Bypass alternator diode to power oil fuel pump safety switch on 1995 4.3LX Mercruiser

[derfxa]

On my 4.3 LX the diode on my alternator is not working ( the stud mount broke and nut/stud spins) and it was the culprit for not powering the electric fuel pump. Once I jumpered the post on the oil safety switch directly to the battery everything works great. Is there an option to keep direct 12v power to the switch and is it OK to bypass the alt diode post, cause I hate to replace a perfectly working alternator. The bypass now works the oil switch as it should functions and cuts off when engine is turned off. The only issue I see now is always having 12 volts to the switch.... Thanks!

 

[alldodge]

I think there is a mix up of issues, correct me if in error

Volvo Penta uses a diode to power the fuel pump relay, but Merc does not. The Merc 4.3LX power to the fuel pump while running comes from the oil pressure switch (item 3)

 

[derfxa]

The 4.3LX fuel pump is powered at the starter only when cranking then cuts off when starter is disengaged and then the power from the diode at the alternator goes to oil pressure switch and with proper oil pressure the circuit goes to electric fuel pump. Not sure if the alternator diagram (VP 4.3 GL) is the same on mine. Not sure

 

[derfxa]

Here's what might work... need feedback... red is my current fix..direct to battery and in green is my idea to have it switched with key ignition so when off it is not live..

 

[alldodge]

The 4.3LX fuel pump is powered at the starter only when cranking then cuts off when starter is disengaged and then the power from the diode at the alternator goes to oil pressure switch and with proper oil pressure the circuit goes to electric fuel pump...

No, it does not work that way. The Mercruiser wiring does not apply voltage using a diode

Hey achris and Fun Times a little help

You can reroute the power so it will always be powered by the key switch, but this has it's own issues

 

[achris]

Here's what might work... need feedback... red is my current fix..direct to battery and in green is my idea to have it switched with key ignition so when off it is not live..

Yep, that will work.... Just once. Because as soon as the oil pressure closes the oil pressure switch, it will keep power the 'switched' side PERMANENTLY!, even when you turn the key OFF. While there's oil pressure, you'll never be able to shut that engine OFF.

First question, is this a Volvo Penta or a Mercruiser?

Chris.........

 

[derfxa]

It's a 1995 4.3LX Mercruiser... well maybe alternator diode is the wrong terminology, but a purple wire from alternator (top right terminal facing the back of alt) goes to the oil safety switch. With the post disconnected to the switch where the purple wire went to I powered that post with 12v and the pump worked as should. See diagram on my prior posts

 

[derfxa]

achris, I thought when the engine is off there is no oil pressure, thus switch does not complete the circuit and that switch has no connection to power engine... not sure how the engine will stay running?

 

[alldodge]

The Yellow/Red wire powers the pump while the engine is starting. The purple wire is powered when the Key is turn ON, and it also turns the ALT ON. Once oil pressure builds up the switch closes and then the purple wire powers the pump. There is enough fuel in the carb to keep the motor running for the 1 to 2 seconds for pressure to come up.

Your motor is wired the way it should be, the issue if the pump is not staying ON when running, is you may need another oil pressure switch

 

[derfxa]

AllDodge, The problem seems to be not getting enough or any power to the switch from the alternator but when I get 12v to the switch works the pump (not a direct 12v to pump but going through the switch). Saw in another post where a guy put in another alternator and the switch/pump worked fine. I'm guessing it's the alternator, thus my question to bypass the alternator as shown in my second diagram. Unless there's an issue with the purple wire routing at the dashboard causing low voltage to that circuit. Are the non OEM alternators ok to use for $70 or so dollars versus the $200+ Mercruiser OEM?

 

[alldodge]

The problem seems to be not getting enough or any power to the switch from the alternator

Power does NOT come from the ALT, it goes TO the ALT

If there is no power on the purple wire then there is a break somewhere. When ever you read a thread about power coming FROM the ALT, this is not a Mercruiser.

When the key is turned to ON, 12V comes to the motor thru pin 5 on the 10 pin connector.
From the connector power goes to
Coil + side
TB Distributor module
ALT
Oil Pressure switch

If there is not 12V on the Purple wire going to the ALT then the ALT will not work. Since the motor does run, you can run a new wire from the + side of the coil to the ALT and the Oil pressure switch. If this is the case I would unwrap the wiring harness and find where the brake is

 

[derfxa]

The purple wire to the oil pressure safety switch had 12v after starting, didn't know to check when running, was checking with just ignition key switched on...ugh. Well, after too many hours farting with the oil pressure safety switch power I realized the issue was really with the fuel pump losing prime, tighten all the fittings, checked with water n soap for leaks when I pressurized it with air and no leaks, but it seems that the fuel backs out of the fuel pump when it sits for awhile thus pumping air. I have to manually prime the pump to get it running. Could the fuel pump have a bad internal seal or impeller housing leaking back out and shouldn't it normally hold fuel from backing down? Or could it be a carburetor issue not holding the prime?

BTW, thanks for all the help!

 

[alldodge]

The fuel pump should not loose prime. If it does leak down then there is an issue, but make sure this is what the problem is. Check to see how much pressure the pump does put out, and should be 3 to 7 psi. Also the antisiphon valve attached to the tank (where the rubber hose is) can get stuck and keep the pump from sucking fuel

 

[derfxa]

The pump has good pressure and the anti siphon valve must be the culprit...geez... all this crap to find out it that it's always been the f..ng anti-siphon valve... When checking fuel pump I did blow back down the fuel pump's exit port and there was no back pressure... #@%&$%#@$! .... Found out there was no anti-siphon valve ... all fixed now! I wonder how many people, rebuild the carb, replace the fuel pump, replace the oil pressure safety switch only to find out it was the valve... Thanks All!