MerCruiser 3.0 MPI Electrical Short

[jnelsonusmc]

Hello - I have been trying to fix this short for a while now. I need some help with a 2011 . The circuit that has a short is for fuse that is for Fuel Pump Relay, IAC and Ignition Coil. I disconnected the IAC turn the ignition switch on and fuse blows - disconnect the Relay and it does not blow. Changed the relay out and still blows the fuse when ignition switch is turned on. I traced it to the Red w/ Black Stripe wire in the fuse holder which is one of the wires for position A in the fuse holder. I found the Red w/Black Stripe wire on the relay as well but cannot find how the wire runs from the fuse bank to the relay. It says the fuse is for the IAC but there is no Red w/Black Stripe wire going to the IAC - there is no Red w/Black Stripe wire going to the ignition coil either. There are qty 3 - 882751 relays all in a line and I rotated them all and same result fuse blows when I turn the ignition on. It is hard to believe that all 3 of the relays are bad but I ordered 3 new ones anyway just in the off chance. If the fuse keeps blowing then there is a good chance that the Red w/Black stripe wire is shorted out somewhere in the harness correct? If I run an independent wire from the fuse holder to the relay, am I going to screw anything up? There is not a lot of wire to play with on either end so if I do this the chances of un-doing it are not good. Any help would be greatly appreciated. Fuse A in the picture below is the fuse that blows on me when I turn the ignition switch on.

2011 MerCruiser 3.0 MPI
Engine S/N 1A631142

Regards,
Jim Nelson

 

[jnelsonusmc]

I forgot to say that I started this troubleshooting because the boat will not start- it turns over fine but with the fuse that keeps blowing running the fuel pump relay - IAC and Ignition coil the boat will not start until I figure this one out. The electric priming fuel pump activates and primes the system - the ignition coil was replaced last year - the coil is the only thing that I did not disconnect and see if the fuse blows when I turn the ignition switch on. Could the coil be shorted out?

 

[Scott Danforth]

think about it. what is the load on the other side of the relay that is now causing the fuse to blow.

is the coil a quality coil or amazon special?

 

[jnelsonusmc]

think about it. what is the load on the other side of the relay that is now causing the fuse to blow.

is the coil a quality coil or amazon special?

I had to switch the mounting rack from old coil to the new coil so it must have been from Amazon - there just are not that many components to a coil and the engine fired right up with it installed - of course until now… most parts on the engine have come from boats.net but I do remember switching the mounting bracket so pretty sure the coil is a cheaper alternative from Amazon. Will check this morning by disconnecting the coil and see if the fuse blows.

 

[jnelsonusmc]

I had to switch the mounting rack from old coil to the new coil so it must have been from Amazon - there just are not that many components to a coil and the engine fired right up with it installed - of course until now… most parts on the engine have come from boats.net but I do remember switching the mounting bracket so pretty sure the coil is a cheaper alternative from Amazon. Will check this morning by disconnecting the coil and see if the fuse blows.

I disconnected the coil and still the fuse blows when I turn the ignition switch on. Steps I have performed-

Disconnected the IAC - fuse blows
Disconnected the Coil - fuse blows
Disconnect the Relay - fuse does not blow -
Switched the Relay out with 2 other same 882751 Relays - fuse blows

Any help for next steps on this would be greatly appreciated.

Jim

 

[jnelsonusmc]

I disconnected the coil and still the fuse blows when I turn the ignition switch on. Steps I have performed-

Disconnected the IAC - fuse blows
Disconnected the Coil - fuse blows
Disconnect the Relay - fuse does not blow -
Switched the Relay out with 2 other same 882751 Relays - fuse blows

Any help for next steps on this would be greatly appreciated.

Jim

In the past couple weeks I have replaced the Starter the 90 amp starter fuse - and the 50 amp main circuit breaker - boat cranks over as it should with new starter. Boat ran fine last Fall - went to start it up and the starter would not engage so I replaced it - then discovered the 20 amp fuse blowing when turning the ignition switch on - before turning the switch to crank position. .

 

[Scott Danforth]

I disconnected the coil and still the fuse blows when I turn the ignition switch on. Steps I have performed-

Disconnected the IAC - fuse blows
Disconnected the Coil - fuse blows
Disconnect the Relay - fuse does not blow -
Switched the Relay out with 2 other same 882751 Relays - fuse blows

Any help for next steps on this would be greatly appreciated.

Jim

the relay is just a switch. so yes, removing the relay removes the load prior to the relay. that didnt tell you anything as there is no load

what is the current you are getting with each load? have you looked at the chinesium coil?

have you did a resistance measurement to determine which item is shorted to ground?

 

[alldodge]

Fuel pump fuse is blowing because most likely the fuel pump is drawing to much current. Disconnect the pump and relay will energize but fuse won't blow

 

[jnelsonusmc]

Fuel pump fuse is blowing because most likely the fuel pump is drawing to much current. Disconnect the pump and relay will energize but fuse won't blow

Disconnected the fuel pump and fuse still blew when ignition switch turned on. The only way the fuse does not blow is when the relay is removed.

Jim

 

[jnelsonusmc]

the relay is just a switch. so yes, removing the relay removes the load prior to the relay. that didnt tell you anything as there is no load

what is the current you are getting with each load? have you looked at the chinesium coil?

have you did a resistance measurement to determine which item is shorted to ground?

