4.3L MPI Dead Ignition Circuit

[Chaparral19h20]

2012 Chaparral 19 H2O 4.3L MPI SN: 2A040699
So I did a dumb thing. Had the OBD service light come on, (no apparent performance issues) and I read that by jumping the OBDM terminals I could pull codes with a multimeter. As soon as I turned the key, the normal startup alarm stopped abruptly and the key went dead. All other circuits function fine (except the trim tabs) and I suspect a fuse blew when I shorted a circuit. I’ve spent 5-6 hrs. all over the boat going through everything I can find but cant find any blown fuses. I’ve tested the three main relays and replaced the 90 amp fuse at the starter for good measure. There are 4 O2 sensor fuses and 4 main engine fuses under black oval caps that all test good. Found a 2 amp fuse under the ECM in a single fuse holder that was also good. I’ve read online that there is a 15amp ignition fuse on the back of the merc 4.3 mpi down by the distributor but can’t locate one.
Appreciate anyone with suggestions for where I should look next. Thanks!

 

[Fun Times]

Hopefully the PCM isn't internally damaged, but if available to your engine application, do you know if you've located the clean power wiring harness which also has a fuse too like noted in the following description,

Mercury-Mercruiser 84-8M0113732 DTS Harness Clean Power 14'

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Items 18 and 19, https://www.mercruiserparts.com/bam/subassemblydetail/36140663/28211310

Circuit breaker number 17 has been reset? https://www.mercruiserparts.com/bam/subassemblydetail/36140664/28211310

Genuine Mercury Marine parts, large inventory, fast shipping. 4.3L MPI ALPHA/BRAVO EC 1A637496 & Up

4.3L MPI ALPHA/BRAVO EC 1A637496 & Up

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The 15-amp fuse you read about on the 4.3's is more of an accessories that comes off of / and rests near the main cannon wiring plug that runs between the boat side to engine side of the circuits... I don't believe they are using that fuse for the newer systems like yours but can't say for certain.

Depending on your location, you might be able to rent a MerCruiser marine scan tool type equipment,

https://www.ebay.com/sch/i.html?_nkw=7+day+LOAN+of+Rinda+Diacom+Marine+diagnostic+cable+and+software&_sacat=0&_from=R40&_trksid=p2332490.m570.l1313

 

[Chaparral19h20]

Appreciate the suggestions. Circuit breaker tested and reset, (never tripped). Can't locate anything like the clean power harness. Info states: Used to reduce voltage drop when battery is located near the helm of the boat. My batteries are in the stern 10 inches from the engine. guessing I don't have this part.
Concerned the scan tool wont work if PCM is not getting 12v from ignition circuit.
Thanks again.

 

[Fun Times]

Appreciate the suggestions. Circuit breaker tested and reset, (never tripped). Can't locate anything like the clean power harness. Info states: Used to reduce voltage drop when battery is located near the helm of the boat. My batteries are in the stern 10 inches from the engine. guessing I don't have this part.
Concerned the scan tool wont work if PCM is not getting 12v from ignition circuit.
Thanks again.

Within reason, that would be correct though keeping in mind connecting to the PCM and confirming power and communication is present would be a crucial diagnostic step especially knowing there were possible codes beforehand...
Without a correct wiring diagram, all guessing would be still just as dangerous to probe wirings.

As a just in case, there are a few places out there that might be able to repair (your) marine type PCM's (a ton of car places too) but it's usually the older model marine PCM's that are repaired where possible vs your model design number PCM so a call would be needed to help know for sure..., Here are two for example,

https://www.maicsalazardiagnostics.com/

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Do you have a version of say smart craft gauges/Mercruiser mobile vessel view, etc. that would plug into an different area engine wiring harness plug connector all to help determine any fault codes popping up now and forever?
If not, now might be the time to consider something as it could help with other steps towards PCM communication too such as maybe something used on eBay at possibly a bit more affordable costs vs new...

Check your local craigslist under marine services, offerup or Facebook marketplace for possible local mobile marine service techs to see if they can swing by to take a second look/connect to the PCM.

Might also be time to consider a Merc service manual as well for additional guidance in which I believe your manual part number ought to be 90-8M0041669

https://www.ebay.com/sch/i.html?_nkw=8M0041669&_sacat=0&_from=R40&_trksid=m570.l1313&LH_TitleDesc=0&_odkw=8M0041669&_osacat=0

https://www.crowleymarine.com/mercruiser/oem-parts/stern-drive/1998/42410pluu

https://www.mercurymarine.com/an/en/gauges-and-controls/displays/vesselview-mobile