1998 Mercury 50ELPT 4S - Starter?

I've been recently trying to fix my dad's boat, and wanted to poll the aldeer knowledge before diving too deep and forking out big bucks on parts.

Basically you turn the key and nothing would happen at all. Maybe 1 time out of 20 the starter would engage if that. Troubleshooting with the multi meter showed input voltage to the solenoid from the battery and voltage from the switch signal to the solenoid. I wasn't getting anything out of the solenoid to the starter though.

My dad replaced the solenoid this weekend and the starter engages every time now, but does not have enough power to turn the wheel over enough to crank the motor ....

Reading different forums it could be that the starter now needs rebuilt/replaced, there could be other mechanical failures binding the motor, or still an electrical problem not delivering enough current to the starter?

Next troubleshooting steps:

1. Remove spark plugs and turn wheel by hand. If it's still hard to turn, then there a mechanical issue binding the motor. How hard/easy should this turn with plugs in/out?
2. Bypass the solenoid with jumper cables and directly send current to the starter to see if it behaves differently.
3. How hard is it to rebuild a starter vs. buying a new one? (Context for skill level: I'm a SW Engineer with some EE classes under my belt back in the day; Worked on vehicles enough to be dangerous Chilton manuals and youtube videos.)

Thoughts and help please

Check the cables first. A corroded ground cable will act the same way. And not just the ends, I've seen them corrode in the middle before.

I guess if it was mine I'd put the jumper cables from a good hot battery on for just a second. If starter turns engine you know you have an electrical problem. If it doesn't you will have to decide if its the starter or the engine. You can take the start off and test it. I have to spray a little white lithium grease on the bendex about every six months just for peace of mind.

Mine did that, but it was a bad (corrosion) connection at the back of the battery switch; the connection at the battery itself looked perfect.

Listen to the experts above.
If it turns out to be a starter, just know that many Mercury motors can use different year model starters. My 2004 90 hp went out about 18 months after I bought the boat. I believe the one I found at a scrap yard was from the early 90's. It is still on there today and original was rebuilt and stays in tool box.

So the saga continued today.

For those speaking about checking the cables for corrosion. How did you determine there was corrosion in the middle of a cable without remove the insulation? Assuming multimeter on each end to measure resistance.

Removed the bendix gear, cleaned, and lubricated with white lithium grease. When the engine is cranked, the bendix gear raises as it should, turns the main fly wheel very shortly (Maybe 2 seconds). Then bendix and flywheel both stop with the bendix staying raised. Once the key is turned off the bendix drops.

I can turn the flywheel by hand even with plugs in. With plugs removed there is a cycle of fairly easy spin and then it's hard to turn like the plugs are in.

Pictures of starter internals.

Can't check resistance with a multimeter because one little good piece of wire will show continuity. Won't tell you anything. I'd be watching the voltage on the starter while it is cranking. I don't really see anything that bad with the starter. I'm not sure why the flywheel isn't spinning free with the plugs out. That sounds strange.

It didn’t look too bad to me either. It was $15 for a brish and spring kit, so I’m just going to clean and replace those while it’s off.

Been researching and watching videos. Need to find a second voltmeter and do a voltage drop test to check the cables.

I thought it should free spin as well. I could spin with one hand for a partial revolution and then really had to pull with two hands to complete.

why do you need a second voltmeter. Just put the leads on the starter and see what the voltage is when it quits. It ought to be whatever your battery voltage is before you try to crank. It may drop to 10.5 while cranking. I'd just want to know if it drops below 8 or 9 or so. Try it while its off and see how it works.

A lot of the videos I've watched have a voltmeter on the battery and a second voltmeter on the starter or solenoid to look at the displays side by side. You could use one and switch the leads, but you're assuming a semi-consistent reading of voltage with crank cycle.

It's about an hour to get to my Dad's house to troubleshoot, so asking questions before heading back this weekend.

Solenoid is a 4 post with the B+, S (Solenoid to Starter), G, and I (Input from Ignition Switch) hookups. B+ is reading 12 volts as input at solenoid. B+ to the ignition switch is also connected to this post on the solenoid with 12v as the switch. (Both non cranking reading). What should the voltage be coming out of the "I" post on the ignition switch and therefore into the solenoid? Assuming 12v. If I remember correctly it's 7.5 volts.

I'd never heard about voltage drop tests while cranking until doing this research, so this could all stem back to the battery while cranking. The only reading I took was 12v non cranking. I'll be performing voltage drop tests this weekend as a new starting point.

I should be 12v. Its just voltage from the ignition switch. If bendix is jumping out I'd think that is ok. Did you test the starter while it is off? If the starter gets in a bind the voltage will drop but all that heat is going somewhere. So don't leave it engaged long. You could run jumper cables to the starter while its off and see what happens. But most of the time when I get into stuff like this when I take it all apart and don't find any thing actually wrong when I put it back together it works fine! Where is the boat actually? I can PM you my number and walk you through it if you need help. Or if close, come help.