Mercruiser 5.0 no power to fuel pump when ignition ON

[Franco_boater]

Hello,
Im writing this post in the name of my friend from the same marina.
He said he was boating with his family then heared a pop and the motor dies...
I suspect also no charging because his batteries are drained too...
When he keys the ignititon ON the pump doesnt work we cannot hear the sound of fuel pump for a while, there is also no power to the electric choke (yellow-red wire). The starter works, it kicks so I assume 120Amp is OK.
Are there any other fuses there somewhere? Its 1998/9 5.0L Mercruiser

Ciao!

 

[Fun Times]

Do you get 12v at the helm gauges when you turn the key on?
Do you have a test light or multimeter handy as well.

There should be a 50-amp circuit breaker with a red button up on the engine, also possibly a white 90-amp fuse right at the starter post where the battery cable connects, plus a 20-amp breaker or fuse right near the ignition key switch plus maybe some other fuses like a fuse box panel for other accessories under/behind the helm where the gauges would be.

 

[Franco_boater]

Yes I have multimeter ready, I need to sneak around I couldn't find any more CB's around... thanks!
For sure there is 120Amp at the + at the starter but I assume its OK if the engine cranks?

 

[Franco_boater]

Can You tell me please how to check/bypass oil switch to eliminate the problem?? Thanks!

 

[Fun Times]

Yes I have multimeter ready, I need to sneak around I couldn't find any more CB's around... thanks!
For sure there is 120Amp at the + at the starter but I assume its OK if the engine cranks?

Unless the fuse at the starter has been changed from original size for whatever reason then the amps should be 90 from new... The 50amp breaker is item number 22 while the 90 is (would had been) one of the item numbers 20

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Again number 8 at the starter shows it as 90 here too,

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oops edit: forgot to ask, we are working on a carbureted engine vs fuel injected? If EFI, then the oil psi switch and info below doesn't apply as it doesn't have an oil switch to power the fuel pump.... < That would be relays that need checking...

Item 11 will be the oil psi switch down by the oil filter location... Port rear lower side. It's powered first by the engine starter while cranking over then once running, the alternator/purple wire which is the ignition side takes over... Which is why the gauge's having power was mentioned above?

https://shortcircuitweb.com/mercruiser-oil-pressure-switch-wiring/

But yes, if the engine cranks over then the 90 ought to be good... To know best once there, check for 12v right at the orange wire at the back of the alternator...Find the 50-amp breaker and check both sides for 12v....With the key left on, wiggle the wiring harness especially if the gauges don't power up. Check the back side of the key switch for 12v and see if it gives the purple wire power when switched to on.... also physically move the safety lanyard switch to make sure it's not halfway in the off position.

 

[Franco_boater]

Yes it’s carb, ok I’ll get back when we will be at the boat.
I don’t remember if the gauges were alive when keyed on. I’ll check that.
For sure the engine was cranking a little until batteries die, and also there was continuous beep ( I assume because of no oil pressure )

 

[Bt Doctur]

What are the first 3 lette

Yes I have multimeter ready, I need to sneak around I couldn't find any more CB's around... thanks!
For sure there is 120Amp at the + at the starter but I assume its OK if the engine cranks?

rs of the word "ASSume"
The main battery cable is direct to the starter, no fuse there.
The 90a white plastic fuse was supposed to only protect the alt but they added the main system to it. When it blows you lose the alt and main system feed.
I remove the red system wire from it and wire directly to the main solenoid post. Your sill protected by the 50A CB

No fuel pump power until you have engine oil pressure

 

[Franco_boater]

Thanks I’ll check that fuse too

 

[nola mike]

What are the first 3 lette


rs of the word "ASSume"
The main battery cable is direct to the starter, no fuse there.
The 90a white plastic fuse was supposed to only protect the alt but they added the main system to it. When it blows you lose the alt and main system feed.
I remove the red system wire from it and wire directly to the main solenoid post. Your sill protected by the 50A CB

No fuel pump power until you have engine oil pressure

Yeah all of this. I had a bugger of a time tracing down a partial failure of that 90a fuse. Ended up removing it for the same reasons given by bt. Near as I could tell that fuse only protects the alt from a high amp back feed. Starter circuit isn't fused but ignition is. First check is to make sure you're getting spark.

