boat ground?

I have a question regarding ground on a boat. I have a 2010 Lund Rebel 1725 XL Sport with a 90 hp 4 stroke merc and am trying to hook up a trim gauge. I figured out that my starting battery (which also has all my electronics connected to it) is isolated from my motor, which is grounded to the hull.

Should my starting battery also be grounded to the motor and hull? Would my starter not want to find the path of least resistance to try and ground to the battery when starting?

If not, I will have to run the ground of the trim gauge all the way back to inside the engine compartment.

When I spoke to my Lund dealer yesterday about connecting a trim gauge, they said it was simple just connect power and ground to 12V source behind the dash and connect the trim indicator wire to the gauge and I would be good to go. That leads me to believe that the battery should be grounded to the motor and hull.

Anyone have any experience with this?

Thanks,

Del

Just connect to the negative going back to the battery.

Just connect what?

Is the battery supposed to be grounded to the motor or hull?

If I connect the trim gauge only to the battery ground, it does not work. I have to ground the gauge to the motor or hull to get it to work properly. Or ground the battery to the motor/hull and it works.

you never want to ground the hull if there is a short you will have a ton of problems look to the next gauge and run a jumper off to another guage on the dash. if you ground the hull the electricity will corroed the hull.another thing if you drop a penny in the hull the reaction between the 2 metals will also corode the hull.

I didnt think you were supposed to ground to an alum boat

I guess i had a brain fart the gauge is part of the wiring harness if you have a merc the brown and white and the ground should be in the harness that goes up to the key switch so look for the brown and white and a black off the key a purple wire if your gauge has a light will be a switched wire which you would splice into the other gauges or the purple off the key same for omc except the trim i believe is white and tan

That is exactly how I wired the gauge, but it will not work that way. I have to ground the battery to the hull in order for it to work. The negative battery terminal is isolated from the trim sending unit ground wire. Any bolt on the motor is grounded to the hull and does not have continuity to the negative battery post.

“Let’s try and clarify the difference between “physical location” of ground points and “electrical paths”. So here goes. A ground wire from any electrical device should NOT be “physically connected” to the hull of a tin boat. Those grounds should be connected to one of two points: 1) the negative buss on the fuse/breaker panel, or 2) the negative terminal of the battery. The negative terminal of the battery has only one attachment point and that is the engine block via the large gauge battery cable. Now then — if you use and ohm meter and check for continuity between the negative terminal on the battery and the hull, between the negative buss on the fuse/breaker panel and the hull, or between the engine block and the hull you would indeed find continuity. That means “electrically” a path exists between these elements, but it occurs only through the engine mounting bolts which is not an especially good ground. Even though that “electrical” path exists, it is recommended that you NOT ground anything, including the battery DIRECTLY to the hull. To carry this just a little bit farther, boats typically have two basic wiring systems. 1) the engine harness going to the control box up front, and 2) the boat wiring harness that feeds the fuse/breaker panel. There are ground wires in both harnesses. The engine harness ground terminates at the engine block while the boat harness ground terminates at the negative terminal of the battery. Battery negative, the engine block, and the breaker panel ground buss are from an electrical standpoint, all the same since they are connected together by wire, not the boat hull. “

From a popular boating information resource.

how old is the boat you may have a broken wire or the insulation is cracked if the boat is older you should not have a ground on the hull. you have 2 big cables that go to the celionoid (sp) and ground on starter you then have a red and black going to the fuse block and from there all your accessories run off the fuse block the fuse block is isolated from the hull usually mounted on wood both systems are closed and do use the common ground disconnect the red and black to the fuse block then check if u have voltage to the hull and do the same for the engine harness

how old is the boat you may have a broken wire or the insulation is cracked if the boat is older you should not have a ground on the hull. you have 2 big cables that go to the celionoid (sp) and ground on starter you then have a red and black going to the fuse block and from there all your accessories run off the fuse block the fuse block is isolated from the hull usually mounted on wood both systems are closed and do use the common ground disconnect the red and black to the fuse block then check if u have voltage to the hull and do the same for the engine harness

The boat is a 2010 Lund 1725 Rebel XL Sport.

A little more digging. The trim sender is grounded to the lower unit/cowling, which is NOT grounded to the power head, which is grounded to the battery.

The lower unit/cowling is grounded to the hull, the power head is not.

Does any of this make sense? It really is starting to make me wonder why connecting a trim gauge is so hard.

If I move the ground of the trim sender to the powerhead, everything is OK. I just need to solder a longer wire to the ground lead of the sender and run it with the BRN/WHT wire into the cowling and pick a location on the powerhead to ground too.

It would have been much easier had Mercury just ran both of the wires of the sender into the cowling and connected the BLK wire to a ground in the harness.