98 Speedster- When one motor starts the other dies

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I have a hard time believing it's the MPEM, I could be wrong though.

Any way you can measure the system (battery) peak high and low voltages nearest to MPEM as possible during start up of 2nd motor? I guess it shouldn't dip under 10V and shouldn't rise over 14.4V.

Eliminate battery switch to isolate something going on there?

Have you looked inside the ignition coil box where the starter solenoids also are, corrosion on ring terminals (especially the grounds) in there is pretty common.

When I had my cover off, the silicone gasket was pretty dry which made it challenging to get back together so I had to lube the gasket using some silicone spark plug wire grease. Or maybe I used some sludge on my finger from the bottom of the bilge, I don't recall exactly....

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I’ll check the voltages and inspect the coil boxes, thx for all the help. I have rebuilt several jet skis but this thing has me stumped. And I like you have a hard time believing it’s the MPEM. Everything functions as it should.
 
Just to clarify, there were no issues prior to replacing the starter on one of the motors?
 
I just have this idea in my head this is a grounding issue. Might make up a few ground jumper lead(s) and connect the various grounds back to the battery negative terminal directly? Be careful doing this if the battery switch is wired on the negative side of the battery.

Some strange things have happened with dual MPEM's but I don't recall this particular symptom.
 
I really appreciate all the help on this. I plan to work on it most the day tomorrow. I will make a bunch of ground jumpers and see what happens.
 
If there were no issues prior to replacing the starter, that's where I would look. Not sure how a starter would cause this, but who knows. I would put a Denso starter back in it. You've installed aftermarket crap, so chances are you're going to be replacing again soon anyway.

Where you located Joe?
 
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Hmm, might pay close attention to the start button circuit, maybe as you release the start button the opposite engine gets a shutdown signal? Disconnect the shutdown button of the running engine before attempting to start the other? Corroded terminal in that circuit? I'm sure your boat is like mine, where the start button is also the shutdown button.

Also, those buttons are known for sticking, maybe pressing on the second button somehow is also making the 1st button?

Just throwing ideas out there.......
 
Wish I were closer, thanx for the thought Jts32 that would have been great and a lot simpler.
 
Update: run jumper ground wires everywhere I could think of. Voltage doesn’t drop hardly at all when starting the second motor. It just shuts it off. If you try and restart it will run for 1-2 seconds and die over and over. I changed starter buttons around, same deal. Looked through the coil boxes all is good. They both run fine separately. Just the P motor will die if you start the S. Wander if I should replace the starter again? I have no clue...
 
I started the Port motor, fired right up, started the Starboard, fired right up, Port motor started to die so I hit the starter button again on the Starboard and killed it and the Port motor started to run again. It just won’t let them run at the same time. Afraid it might be a MPEM problem.....
 
It is beginning to sound like something's afowl with the MEPM.... Augh!

Are you absolutely sure there's a heavy ground cable between, tying both engine blocks together?
 
I have noticed that the cranking isn’t what kills the Port motor. It’s only when the Starboard start that the port motor dies. What about the stator? Or charge coil?
 
Could the stator be (bad) grounding out on the S and causing erratic problems? Do you know if you can convert this over to two separate Mpems?
 
They can be converted to 2 ski mpems and eboxes, but you will need to do some wiring for the added length from the engine compartment to the helm, you will need to add a second dess post, and will lose the neutral safety switch. I have never done it, just read how to's about it
 
Just to confirm for myself, if you start the S then start the P, the S dies. Or if you start the P then start the S, the P dies. Both are the case.

If so, I can't imagine how a stator or ignition pickup coil could be the issue.

If the problem seemed related to vibration, I'd say maybe check the associated stator connectors, inclusive to the stator housing, internally.
 
Sportster, the Port motor is the only one that will die. It acts like when you start the S it just clicks a switch shutting the P motor off. I thought about taking the new starter off and replacing it since that’s where all the trouble started but I have no clue what a starter would have to do with it.
 
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