Thread Tools Search this Thread Display Modes
Old 11-05-2017, 02:27 PM   #1
Join Date: Sep 2017
Posts: 7
Default Mercruiser 4.3 Carb - Wiring Question

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.
Attached Thumbnails
WiringHarness.jpg   PressureSwitch.jpg   unknown.jpg  

lree12 is offline   Reply With Quote
Old 11-05-2017, 02:40 PM   #2
mmwjr's Avatar
Join Date: Feb 2012
Location: Essex, Maryland
Posts: 7,748

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.

1997 2400 SCR
5.7 Vortec / Bravo 2
mmwjr is offline   Reply With Quote
Old 11-05-2017, 02:47 PM   #3
Join Date: Sep 2017
Posts: 7

Thank you Sir, that explains it perfectly.

Hopefully that is the last of his "mods".
lree12 is offline   Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Powered by vBadvanced CMPS v3.2.3

All times are GMT. The time now is 05:54 AM.

Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2020, vBulletin Solutions, Inc.