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TOPIC: Instruments and Merc 300

Instruments and Merc 300 1 year 11 months ago #128758

  • Merccooper
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I was just reading a post questioning where to buy vintage instruments. This made me start thinking about installing instruments (Temp? Voltage? RPMs?) for my Merc 300. Can this be done and does anyone know of a link that shows how to do it? I did a search but didn't find anything.

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Re: Instruments and Merc 300 1 year 11 months ago #128761

volts and RPM can be bought new. the new RPM hooks to the rectifier yellow wire. the volts hooks to negative on the battery or block ,the positive hooks to any positive the goes on and off with the key.there is a temp gauge that you can screw into the water jacket, top of block should have a 1/8 pipe plug for that. some have a plug or a place to drill and tap in the water jacket cover near plugs 2&3.

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Re: Instruments and Merc 300 1 year 11 months ago #128786

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Thanks for the details Dave.
This could be something I include in my winter project.
Do you think vintage gauges like these would work rather than going new? (I know the PSI wouldn't be of use)

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Re: Instruments and Merc 300 1 year 11 months ago #128788

no the only one to work is the temp IF you had the sensor and the AMP gauge. tach is for a IO oil not needed.

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Re:Instruments and Merc 300 1 year 11 months ago #128812

  • RickG
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I've been wondering lately if monitoring water pressure might be more useful than temperature. You'd be able to know right away if the impeller failed or something clogged the intake.
From what I've been able to read Mercury outboards run about 12-14 psi in the water jacket at speed. SW makes a 0-15 psi gauge and an electric sending unit that would fit in the 1/8" plug you mention (...it's a fuel pressure gauge so the label would be wrong, but I think I can fix that).

Anyone tried something like that?

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Re:Instruments and Merc 300 1 year 11 months ago #128817

merc even had a water pressure gauge.

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Re:Instruments and Merc 300 1 year 10 months ago #128819

  • ed-mc
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The ebay item id given is invalid, no result has been sentLots of mechanical water pressure gauge kits are available. You can utilize the fitting at the top of the block, and if you put a 90-deg elbow in there you won't have any clearance issues with the tophat.

If you wanted to get really fancy you could mount an electrical transducer on a bracket in the powerhead, and plumb the pressure source to the 'ducer. Then up to an electric gauge in the dash.

But you'd have to rig up some sort of power switch on the dash since there is no switched electrical power to the 300, just power for the electric choke & starter solenoid. On an engine that has switched 12V running around in the internal wiring, it'd be simple to tap into the switched source to power the 'ducer.

Quite frankly, way overkill for this application and a manual gauge is the best bet.

BTW a 0-30 psi range would be overkill as well, O-15 is more than you'll ever see. The 300 might develop 12 psi if you're lucky. But that's more than enough for good cooling flow. And you'll have an excellent handle on the condition of your cooling system; when the max pressure starts to slide down a bit, you'll know it's time for a new impeller.

Here's a gauge kit I found on eBay for around $46 shipped, it has a 0-15 psi gauge with fine increments, mounting bracket, and a nice long length of hose. You should be able to find the correct brass fittings at a box store or NAPA etc.



A whole bunch of pressure gauge stuff on eBay, manual and electric, if you want to peruse:



Quicksilver OE gauge kit complete with fittings, $69.99 shipped:



HTH.........ed

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Re:Instruments and Merc 300 1 year 10 months ago #128877

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Thanks Ed, that's extremely helpful. Don't really need to monitor water this way but I'll have 2 gauge holes to fill and this seemed like it'd be useful.

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Re:Instruments and Merc 300 1 year 10 months ago #128951

  • 63g3
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Absolutely put a water pressure gauge in....I would never run an outboard without one, lack of cooling is catastrophic within seconds. The blocks are so light there is no mass to heat up giving more time till pistons get overheated so a gauge is indispensible in monitoring.
As far as other gauges, a voltage (not amp) gauge or a fuel gauge are easy to wire or possibly a small display depth gauge? I try to stay correct vintage but early 60's gauges are very hard to come by so you might just have to settle on later year gauges if you have holes to fill.
I always restore dashes so they have no holes in case I can't find correct gauges but at a minimum I'll do a later year water pressure gauge in an under dash gauge holder so i don't have to punch a hole for it. Correct steering wheels are much easier to come by if needed. Once propped correctly with a hand held tach, a permanent tach is really not needed and you can use your phone now for a speed check.
Just my 2 cents....
Randy

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