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lree12 11-05-2017 02:27 PM

Mercruiser 4.3 Carb - Wiring Question
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Hi All,
Looking for some advice re: wiring of a Mercruiser 4.3 vortec carb engine.

I bought the 2001 1900 SC boat this year and noticed that the fuel pump was constantly running when the ignition was on - but not cranking. Looking at the wiring harness diagram - (attached) a purple wire goes to the oil pressure switch then the output of that goes to the starter circuit and the fuel pump.
It appears as though the previous owner bypassed that switch - looks like he has the purple/yellow tied together and some ground wire going to the sensor terminals and back to the battery.

My question is:
When the engine is off, should I have continuity between the two terminals on the oil pressure sensor/switch?.
Also, what generates the oil pressure, is it via a separate oil pump or is by cranking the engine?.

Thanks for your help.

mmwjr 11-05-2017 02:40 PM

The previous owner has created a safety issue that needs to be fixed. Probably a bad oil pressure switch.

When the engine is off the oil pressure switch is open due to no oil pressure and this removes power from the fuel pump. When starting there is no oil pressure so power is supplied by the starter soleniod, once started the oil pressure switch closes and provides the power. The switch is there in case the engine stalls to prevent flooding of the carb that is an explosive situation.

lree12 11-05-2017 02:47 PM

Thank you Sir, that explains it perfectly.

Hopefully that is the last of his "mods".

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