Thunderbolt tachometer wiring???

anyone no the wiring for one of these??? according to the mercury diagram .there are three wires, black, white and brown. i assume black is negative, white is positive and brown is the trigger...now the back of the tacho has 4 terminals. one pos, one neg and another terminal above each of these,,now the piece of wire that is still attached has one wire to pos which is then linked to the terminal above it. the second wire is linked to neg with nothing on the terminal above it...any ideas???

Re: Thunderbolt tachometer wiring???

My guess is for the tach ilumination light or anouther power connection for a second gauge.

Re: Thunderbolt tachometer wiring???

How about some more info so we don't have to guess. What motor? Is the gauge the same age/vintage as the motor?

Re: Thunderbolt tachometer wiring???

yes its vintage, its to attach to a mercury 1000 which is on the boat, although i am thinking of puttin a mercury 500 on the boat instead as the 100 hp is a bit too big....iv attached some photo's any help appreciated...

Re: Thunderbolt tachometer wiring???

You are correct regarding the wire color codes. Black = NEG, WHT = POS, BRN = SIG

Can't be certain about the connections but this is my best guess.

The two top terminals were for the backlight bulb, one POS and one NEG. That is why the POS wire goes to two terminals. The bulb and holder are missing.

There is a marking at the bottom center called TBOLT next to what looks like used to be a terminal. Does that look broken or maybe a plug that somebody stuck in there? Can't tell from the photo. I would say that would have been the location to hook up the BRN signal wire.

Is this a new to you boat? I have seen those tach's for sale on ebay quite frequently if you are interested in replacing yours.

Re: Thunderbolt tachometer wiring???

iv checked next to the tbolt markings. i see where you mean but it just looks like a rivet, it doesn't look like any kind of fixing and certainly doesn't look like its damaged