a beautiful boat should sail forever.
As always, I am not a professional, and just sharing what I learned, which may be helpful, but always do your own research. I am getting the DC wiring in shape, and since I have TWO HELM STATIONS a modification to the temperature and the oil pressure gauges was required.
First, I AM DISCUSSING TWO DIFFERENT SYSTEMS. One is the GAUGE SYSTEM and the other is the WARNING SYSTEM that sets off an alarm when the temperature or the oil pressure is screwing up. IF you do not recognize that these are two different systems (circuits) you will be continually bewildered.
So starting with either the oil pressure or the temperature GAUGE SYSTEM, what they have in common is that they consist of a SENDER (sensor on the engine) and a GAUGE THAT MUST BE CALIBRATED FOR EACH OTHER!!!!!!!! To further complicate the issue, there are single gauge senders and dual gauge senders (in case you have two helm stations) - and the gauges must be calibrated accordingly - so basically you do not just order a gauge, or a sender, - you order a matched set of gauge and sender. I ordered mine from Sierra, who has a great chart on their web site well worth studying.
The underlying reason the gauge and sender must be matched up is that these senders are "variable resistors", ie the temp or the oil pressure creates a resistance in the circuit that determines the amount of current flowing through the gauge, which through an electromagnetic coil in the gauge determines the deflection of the needle. The sender terminal on the back of the gauge provides positive current to the sender, which is grounded to the engine block.
The actual circuit on my original, rebuilt Ford Osco 172 cubic inch diesel comes off the "IGN" terminal on the ignition, goes over to the gauge "positive" terminal , then out the gauge "sender" terminal down to the sender on the engine block, ie the ground.
THE WARNING OR ALARM SYSTEM IS A DIFFERENT CIRCUIT! It has a "switch" instead of a sender, that is spring operated. In the temp switch high temp (pressure) will force the switch to close, setting off the alarm, while in the oil pressure switch the oil pressure keeps the switch open, but if the oil pressure drops below a certain level the spring forces the switch closed and completes the circuit, setting off the alarm. (that is why oil pressure alarm comes on when you are starting the engine, until the pressure builds up. Oil pressure switches come in 6 psi and 15 psi, depending on when you want to be alerted.
The actual circuit goes from positive source to buzzer to switch to engine block. On my engine the original mechanical oil pressure was a copper pipe to the stb side of engine in front of the water pump (about center ofengine side). For the new system just screw in a 1/8" pipe (remember, when you are talking pipe threads 1/8" pipe is quite a bit bigger than 1/8"), then add a T junction so the sender and switch can both be installed. Parts are available from ace hardware, but there are clearance issues to deal with. Two are shon in the photo - I ended up using the longer pipe but the smaller T.