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79 Evinrude 75HP (75943R) - Starter engages immediately, but only when solenoid is mounted

J

jhub2026

New member
Important, I'm no mechanic! Just someone trying to help out. Anyhow, I'm troubleshooting a strange starting issue on a 1979 Evinrude 75 HP (Model 75943R, S/N J0001903) and I'm hoping someone familiar with these older OMC electrical systems has seen this before.

Last year the boat developed a no-crank condition. The owner assumed it was the starter and had a new starter installed, but the problem remained. I wasn't involved in that repair, so I can't verify the starter wiring.

When I started troubleshooting, the starter solenoid would not click. If I jumped across the two large terminals of the solenoid, the starter cranked the engine normally, so I replaced the solenoid.

I'm now on my second brand-new Sierra 18-5808, and both exhibit the exact same behavior.

Here's the strange part:

If I connect all wiring (battery cable, two red harness wires, starter cable, yellow/red wire, and both dark wires) but leave the solenoid hanging free so it is not touching the engine anywhere, everything works perfectly. Turn the key and the solenoid clicks, the starter engages, and the engine cranks normally.

Now I disconnect the negative battery cable, bolt the solenoid into its factory mounting location, and reconnect the negative battery cable without changing a single wire.

The instant the battery is reconnected, the starter immediately engages and cranks the engine—even if the ignition key is completely removed from the boat.

I can reproduce this 100% of the time:
  • Solenoid hanging free = normal operation
  • Solenoid mounted = starter immediately engages as soon as the battery is connected
The wiring never changes. The only variable is whether the solenoid is physically mounted to the engine.

I bench-tested the replacement solenoids before installation and they behaved normally.

At this point, the only thing I can conclude is that mounting the solenoid to the grounded engine is completing an electrical path that does not exist while it is hanging free. I don't understand what internal or external electrical path would allow that to happen, and that's what I'm hoping someone can explain.

Any ideas would be greatly appreciated. Thank you!
 
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You have + battery cable going to one big terminal.-----A ground wire from one small terminal?----And where does the wire on the other small terminal come from?
 
Its the yellow/red wire that comes from the Large Red terminal. See the blue arrows in the image below Thank You!
BlueArrows.jpg
 
These solenoids should not be grounded to or thru the case, but strictly thru one of the 2 small studs. The 2 small studs simply go to the energizing coil, so which wire (yellow/red & black) goes to which small stud is irrelevant. Actually, the case is plastic and has a rubber sleeve so how could it ground? And of course, the yellow/red is only powered when key is in the start position.

If wired as shown, then it's right. The Sierra part number is correct for the application. I have never had an issue using real Sierra parts. BUT on EBay I see a bunch of Sea Sierra parts. Similar logo but Chineesium knock off junk.

Another thought(s)?
You got into this thing after the PO did some bastardizing. Is this solenoid mounted horizontally? Early ones were with no ill effect.

Is is mounted vertically? If so, round end up or square end up? Is it possible it's upside down & gravity is pulling the contacts together? I'm thinking it really shouldn't matter but.... Especially if it's a Chineesium knock off. Could you have a defect? Yes but 2 defects seems unlikely.
 
????---The yellow / red wire comes off the ignition switch !----Energizes the solenoid when key is in crank position.-----Review the wiring diagram.
 
Droid said:
These solenoids should not be grounded to or thru the case, but strictly thru one of the 2 small studs. The 2 small studs simply go to the energizing coil, so which wire (yellow/red & black) goes to which small stud is irrelevant. Actually, the case is plastic and has a rubber sleeve so how could it ground? And of course, the yellow/red is only powered when key is in the start position.

If wired as shown, then it's right. The Sierra part number is correct for the application. I have never had an issue using real Sierra parts. BUT on EBay I see a bunch of Sea Sierra parts. Similar logo but Chineesium knock off junk.

Another thought(s)?
You got into this thing after the PO did some bastardizing. Is this solenoid mounted horizontally? Early ones were with no ill effect.

Is is mounted vertically? If so, round end up or square end up? Is it possible it's upside down & gravity is pulling the contacts together? I'm thinking it really shouldn't matter but.... Especially if it's a Chineesium knock off. Could you have a defect? Yes but 2 defects seems unlikely.
Click to expand...
Here's an actual photo of the new Sierra 18-5808 Solenoid wired. You'll see the cushioning band is completely degraded and so I've removed and wrapped the solenoid in electrical tape where that cushioning band is.
  • Wiring.jpg
  • A legend for this photo:
    • Blue Arrow = Starter side of solenoid
    • Yellow Arrow = Yellow/Red wire from wiring harness
    • Green Arrow = Neutral Safety from wiring harness, Light purple line tracing the ground wire to the motor
    • Red arrows show the battery side terminal (kind of buried) and another just showing the battery wire.
 
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