Better understanding the Mercruiser 470 starter setup

[Oshawapilot]

With the ignition system issues on the problem child engine finally (hopefully) coming to a close (see my other threads), I'm starting to turn my eye towards the start system.

The engine has always been a bit slow while cranking...and while looking at the slave start solenoid yesterday I noticed that it is getting old and crusty. Yes, the main cables coming from the battery are not in A1 condition either, so they will get some attention as well.

So, I have a basically new unused starter relay in my garage that I was contemplating brining up to swap it out with just to rule out the old and crusty solenoid as a possible reason behind the cranking issues..however, when I started reading things more here I soon realized that the starter solenoid on the 470's is actually on the starter itself. Not sure how I missed that during visual inspection, but anyhow.

My question is WHY is there two start solenoids - the slave, and then the actual main solenoid on the starter itself?

Can the slave be eliminated removing a possible point of failure/issues, and just wire the ignition switch trigger wire directly to the starter solenoid like most vehicles have it?

In short, I'm not fully grasping what the separate slave solenoid does aside from provide a full 12V source of power to the coil during cranking, but that would be very simple to route from elsewhere during starting, no?

 

[Scott Danforth]

clean the battery terminals at the battery and at the starter and the block. being between 31 and 40 years old, you may need to replace your cables.

the pilot relay is because the purple ignition wire is making a 25' run or longer and wouldnt have enough voltage to trigger the starter solenoid. All I/O boats use a pilot relay. every single one. your issue is most likely bad connections, bad cables, bad battery or bad starter in that order.

 

[Oshawapilot]

the pilot relay is because the purple ignition wire is making a 25' run or longer and wouldnt have enough voltage to trigger the starter solenoid..

I'm not doubting your knowledge, just wondering the logic since they're both relays.

So one relay is triggering another relay, basically.

 

[Scott Danforth]

As stated prior, voltage drop.

Also as stated prior, clean your contacts

 

[achris]

The solenoid on the starter requires about 8 amps to pull. Running that sort of current through the loom and the key switch causes them to heat up, quite quickly. The other relay draws less than an amp.

 

[stonyloam]

Scott is right, the slave solenoid provides full voltage and amperage to the starter, it is best to keep it. It is a feature on (as far as I know) all mercruiser engines. But the story is a little more complex with the 470 (seems like it always is). the Original 470 engine used a starter with no built in solenoid so it was driven by (what is now) the slave alone. I guess that did not work out too well since the changed to a conventional type starter with a mounted solenoid. That is why the 470 slave sends 12 volts directly to the coil during starting, while on other Mercruiser engines 12 volts is sent from the starter solenoid to the coil. Now on to the slow turning. The original starters had a tendency for the armature to become bent and rub against the field coils slowing it down. If cleaning the terminals doesn’t help try a PMGR starter, they work great, I have been using one for years. About 2/3 the size and way more power.