 

[Franco_boater]

Unless the fuse at the starter has been changed from original size for whatever reason then the amps should be 90 from new... The 50amp breaker is item number 22 while the 90 is (would had been) one of the item numbers 20

Genuine Mercury Marine parts, large inventory, fast shipping

Genuine Mercury Marine, Mercruiser and Mercury Racing engines, parts, propellers. Authorized dealer with large inventory, great prices, fast shipping!

www.mercruiserparts.com

Again number 8 at the starter shows it as 90 here too,

Genuine Mercury Marine parts, large inventory, fast shipping

Genuine Mercury Marine, Mercruiser and Mercury Racing engines, parts, propellers. Authorized dealer with large inventory, great prices, fast shipping!

www.mercruiserparts.com

oops edit: forgot to ask, we are working on a carbureted engine vs fuel injected? If EFI, then the oil psi switch and info below doesn't apply as it doesn't have an oil switch to power the fuel pump.... < That would be relays that need checking...

Item 11 will be the oil psi switch down by the oil filter location... Port rear lower side. It's powered first by the engine starter while cranking over then once running, the alternator/purple wire which is the ignition side takes over... Which is why the gauge's having power was mentioned above?

View attachment 409376

https://shortcircuitweb.com/mercruiser-oil-pressure-switch-wiring/

But yes, if the engine cranks over then the 90 ought to be good... To know best once there, check for 12v right at the orange wire at the back of the alternator...Find the 50-amp breaker and check both sides for 12v....With the key left on, wiggle the wiring harness especially if the gauges don't power up. Check the back side of the key switch for 12v and see if it gives the purple wire power when switched to on.... also physically move the safety lanyard switch to make sure it's not halfway in the off position.

12v at the orange wire should be with IGN on or off? Thanks in advance! I will keep you friends updated

 

[Fun Times]

12v at the orange wire should be with IGN on or off? Thanks in advance! I will keep you friends updated

Engine off...

Purple wire just key on...

 

[Franco_boater]

I remember we have noticed no 12V to the choke as well, should it be HOT with ignition ON or when the engine is running? I Think in my V6 its ON when my ignition is ON from COIL +

 

[Fun Times]

The choke should be hot when the engine is cranking over to start via the starter post until the oil psi switch builds about 4psi then purple takes over when running...The choke is on the purple/yellow wire in the diagram above.

 

[ste80]

sorry, if is not efi, or MPI when ingniction is on you can not hear fuel pump just when injiction is on you need to crank motor or supply 12v
if you have a LX like the above schema ,it is very easy the power on the pum will arive only when you will turn the motor on and starter motor will run. And then during running when oil switch will be with contact clooses ( once you have pressure)
to bypas this you have to baypass the oil pressure switch or give +12 to yellow purple wire with 12 v
If pump will not run then replace it

 

[Franco_boater]

Engine cranks but without starting, even when spraying start fluid doesn’t work…
I’ve got 9V at the coil +, 12v at the orange at alternator.
Kill switch is in RUN, I don’t have red circle CB at the top of the engine. Maybe fuse at the starter how to check it? All instruments works at the dash
It has TB5


Ciao from Italy

 

[Scott06]

Engine cranks but without starting, even when spraying start fluid doesn’t work…
I’ve got 9V at the coil +, 12v at the orange at alternator.
Kill switch is in RUN, I don’t have red circle CB at the top of the engine. Maybe fuse at the starter how to check it? All instruments works at the dash
It has TB5


Ciao from Italy

It looks like you’re losing voltage through the key switch indoor helm if you only have nine votes volts at the coil I don’t think that’s enough for a thunderbolt ignition

I would try to jumper 12 V two positive side of coil directly from the battery and see if you get a spark. If you do at least you know somewhere in the ignition circuit from the helm and key you are losing voltage.

 

[Franco_boater]

Can shift interrupt switch block the ignition maybe? I don’t have any tools to check if the coil is making spark to the distributor…

Can disconnect gray (-) wire from coil help isolate the failed tacho?

 

[Scott06]

Can shift interrupt switch block the ignition maybe? I don’t have any tools to check if the coil is making spark to the distributor…

Can disconnect gray (-) wire from coil help isolate the failed tacho?

Yes shift interrupt can be tripped and block spark. Just disconnect the wires

take wire from coil off dist cap hold close to a ground like engine block, crank engine you will see spark if coil is making a spark.

definitely disconnect the tach if it is broken . Some mercruiser have separate tach wire to coil where there will be two grey wires one to tach other goes to ignition module. others will have just one wire to negative side of coil that splits off to tach and module. If this is the case disconnect the tach signal wire on back of tach. A bad tach can definitely kill the spark

 

[Franco_boater]

But on the other hand if it’s 9V at the coil the ignition should work…. I think of putting 12V from batt to + on the coil