1987 Johnson 70-HP Engine Gauges on 15-foot Boat

[Maboter]

I have a 1985 Johnson 70hp engine. My current wiring harness has a light switch, tachometer, and trim tilt gauge. I’d ideally like to add more gauges but am looking for comments how to do so. I’ve read a number of threads on here discussing this. My questions are as follows:

Q1: Can the existing engine wiring harness support more gauges? It’s OMC controls.

Q2: If I were to add more gauges what is the best way to do it?

Q3: Does daisy chaining off the trim tilt wires work or is it better to do it another way?

Q4: I will be adding a Garmin 54cv UHD2 chartplotter. Will this display have a voltmeter function?

Q5: How do you add a fuel gauge if I’m using a 6 gallon external tank?

Q6: Will the existing engine wiring harness support a water pressure gauge?

Q7: How would I add an hour gauge to my setup?

My 1985 Johnson has been remanufactured with a new power head and I’d like to use the gauge for break in and overall maintenance.

[jimh]

Q1: Can the existing engine wiring harness support more gauges?

Any new electrically-operated gauge will probably need new wiring between the gauge and the sensor that provides input to the gauge

Q2: If I were to add more gauges what is the best way to do it?

I don't know what you mean. Ask a more specific question. Read this article:

Conventional Gauge Rigging
https://continuouswave.com/whaler/refer ... gging.html

Q3: Does daisy chaining off the trim tilt wires work or is it better to do it another way?

Q8: What circuit in the TRIM gauge are you asking about?

It is very common that the 12-Volt ACCESSORY circuit from the ignition key switch and the BATTERY NEGATIVE circuit are daisy-chained between electrically-operating gauges.

Q4: I will be adding a Garmin 54cv UHD2 chartplotter. Will this display have a voltmeter function?

Read the GARMIN manual. Typically a display device will often have a means of monitoring its INPUT VOLTAGE.

Q5: How do you add a fuel gauge if I’m using a 6 gallon external tank?

Generally ALL remote fuel tank level gauges work on an electrical basis. Generally a particular fuel gauge will be designed to connect to a particular resistive sender. Generally an on-deck 6-gallon fuel tank will be made of plastic. Generally on a 15-footer you would check the fuel tank level by leaning over and looking at the the indicator on the tank, either a transparent fill level molded into the tank or perhaps a mechanically-linked fuel gauge that uses a float to operate the dial pointer. To make an electrically operated remote fuel tank level gauge you would have to find a suitable sender unit, adapt that to your plastic fuel tank, and then wire the sender to the gauge. This will be awkward if you ever plan to remove the six gallon on-deck tank from the boat, as you will have to be able to detach the fuel gauge wiring from the sensor. On a 15-foot boat this seems a bit unwieldy.

Q6: Will the existing engine wiring harness support a water pressure gauge?

A water pressure gauge is generally not electrically operated. The typical water pressure gauge uses a small semi-flexible hose to connect the gauge to a water-pressure tap fitting on the engine power head. Consult the engine manual for your engine to see if a pre-drilled and threaded boss has been provided in the power head where you could attach a hose barb fitting to tap into the water cooling jacket.

Q7: How would I add an hour gauge to my setup?

A simple method to record engine run time with an hour meter is to wire the hour meter to be supplied with 12-Volts from the ignition key switch ACCESSORY circuit. The drawback to this approach is that if the ignition key switch is left in the RUN position with the engine not actually running, the hour meter will accumulate this time, too.

[Maboter]

It is very common that the 12-Volt ACCESSORY circuit from the ignition key switch and the BATTERY NEGATIVE circuit are daisy-chained between electrically-operating gauges.

hanks for all the responses. They are all very helpful.

I’m wondering if I can add on a voltmeter off the same wiring used for the trim tilt. The trim tilt gauge has a [violet], white, and black wires. I’m assuming the [violet wire] is off the ignition circuit so if I just spliced it or daisy chained it to a voltmeter and/or other gauges I’m wondering if that would work.

[jimh]

I’m assuming the [VIOLET wire] is off the ignition circuit so if I just spliced it or daisy chained it to a voltmeter...I’m wondering if that would work.

As already mentioned above in my lengthy reply, to daisy-chain the positive 12-Volt circuit and the battery negative circuit among several electrically-operated gauges on an instrument panel is very common. If you read the article linked in my reply earlier, you see an example of such wiring in the instrument panel I constructed.


Fig. 1. The back of an instrument panel with conventional engine gauges. The VIOLET conductor carries the ACCESSORY power from the ignition key switch to several of the gauges by a daisy-chain wiring method. The BLACK conductor carries the battery negative circuit in a similar manner among several gauges. The BLUE conductor carries the gauge lamp illumination circuit among several gauges.

Regarding getting an accurate measurement of the battery voltage, measuring the voltage at the ACCESSORY circuit should give you a very close reading of the actual battery terminal voltage as long as all the many connections in that long circuit are good connections and there is very little voltage drop in the wiring. Typically the usual Voltmeter gauge is not very high resolution anyways. About all it will be good for is to discover if the battery voltage is much too high, much too low, or somewhere close to normal.

If you are really interested in knowing the battery voltage with any accuracy on a 15-foot boat, you would need to use a digital voltmeter and connect the SENSE input directly to the battery with nothing else on that circuit. But again, we are talking about a 15-foot boat, so the usually very coarse readings of a $30 voltmeter gauge is probably all you really need. Let's not get carried away.

ASIDE: I ran a 15-foot open skiff for several years and there were no engine gauges except a tachometer. For water pressure I relied on the overboard discharge flow of cooling water as an indicator of proper water pressure. For engine temperature I relied on the engine OVERHEAT warning to alert me of a problem. For battery voltage I judged the battery by how fast it would (or wouldn't) crank over the engine at starting. For trim, there was no power-trim so the engine trim setting was fixed.

[Maboter]

Thank you very much. That’s exactly what I was looking for and is very helpful.

Sounds like most of it is unnecessary but nice to have. I might just add a voltmeter to the tach and trim as it seems pretty straightforward and forgo the rest of them.

Appreciate the thoughts and advice.

[jimh]

Thank you very much...Appreciate the thoughts and advice.

Your are most welcome. Your acknowledgement of the my reply is also much appreciated.

ASIDE: I often write rather elaborate replies to questions and never hear a peep from the person who asked the question, thus not knowing if that person ever even read my reply.