I also posted this in electrical section...

So as the title says. I have the Merc 383 stroker scorpion bravo with the ECM 555. after changing wires and HVS cap the motor started right up and sounded great for a couple minutes. then it started to idle rough and die. It also keeps making a beeping noise like the key is turning back on to wake up the ECM and the fuel pump reprimes itself.

So far we have replaced the coil pack and voltage regulator, completly disconnected the alternator. hardwired both the key and the kill switch. we have replaced the crank position sensor. we have also put two new grounds to the ECM and a 12v source directly from the battery yet it keeps acting like its waking up during the engine running. we have not replaced the TPS.

with the key on, engine off and the diacom software hooked up the ECM will stay on and not restart itself. but with the engine running the screen on the diacom goes blank and shows the ECM restarting.

what could possibly be causing this?

You more than likely have an INTERNAL ECM 555 issue. You are more than welcome to send your ECM into me to have it tested. I do ECM testing and repair. A BRAND NEW ECM 555 is 2K and app specific ONLY meaning one can not just go buy any ECM 555 and work correctly. It wont and a huge waste of money. Repair is one third of the cost of new --- if not less.

It is not UNCOMMON to have an ECM internal failure on these marine apps. I would not do any more parts changing until at the very least having your ECM tested. Also if you keep running the engine tthis way -- possible engine failure could take place. And that will be expensive.

Do not disconnect the alternator on a running ECM engine because that is one way to knock out your ECM. I have to ask why did you do that ? It most likely cause more harm than any good. Just saying.

How much is your testing and repair? what kind of turn around time could i expect?

600 to 750 bucks depending on how much damage. one week in my hands seems to be the norm then 2 day shipping. Sometimes sooner repair but depends on the damage and what parts replacement. Like I mention min one third the cost of new your s is 2 K ecm 555 for BRAND NEW

Might be a bad connection, and may have happened while replacing wires, don't know

Have you tried replacing or swapping the relays? Main power, Fuel pump and Start relays are all the same and can be swapped

Yeah we swapped all the relays and it didn't change anything.

It still reminds me of a bad connection or weak battery. If there is a bad ground power will pass enough current to turn things on, but soon as the motor starts or is cranked, more power is needed, and voltage is dropped from a bad connection. With meter on the terminal 86 of the MPR, read power, then crank and see if voltage drops or you loose it.

If that's steady, move probe to terminal 85, with key ON and not started, 85 should go from reading voltage to zero. Once started, if power jumps back up and then down, the ECM is turning off/on

Well over the weekend we noticed that the main power relay was getting very hot. once it got too hot then the motor would shut off. if I swapped the relay with one that wasnt hot the motor would continue running until the new relay got hot. So it looks like i need to find a dirty connector, bad ground, short, or some reason. Any ideas on where to start looking?

With it getting hot I wonder if someone replaced the fuse feeding the MPR with one to large or a jumper.