Need some advice on a weird electrical problem

[seapro17]

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 wiring harness, 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

 

[alldodge]

Re: Need some advice on a weird electrical problem

Unplugging the harness and re-plugginf is a wierd one. Suggest ringing out the harness and I'm guessing one of the pins is barely making connection, probably the purple wire. When its dead and won't start check the coil for 12V, if its not there work backwards to find out where it shows back up. Also "an maybe do first" check your grounds, you could be loosing one.

 

[Mischief Managed]

Re: Need some advice on a weird electrical problem

I'm thinking a floating ground that's somehow related to the field coils on the alternator. Seems like it craps out when the alternator starts making juice.

 

[tfret]

Re: Need some advice on a weird electrical problem

without a doubt you have a poor electrical connection somewhere. If unplugging / re-plugging your ignition harness corrects the non-start problem then that is a good place to start. However, the harness thing could just be a coincidence. Make sure there is no corrosion on the pins and sockets. I use tye wraps to keep my harness on tight. Closely check all your electrical connections. This takes patience. Don't forget about your ground connections. They are equally important (lots of people don't understand that). I don't think it's related to the shift interrupter switch, but you can jump out that switch with a jumper wire temporarily to test it. It is possible your ignition switch is getting a flakey connection also but that's less likely.

 

[Don S]

Re: Need some advice on a weird electrical problem

A "wiggle test" of all electrical connections while underway would be a good start in finding the problem.

 

[Silvertip]

Re: Need some advice on a weird electrical problem

A voltmeter that goes from 12 volts to zero in sync with the engine stumbling points to some bad connections. Yes -- connections at the battery. Battery cable connections at the engine. The main engine harness connectors. When it doesn't start, break out the voltmeter and follow the juice. Bet there isn't any at the "B" terminal or the "I" terminal on the switch at that time.

 

[tfret]

Re: Need some advice on a weird electrical problem

a test light is very helpful. You can poke the sharp point right through the wire insulation so you don't have to disconnect anything to take a reading. and they don't cost much either.

 

[spdracr39]

Re: Need some advice on a weird electrical problem

a test light is very helpful. You can poke the sharp point right through the wire insulation so you don't have to disconnect anything to take a reading. and they don't cost much either.

And thus exposing the wires to future difficult to find corrosion. Don't do this.