the battery to motor, * I added a second set of cables leading from pos & neg battery to the dash area for powering the cluster as needed.
* I have also added an electrical power kill switch near the battery compartment that is just a turning T-handle which breaks the positive cables as they leave the battery.

* The questions I have are about the dash instruments wiring and grounding.
The 2 new fused cables from battery...1 Pos x 1Neg...Lead to under the dash.

+ I have ran the Pos cable to a multi-fused power block for dispursing power to needed gauges/gauge lighting items/horn/boat lights/12v dash adapter.
- The neg cable is ran from battery to dash to a multi-connector block for neg needed connections.

Question: On dash items requiring grounding wires, do I run these ground wires from the dashelectrical items using them to the neg multi-connector block that the new neg battery cable is connected to? ...or do I need to create a new common ground somewhere under the dash?
There is no real place to ground under dash because of lack of metal objects. The only large metalic item around is the fuel tank and I would imagine this would be a no-no for electrical grounds to lead to.

Question: What would be the maximum amperage fuses for electrical dash components like I have listed?

 

[wifisher]

Re: 74 Merc 850- Dash Gauge Wiring & Grounding

You can run all of your grounds to the block leading to the negative battery post. That is what is meant by "ground" in a boat, car bike, etc. There is no true earth ground.

The fuses are not there to protect the electrical items. They are not sized this way. The fuse is ONLY to protect the wire. As long as you are using 18AWG or bigger, I would just use a 10A fuse to power all of the gauges. You could use a bigger fuse, if needed, but I don't think that you will use anywhere near that much current.

 

[Bradmph]

Re: 74 Merc 850- Dash Gauge Wiring & Grounding

Thank You Sir, that was the answer I needed to hear...

 

[Silvertip]

Re: 74 Merc 850- Dash Gauge Wiring & Grounding

So you switched the wiring from the way it "should be" to the way it "should not" be for what reason? The gauges now remain active every minute the main power switch is on whether or not the engine is running. Yet you retained the tach wire via the engine harness. To befuddle this even more, the main power switch is not even on the helm. I really question the reasoning for this. Your car doesn't operate like that, your truck doesn't operate like that, and your boat shouldn't operate like that. Is it against the law? Of course not. Does it defy logic -- you betcha. Whomever owns the boat after you is going to also wonder -- what was he thinking. Some of the instruments, like your tach for example, have electronics in them. Should you forget to turn the main power off (and I guarantee at some point you will), every one of the instruments continues to draw power needlessly. Wouldn't it be a lot simpler to just have them turn off when you turn the key off? Of course you can do what you like or feel is right but this is not one of those "feeling right" issue.

 

[Prophammer]

Re: 74 Merc 850- Dash Gauge Wiring & Grounding

Silvertip sounds hard but sometimes truth hurts, I'm waiting for mine! How is amps fed to your amp gauge? Where does your Tach get it's signal from? Does your fuel tank gauge sending unit have 2 wires, if not it probably is grounded through the tank, and I think a steel gas tank is supposed to be grounded anyway. Did you put in main fuses near the battery? in case something does short out, less wiring wil burn. Is MPH managed by way of air pressure to the back of it ? All questions just a matter of curiosity on my part. Please post updates on how its going!

 

[Silvertip]

Re: 74 Merc 850- Dash Gauge Wiring & Grounding

I was not born and raised in "mamby-pamby land" and have always questioned and will continue to question those that seem hell bent on creating a problem where none exists. My comments were not harsh -- but rather simple, truthful, conversational English which is becoming a thing of the past as people tend to take a counter-opinion as a personal attack. Good grief folks -- pull up your big boy (or girl) panties and get on with it. This is a forum and you have the option to take advice and give advice (right or wrong) but as such, you will run into differing opinions. If you weren't looking for advice you wouldn't be here. Actually there are folks that look for confirmation of what they want to do knowing full well is it not safe or that it is against the law. They simply want someone else to say it's ok and then take offense when someone actually tells them it's a bad idea.

 

[Prophammer]

Re: 74 Merc 850- Dash Gauge Wiring & Grounding

I'm glad there are still people out there who will stand up for what they believe. I hope those who don't, can take it.... from those you do! But what about wiring that !@#$% panel. I think even my perko switch should've been on the helm where I could get to it! To late to change it I sold that boat!

 

[Silvertip]

Re: 74 Merc 850- Dash Gauge Wiring & Grounding

I think even my perko switch should've been on the helm where I could get to it!

In a perfect world that's where it should be but unfortunately, the expense and complexity of routing the long runs of large gauge battery cable makes that impractical in most cases.

Most folks don't realize that smaller recreational boats have two electrical systems, not one. The first runs from the battery via the large battery cables to the engine, and from there power is distributed to the helm via the control harness. The "A" (accessory) terminal on an outboard ignition switch powers the guage cluster ONLY. The second system consists of a pair of 10 gauge (usually) wires that run from the battery to the helm where they terminate at the +12 volt and ground busses on the fuse panel. A circuit breaker or fuse is normally provided on or within six inches of the POS terminal at the battery. Back at the helm, the +12 volt bus feeds the switches via fuses for the circuits they control. All grounds come back to the ground bus. The reason the fuse panel is not fed by the ignition switch is because the switch contacts are not designed to handle the high current loads some folks tend to overload their boat with. Like killer stereos, downriggers, spot lights, etc. This is sort of what the OP was trying to do but not understanding how life should be, decided to be creative.

There is a generic boat wiring diagram in the stickies at the top of this forum. It represents how the vast majority of recreational boats are wired.

 

[Prophammer]

Re: 74 Merc 850- Dash Gauge Wiring & Grounding

I finally put in an aux battery, and all non-essentials run from that, I can leave a light on until she's dead, the motor will still start! Only problem is twice as much battery maintenance!

 

[local825]

Re: 74 Merc 850- Dash Gauge Wiring & Grounding

In a perect world- Good info Silvertip, gonna see if i can simplify this even more, (if possible) there are two"thick" wires from the battery to the fuse panel one Red, one black. The wires are + and - they give power to the accessories, aerators,bilge lights ect.... Now you have gauges, which (Are Not) Not, "ran to the fuse panel" Are not "ran to a battery direct, Are not ran to the accessories, . On most if not all your power for gauges will come from the control box which will also have a plug in harness to the engine.............

 

[local825]

Re: 74 Merc 850- Dash Gauge Wiring & Grounding

In a perect world- Good info Silvertip, gonna see if i can simplify this even more, (if possible) there are two"thick" wires from the battery to the fuse panel one Red, one black. The wires are + and - they give power to the accessories, aerators,bilge lights ect.... Now you have gauges, which (Are Not) Not, "ran to the fuse panel" Are not "ran to a battery direct, Are not ran to the accessories, . On most if not all your power for gauges will come from the control box which will also have a plug in harness to the engine.............

If I tell you, a mosquito can pull a plow" Hitch em, Up..................