Mercruiser v8 cuts out when hot

[jakwi]

Thanks again for the help guys, Just having someone to bounce ideas off is tremendously helpful. Yesterday I pulled the shift interrupt switch again. It is a sealed plunger type switch, but I sprayed some contact cleaner into the gap around the plunger and worked it for a while. I was able to get the closed resistance to .3 or .4 ohms and the open resistance is still OL. So as far as I can tell there is nothing wrong with the switch. I also inspected the harness as well as I could and rewrapped it in spots that I felt were at risk for rubbing through vibration. One spot in particular was back by the coil on the oil pressure sensor. I didn't see any exposed wire, but I wrapped the harness in that area with rubber tape, and ziptied it in a way to keep it from rubbing. I also double checked the grounds, and rearranged the wiring to the coil so that there was no risk of rubbing through the insulation.

As far as the knock sensor. That never occured to me. I'm pretty sure I can eliminate that on my next test run by jumping the purple/wht wire to ground, but I supposed I could always just unplug the sensor. I did try that back when I was having issues with the coil arcing, but I think that was a different issue. Luckily that sensor is fairly affordable. $30ish if I go with the one from Autozone, or $55 I think for the mercruiser part. ... any reason I can't use an automotive one? They look identical.

So for my to do list. On my next run, I'll disconnect the shift interrupt switch, Knock sensor, and Tach. The weather isn't looking great, so it might be a couple of weeks, until I get the next chance.

 

[jakwi]

Well I took it out on Saturday, and it ran like a champ, not a single hick-up, so as best I can tell it was either a rubbed through spot in the wiring harness that I corrected or the fact that I cleaned up the contacts on the shift interrupt switch. I kind of lean towards the latter, but I don't really have any way of knowing.

The one bad thing that happened was that on the way back I noticed my voltage had dropped from the usual 14.4 to 12.3. So I figured I had lost the alternator. I tied up at the dock while I went to get the truck and trailer, and when I went to restart I had nothing, no click of a dead battery, just the alarm buzzer going off.

So we pulled it to the trailer by hand and took it home. I found the 90 amp starter fuse was opened. I couldn't find one locally, but I did find the volkswagon fusible links, a whole variety of them for $5 at the local autoparts store. I was able to use one of those in the fuse assembly, believe it or not and reassemble.

I turned on the battery switch and I could hear it sizzling. That's not good so I turned it back off and felt the wiring. The battery wire to the alternator was warm to the touch. So I disconnected that one and tried again, No sizzling, and it started right up. So... on to the next thing I guess. It's the Mando 55 amp mercruiser alternator. I found one on amazon for $80, not the end of the world I guess. In my research I did find a westerbeke manual that had several pages on troubleshooting the mando alternator, so I'll attach it here for reference if anyone is interested. I'll probably try to rebuild the old one since I don't have to send the core in. It's too bad this sort of information is so hard to find.

Anyway I wanted to leave an update on my problem, that I'm happy to report is resolved.

Edit: oh and I did want to point out for anyone reading this down the road that the purple and white wire is to place the Thunderbolt ignition in base timing mode, I don't know for sure that would eliminate the knock sensor, the better way is to simply unplug the knock sensor.