I inspected the coil - there is nothing indicating anything wrong - when i disconnected the two connectors to the coil and the coil wire to the distributor the fuse still blew when the ignition switch in the on position. I did trace the Red with Black stripe wire from the fuse bank as the wire that is shorted out - I have not measured the current because the fuse keeps blowing - so there should not be any current to measure correct?

 

[alldodge]

The 3.0 is the only motor I've found that uses one fuse to protect ignition, IAC and FP. Other motors those items have separate fuses.

So long as the fuse keeps blowing your not going to be able to measure the current. If a larger fuse is installed it will most likely melt the wiring

Need a wiring diagram, or need to open the harness up to trace the wiring

 

[alldodge]

Could measure ohms with everything disconnected and see if you can get zero ohms from any connection to ground. Once you find which shows zero, start tracing

 

[jnelsonusmc]

I have found several diagrams online and none of the ones I found show the Red w/Black stripe wire. I have the SELOC manual but the diagrams are pretty useless in it. I am at the point of trying to run a separate line from the fuse to the relay - replacing the Red w/black wire - will I do damage to something I am not thinking about by doing this? I have positively identified the wire that is shorted and I know the start and ending points - (fuse and relay) - will I cause more damage doing this?

 

[alldodge]

Don't run another wire unless the original wire is cut off from original connections. This is a simple diagram of how it's probably connected. Note the fuse blows if anything in on the other side has an issue. Wire or component

 

[jnelsonusmc]

Don't run another wire unless the original wire is cut off from original connections. This is a simple diagram of how it's probably connected. Note the fuse blows if anything in on the other side has an issue. Wire or component

View attachment 393277

all components disconnected - and fuse keeps blowing - my assumption is wire from fuse to relay is shorted out inside the harness somewhere. Really would like to know why it shorted honestly - guess I am unwrapping the harness to trace the short down. They do not make anything easy to get to on this 3.0 engine....

 

[jnelsonusmc]

unwrapped the harness and traced the Red w/Black stripe wire from the fuse to the relay - nothing wrong with the wire. The second wire in on the fuse is Pink with White Stripe and it goes to each of the 4 injectors - later today after work I am going to disconnect all the injectors and see if the fuse blows then connect one at a time if the fuse does not blow - I checked the Red w/black wire with a fuse in the and relay off - the pink with white stripe wire would have been connected through the fuse when I tested for short. With the fuse removed the Red with Black stripe shows good from fuse holder to relay and shows 12.9 volts at the fuse terminal with switch on - switch off good continuity from fuse to relay on Red w/black wire. Glad I did not cut and run new wire for Red with black stripe.... So every wiring diagram i have found does show the pink and white stripe going to the ECM and then to each fuel injector. If I disconnect the injectors and the fuse still blows that will point me back to the ECM correct? I have not heard of an ECM causing blown fuses - am I going in the right direction here?

Jim

 

[dubs283]

Proper testing for a short to ground is with an ohm meter. Locate a solid ground and test circuits that are not supposed to have ground. Once you find the circuit that is grounded you can disconnect components to see when the ground goes away. This is a static/unloaded test. A proper wiring diagram will aid greatly in verifying which part of the harness needs inspection once the short is narrowed down to location

The method of testing under a load and verifying if the fuse blows or not does work but wastes fuses and causes undue stress on the wiring and can harm the circuitry in the ecm.

 

[jnelsonusmc]

Proper testing for a short to ground is with an ohm meter. Locate a solid ground and test circuits that are not supposed to have ground. Once you find the circuit that is grounded you can disconnect components to see when the ground goes away. This is a static/unloaded test. A proper wiring diagram will aid greatly in verifying which part of the harness needs inspection once the short is narrowed down to location

The method of testing under a load and verifying if the fuse blows or not does work but wastes fuses and causes undue stress on the wiring and can harm the circuitry in the ecm.

I absolutely agree with your advice - I just can not find a good diagram for the Mercruiser 3.0 MPI - plenty online for the 4.3 and 5.0 engines but not a lot that I could find on the 3.0. Welcome any recommendations on where I can purchase a Manuel that has a valid wiring diagram. You are also absolutely correct in that I have gone through a box of fuses already….. but without a good wiring diagram I am trying to work through this a piece at a time.

 

[Scott Danforth]

I absolutely agree with your advice - I just can not find a good diagram for the Mercruiser 3.0 MPI -

its in the factory service manual available here: https://publications.brunswick.com/

 

[alldodge]

Let me start with there just might be an issue with your pic. It wouldn't be the first time a manual or sticker was printed incorrectly, but that said it does follow the PCM555



Most all Merc MPI's have 3 relays
Main Power (MPR)
Fuel Pump (FP)
Slave (SR, also called start assist)

With the relay your looking at, and it having a pink/white wire that goes to the injectors, this just might be the MPR. The MPR connects power to same points as listed for item "a"

Key supplies power to MPR and the PCM. The PCM then applies a ground to the MPR coil which then sends power Injectors, coil, FP relay and other items. When PCM checks and all is good it applies a ground to the FP relay coil and pump turns ON




The SR is no used until key is turned to start and has no effect on PCM

In this above it has a separate fuse for alternator input. You mentioned the relay has a Red/Blk which would make it MPR