Potentiometer specs for Trim Sending Unit Needed

[neilmack]

Is this spec in the repair manual?

I have a 1979 SeaRay, 305 OMC, 800 Stern Drive. The trim position gauge on the dash has never worked, althought the electric trim motor & the trim movement works very well. I just can't tell the trim position on the dash. (Tilt works great now too thanks to posts on this site.)

I've pulled apart the trim sending unit and found that the small wirewound potentiometer needs replacing. I can't find the complete unit for sale anywhere but potentiometer should be easy to R&R.

The potentiometer appears to be a 100 ohms resistance which is rare in electronic stores. They want to know the wattage of the potentiometer. Only a OMC part number on the potentiometer.

Can someone with an old OMC repair manual see if the specifications (ohms, wattage) for the potentiometer in the trim sending unit are listed in the manual.

Much appreciated.

neilmack

 

[bruceb58]

Re: Potentiometer specs for Trim Sending Unit Needed

I have never seen one of these sending units but I can give you some advice on your other questions.

To determine the potentiometer needed, get a pot that goes to 1K Ohms or so and adjust the pot so that you get a full up and full down reading. Measure the ohms of the pot at both of those indications and you will have your answer. Make sure you disconnect the pot from the circuit when measuring the ohms.

To determine the wattage, you can measure the voltage across your pot and get the wattage.

Wattage = Voltage(squared) divided by the resistance.


I think your main problem in finding a pot that will work will be the fact that the pot on there now has a certain amount of resistance per degree rotation of the pot. That will be the hard part. It is not the absolute value of the pot itself.

 

[neilmack]

Re: Potentiometer specs for Trim Sending Unit Needed

Bruceb58, thanks for your tip using the 1,000 ohm pot. I'll use it if the 100 ohm, 5 watt pot that I bought from www.DigiKey.com doesn't work.

Based on your formula for wattage, I only needed a 2 watt (100 ohm) pot but the 5 watt pot fit better in the sending unit because it had a smaller can. I had to cut the shaft down a little.

I plan to test my 100 ohm, 5 watt, pot on June 2. See photo attached of sending unit with new pot installed. Orange plastic gear moves up and down on the jack screw that raised and lowers the engine craddle - this is how the trim works on this boat.

thanks again,

neilmack