Boat Motor Trim Sender Design Question

[metrologist]

The wiper rotates with the trim (tilt) of the motor. Why would they short one of the coils?

[Manul]

Sorry, I don't understand the question. Could you elaborate on that?

[Circlotron]

Parking here to see a good explanation.

[Manul]

You mean why the right one is connected throught both it's ends? Well, for example, when the right slider moves up, the left slider moves down, symetrically. So if the right one would not be "shorted", then the measured sum resistance would be same at any slider position, because when left side resistance increases, right side resistance decreases and vice versa. But this design seems ridiculous for many reasons. What the hell is that??

[Circlotron]

The effective resistance of the left element would vary linearly with change in wiper position. The right element resistance versus position would be nonlinear because of the parallel resistance of the wiper to each end. Maybe there is some resistance vs rotation curve that matters?

[Kim Christensen]

Probably widens the dead zone in the middle of the movement. Maybe so an ADHD skipper doesn't keep fiddling with it...

[Circlotron]

Assuming each resistive element is 1000 ohms, this is the total resistance vs clockwise rotation. Definitely non-linear. Maybe the effect of the motor angle in the water is also decidedly nonlinear and this is a low tech method of somewhat straightening things out?

[metrologist]

Yeah, that's close to the plot I got. Both coils are symmetric and total sender resistance is supposed to run from ~0 to ~170 ohm. I think there are 80 turns of 38ga nichrome wire, or ~4 feet for each coil.

~0 ohms is supposed to be the full down position, or just past having the propeller axis horizontal. I imagine you want greater sensitivity in the range when the prop is in the water, and when the prop comes out of the water you don't really care much exactly where it is, you'd just want to know when the motor is fully up for trailering, so the sender is less responsive on the up side. It actually matches the trim controls. When trimming up, there is a limit switch that stops upward trim just before the prop comes out of the water, then to continue up you have press the "Trailer" button. It's a safety feature because you wouldn't want the prop coming up above the surface hacking your water sports enthusiasts up, putting a damper on things.

The sender is 36 years old and a couple of the loops on the open coil next to the connection node were broken. When I unscrewed the coil, the bakelite insulator started splitting into pieces. Both coils have lose floppy coils and are shorting out. I wanted to remake it. Before taking it apart I was expecting that the coils would be shorted or in series at the extremes, until I saw the shorting bar under one of them, so that makes using a single coil less desirable.

[Circlotron]

I admire those electrical engineers who back in the day didn't have stuff like a single pot feeding an A/D in a micro with a lookup table, so they did it using basic principles that many nowadays may have not considered.

[Kim Christensen]

Yup... It would be hard to get a non-linear response out of a single wirewound pot. Ganging two elements this way is a clever way of doing it.

[metrologist]

Can you imagine how they did the gauge? It's broke too. I suspect there are two magnetized discs and they have pulled off of their white bushings. The coils produce the magnetic field to rotate the discs, shaft, and needle accordingly...

The orange plastic was crumbling so I had to glue many pieces together...

[floobydust]

I would suspect they are using off the shelf automotive fuel gauge parts, at least for the gauge. Look closely for any lettering or markings on its plastics etc.
The tank senders are common resistances. American common value is 240Ω-33Ω, GM 90Ω-0, Ford 158Ω-16Ω closest to what you read, and some European one is 190Ω-0.
I'm not sure why the sender is a rheostat, you normally don't have that much power going through it assuming a 12V system.

[Kim Christensen]

I'm not sure why the sender is a rheostat, you normally don't have that much power going through it assuming a 12V system.

Wire wound was more reliable than carbon and other technologies of the time. This is especially true in mechanical sensors where the wiper will sit and "jiggle" in one spot for long periods of time.
This changed when they started making POTs out of conductive plastic which is even more durable than wire wound.

[Circlotron]

I would suspect they are using off the shelf automotive fuel gauge parts,

That would account for the nonlinearity. Fill the tank up and the first quarter of fuel takes forever to consume so you think hey, this car is great! Then you forget about it until the last quarter goes yoink!

[CaptDon]

Typically the most interesting areas of trim are from parallel to the bottom of the boat to an area of maybe +/- 15 degrees from parallel. Once trimming in a direction more positive than 15 degrees the prop is coming out of the water. Good resolution on the meter scale in the trim range would be a good thing. From +15 on up to about +60 you don't need much resolution because you don't run the tilt in that position while underway.
Non-linear in this case would make sense.