A real riddle for Don

[Muskiebri]

Don, I have a 300XS built in Dec-2009. All wiring was installed brand new when I got the engine in April-2010. There's no "beep" when you turn the key to the "on" position, the MercMonitor (Level 1) I bought from you says "engine com lost", 0.0 volts, 32 degrees F (when it's really 80), 0 engine hours, etc. I unplugged and reconnected every connector I can find. Unhooked cables from starting batteries and reconnected. There's no fuel pump noise when you turn on the key, but the engine will crank. Found one blown 20A fuse in position #3 in the fuse holder just below and behind the ECU (the holder with three 20A fuses in). The 2 wires running to that fuse are red and red with a fine violet stripe. All other fuses I could find check out. I measured 12.9V from the location of the replaced fuse to ground with the key off. With the key off or on, I didn't measure any voltage at the location of the other two 20A fuse wires. The relays below the ECM all work (I swapped the rear one with one from the trim (right in front of it) to no avail. There's power at the alternator. No blown fuses under the dash. Am I missing any breakers or fuses? Could this be an ECU issue? I haven't run the engine yet this year and didn't do any engine work other than change plugs and LU lube. But I've turned the key to the on position many times over the past several weeks to get the Lowrance HDS data overlay to look the way I want. I also have Sirius weather and structure scan. But the MercMonitor, key/horn, and fuel pump had been working properly very recently. This is a mystery that has me really frustrated!

Thanks,,,
Brian

[EuropeanAM]

Not really that much of a riddle.

If indeed it was the #3 position fuse that was blown (lower left position in the lower fuse holder)... that fuse protects the circuit for: Electric fuel pump/ECM driver power/oil pump circuit/injectors ‑ 20 amp fuse.

A short in ANY of these items on this circuit will blow the fuse, or prevent ALL of these items from functioning (or both).

I would strongly recommend that you get this engine in to your local (well qualified) Authorized Mercury Dealer for PROPER DIAGNOSIS and repair.

As for the PCM.... it may (or may not) have been compromised. Once the shorted component is located and replaced, it will work (or not).

IF it is determined that ACTUALLY the RIGHT LOWER fuse is the one that blew- have your dealer reference SB 2007-03 (Racing) and 2009-026 (Outboard).

Good luck!