Re: Plagued with starting issues
 

Thank you, that makes sense.

Great idea! Definitely going to try this. Thank you.

I'll keep you guys posted, I want to get this sorted out so bad.

Re: Plagued with starting issues
 

Keep in mind that jumper cables won't carry as much current as the ones in the boat, but if it helps somewhat, that's a good indication that a cable or two is bad. They don't last as long in a boat as in a car because of the dampness.

Re: Plagued with starting issues
 

The safe way to go about this is to use a voltmeter. Connect the black lead of the voltmeter directly to the negative terminal of the starting battery. Connect the red lead of the voltmeter to the heavy red wire connected on the starter. The voltmeter should read battery voltage (>12V). Turn the key to start and observe the voltmeter, it may dip below 6V for a brief moment while cranking but should recover to above 9V quickly. If the starter does not crank and the voltage stayed above 9V then the problem is in the start signal. Move the red lead on the voltmeter to the "start" terminal (Yel/Red) on the starter solenoid. Turn the key to start and observe the voltmeter, it should read battery voltage. If it reads 12V and doesn't crank then you have a bad starter or starter solenoid. If the voltmeter reads less than battery voltage when turning the switch to start then the next are of interest is the slave solenoid. Move the red lead on the Voltmeter to the heavy red/purp wire on the slave solenoid. The voltmeter should read battery voltage at all times. If it reads less than battery voltage then the problem is the circuit breaker with the red button on the motor. If it reads battery voltage at all times then move the red lead to the heavy Yel/red wire on the slave solenoid, turn the switch to "start" and observe the voltmeter. You should hear the slave solenoid click and the Voltmeter should read battery voltage. If you hear the click but do not read battery voltage then you have a bad slave solenoid. If you do not hear the click then move the red voltmeter lead to the small yellow/red wire on the slave solenoid. Turn the switch to "start", the voltmeter should read battery voltage. If it does not read battery voltage then move the red voltmeter lead to the "S" terminal on the back of the starter switch. Turn the switch to "start", the voltmeter should read battery voltage. If it doesn't then the start switch is bad. If it does the the problem is either the wiring from the switch to the big connector on the motor or the neutral safety switch in the shifter. Good luck.

Re: Plagued with starting issues
 

I struggled with starting issues for years with a used boat, 5.7L Mercruiser MPI. Here is what I found and replaced in order:

-bad connection at the ignition switch, I temporarily ran a new wire directly from the battery. Not recommended for long term as you bypass the fuses, but this got it started and going on one day stranded on the lake. I eventually re-wired it.

-You will either have a slave solenoid or a relay on the top of the engine, those are cheap and easy to replace. My relay was bad and caused another long day on the lake.

-There is a 90AMP fuse on starter solenoid, that was burnt and corroded and prevented my starter from engaging (clicked only) unless the battery was above 75% charged. I replaced that and replaced the cable ends, removed all corrosion at all points and reconnected. This fix, I feel, had the biggest improvement. I saw it looked bad but never did anything about it. With the other repairs, I would have to turn the key several times before the click would amount to anything. This fix made the biggest impact.

After those 3 things the boat turned over smoothly every time.

By they way, these repairs occurred over several years, and several years of testing solenoids, measuring / observing voltages over this, from here to there, upside down in the engine compartment, getting it to work sometimes but not every time. The slave solenoid/relay is a cheap quick fix. Running a wire from the battery to the ignition took 5 minutes. The starter fuse, and cables took a couple hours. I wish I skipped the many many hours of diagnostics, sweat, knocks on the head and knuckles etc, and instead just fixed those 3 things.

Re: Plagued with starting issues
 

I just noticed mine is not currently using the 90amp fuse on the starter that I see in most pictures. How will this affect it?

Re: Plagued with starting issues
 

It's protective, it may be somewhere else on that line but generally it is right at the solenoid on the starter. There is usually a 30A breaker at the top of the engine as well, right at the slave solenoid or relay. It should not be a contributor to your issues if it is not there.

Re: Plagued with starting issues
 

Check the ground connection at the back of the motor. And make sure the starter is tight. I had problems starting for a few months. After changing starters , solenoids and ignition switches turned out it was a bad ground connection

Re: Plagued with starting issues
 

Hello, Similar issues first time out this year and will share what I've learned/determined. My boat is one of the last Diablo's with key switch btw, yours may be similar.

Several years ago I had my slave solenoid (on the motor) just click at times. But usually if you turned the key and let it click a few times the boat would start. I took it all apart and cleaned everything up and had not had any trouble again until recently.

And my key switch was always a little flaky but had been getting worse. Then trip before last my brother-in-law was helping me and a bit too aggressive and the key just went round and round so I knew it was bad for sure now. I tried jumping from the battery to start but it still just clicked or buzzed and would not start.

Once I had the ignition back on, gauges come alive, I then just bridged from the battery cable at the starter to the coil energize input and the boat started fine every time. So the problems were ahead of the starter/solenoid assembly.

Before the next trip I replaced my key switch with a Sierra MP41040 4 position key switch. Identical to the one in the boat and if you look closely there is a little hole that you can insert a paper clip and pop the tumblers out. This may be true of most variations on these switches. In any case I was able to put my original tumbler with my nice gold plated Mariah key in the new switch assembly. It was a little tricky to hold the metal sleeve in place while inserting the other tumbler so it would click in place but not difficult. Problem #1 solved.

The next trip out everything seemed A OK until late in the afternoon and wouldn't you know it is back to the click but no start. Sometimes clicking rapidly like a dead battery. I thought no way we did not even have anything on but the depth finder. So I try the bridge from the batt cable to the solenoid post and it fires right up.

Bridging the two posts on the slave solenoid and it will also crank just fine. Problem #2 is the slave again. My opinion is that sometimes when the motor is hot the slave solenoid acts up and gives a weak connection. It may just click it may buzz a little but that thing is troublesome. I could bend the tabs and pull it all apart again but is there another brand that is a better alternative and more reliable than the OE Mercury? I will rather replace a $20 part this time.

I should mention how while troubleshooting my No-Start back home it took me a while to realize my lovely wife who pulled the boat back on the trailer didn't bother to put the shift all the way into Neutral. I was tracking the signal again thinking what now it has a brand new switch what else is bad, the safety switch now?, before it came to me. It looked like it was in Neutral but the button had not popped out so check for the obvious also.

My boat ran great once started, the trim tabs are working lovely, all but my tach which has frozen in place again is in good shape. I will attack that and the slave in a couple days, then hope to get back next time with a trouble free boat.

Re: Plagued with starting issues
 

Update! I can't thank you guys enough for your responses and ideas.

After running some tests we found extremely high resistance through the battery switch, so it's going to get replaced first then retest. I'm taking this opportunity to convert to a dual battery setup! New switch and deep cycle battery are getting added in tonight. I hope that's all it needs.

Re: Plagued with starting issues
 

MTH,

Take some pictures, I've been wanting to go dual battery for awhile now...but haven't quite come up with a layout/wiring diagram.