[mike523]

Long story short, bought this boat and almost immediately ended up replacing the motor. 4.3 Mercruiser. Alpha 1 Gen 2.

Upon getting the boat back. hooked it up to the hose in the yard and noticed tach and temp gauges were not working at all. Turned our thermostat sending wires were backwards, switched them around and temp gauge worked again.

Still no tach but wanted to get the boat on the water and make sure she ran ok so I took her out and everything seemed ok for about a half hour and then suddenly gauges went berserk. PSI went from 40-80, temp suddenly maxed out, fueled went from 1/2 tank UP to 3/4 tank. It scared me a bit and I shut her down right away. Then noticed that 3 of the gauge needles were still bumping and wiggling even with the boat turned off so I figure there is possibly a loose wire or a ground issue somewhere? Started her back up and headed back to the dock, halfway there the gauges went back to "normal" (still not 100% functional but reading normal temp and PSI at least).

So I've got 2 issues:

1. How do I troubleshoot my tach?
2. How do I troubleshoot this apparent wiring/ground issue?

[Hudson River Steve]

You can start by removing the large plug on the engine. All of the instrumentation wiring passes through the large plug. Check the pins for corrosion. If you see any corrosion you can purchase tools online to clean the pins...or you can simply try plugging it and unplugging it multiple times to wipe the pins.

Do you have access to a DMM? If so, you can start troubleshooting by reading the voltage across the Batt or IGN terminal and the GND terminals of your gages when they are acting wonky. You should see a steady voltage between 12V & 15V. Anything else and you have a poor connection.

As for your tach, start by spinning the 4, 6, 8 selector switch to clean the contacts on the printed circuit board within the tach. Make sure you set it back to 6 cyl. If that doesn't solve your problem then make sure that you have battery voltage across the Batt and GND terminals. If you have battery voltage on these terminals then the only other signal you will need to make the tach work is the actual tach signal. The tach signal can be found on pin 2 of the large plug on the engine which is a Grey wire. You didn't mention if you have a carburated engine or a fuel injected engine. If carb then the grey wire goes back to the negative terminal of the ignition coil. If fuel injected, the grey wire goes back to the injection computer. If your DMM has a frequency setting on it you can check the tach signal at the back of the tach, it should ready approximately 30 Hz at 600 RPM.

[mike523]

Thank you for the detailed reply! It is carbureted and I do have a DMM. I used it yesterday to verify the alternator was working. I got 12v across the battery terminals when engine was off and also at idle. When I gave it a little bit of gas it went up around 14v, so alternator should be good. Can you give me an idea where the large plug would be found?

[Hudson River Steve]

The large plug is usually mounted near the rear of the engine. It is usually close to one of the exhaust elbows. It has a large rubber boot that protects it and looks something like what is shown in this picture https://www.perfprotech.com/blog/art...ness-schematic

[mike523]

ugh that sucks lol