svonmiller

Howdy all,

I have a bizarre electrical problem I'm hoping someone can help me with. The motor is a 1989 5.7 liter Mercruiser built by Mercury Hi-Performance (before they became Mecury Racing). It's a 360 horse EFI motor. Last summer, I pulled it out of my twin IMP, rebuilt it, and dropped it into a 1985 24 footer. The only mod I did was that it was designed for a bravo drive, and the 24 footer has an alpha, so I put a shift-interrupter on it. I set it up on the ignition, not the ecu.

It ran great all last summer, and it ran great this season until recently. It started developing a problem where it wouldn't start. Turn the key on, nothing, no volts, like a bad battery connection (but it's not that).It's like the battery is disconnected. But it's not the batteries or connections. If I unplug the pigtail and plug it back in, then I get volts to the dash and it starts.

Last week I put in new batteries and a new starter (grasping for straws). It starts fine, and runs fine below 1500 rpm. If I throttle above 1500 rpm and put a load on it, it coughs and sputters and dies. While it's coughing and sputtering the needles on the instruments jump up and down, especially the volt meter, going from 12 to zero and back to 12 very quickly, just like someone is pulling a battery cable off and on real fast. If I throttle back, it behaves. If I don't it dies and then I get nothing on the dash, just like the battery is disconnected - but the batteries and cables are fine, I've got blower and bildge and power hatch. If I unplug the pigtail, that "resets" it, and I can start it.

This is repeatable - over 1500 rpm it dies, then I get no voltage to the dash, then I unplug the pigtail then plug it back in, I can start it. If I don't unplug the pigtail and plug it back in, it stays dead. Turn the key, nothing, no volts. I don't even know what to look at with this kind of problem. I've checked all the wiring and it looks fine. I do have the other motor that was in the twin, so I could swap out the alternator, ecu and ignition with the ones from the other motor, but I'd rather figure out what the problem is. Has anyone seen anything like this before?

Thanks in advance for any tips!
Stan

Questofpower

Sounds like a bad ground somewhere, try finding the common ground on the gauge cluster and the ignition switch and just run a new wire or a jumper to see if that helps, my guess Is, it is going to be the ground in that pig tail you pulled off. Not sure why 1500rpm would be a problem, might just be the right vibration to knock the ground loose. I wish I could tell you what wire is the ground in the pigtail, but I know nothing about that wiring harness .

Might also be a short in the ecu, try and swap with the other motor

svonmiller

I have schematics and looking at them it looks like the only thing the ground is used for in the dash is for the instruments, it's not used on the ignition. But, if you had a poor ground to the instruments, I'm not sure if that would effect everything else, seems like maybe it could. I wonder if a bad ground on the ignition amp would cause and issue like this. A poor ground somewhere would make sense.

Unlimited jd

Sounds like a bad connection in the cannon plug

whoya

Is your alternator charging?

svonmiller

I haven't checked that specifically, but I put in brand new batteries and charged them over night. I putted back to the dock at 1500 rpm which took about 30 minutes, and it did fine. But checking the alternator for sufficient charge isn't a bad idea, although I wouldn't imagine it making the needles jump and the whole system going dead, and then being able to reset it. Definitely worth a look though.

Sonic30ss

I had a similar no voltage problem, turns out the main power wire from the plug to the dash had a
Break in it somewhere, would work fine then nothing. Ran a new wire and it solved the problem