Starter Circuit Questions

[ryastu]

I’m new to this and am trying to better understand how a starter relay fits into/ benefits a starter circuit system. I’ve done some reading but still have questions. In my layman brain, it seems like the relay does the same thing as the solenoid mounted on top of the starter; creating a circuit when a small amount of current is generated by turning the key. Why does it need to be involved, or what benefit does it have? Why can’t the wire carrying the current from the ignition be directly wired to the ignition mount terminal on the solenoid; it sounds like it some application it does connect directly to the solenoid.

Secondly, on the solenoid positive power post where the heavy gauge wire is connected to the battery, also resides a ring connector that combines two wires, a white wire going back to the relay (assume to complete the circuit), and a heavier gauge red wire that connects to the glow plug relay. How does this power to the glow plug relay fit into the system? In my mind I need heat to the glow plugs prior to engaging the engine and do so by partially turning the key prior to starting.

I should note that my engine is an older Volvo Penta engine from the 80’s.

I appreciate you guys chiming in to help me understand! Thanks, Ryan

 

[Eli27]

I dont know the answer to this and cant wait to find out but if you ask me, i think its to protect the ignition circuit from a back feed from the starter.

Instead of your whole electrical system connected to the start circuit becoming a heating unit the solenoid takes the fall.

Lets see if i know something from working on cars for 20 yrs

Eli

 

[GoneDiving]

Some solenoids still require a large input current to activate the starter. Add in a potentially long distance to the key start and a intermediate relay can reduce voltage drop and wire size.

In my case it's 12amps, 10m/30ft away from the key.

 

[Bacchus]

My thinking is similar to G D
Starter is very hi amps and you dont want to run LG cables any more than min distance. Starter solenoid contacts eliminate need to run LG batty size cable to helm and the need for hi amp switch for starting. The solenoid also engages the drive / gear and takes a moderate amp to operate. A secondary relay further reduces the amp reqd at the helm start switch.

Second point... the power post is just a common point to puck up always on hi amp 12V power so no need to run a separate pos for pre heaters. The switching (partial on) uses that pos and makes the preheat circuit through a separate ground than starter.
Same thing with many alternator wiring schemes. Alt output frequently just runs to the starter terminal and uses the starter cable for connection to the batty.

 

[Eli27]

Some solenoids still require a large input current to activate the starter. Add in a potentially long distance to the key start and a intermediate relay can reduce voltage drop and wire size.

In my case it's 12amps, 10m/30ft away from the key.

I like it.

Eli

 

[Lepke]

A couple more reasons for using a relay is smaller wire needed for the run to and from the engine. And the Starter solenoid gets full voltage saving contacts.

I've noticed over the years wire has become smaller on new engines and vessels. I suppose electrical engineers calculating the minimum wire size necessary to save copper and keeping busy to save their job.

 

[ryastu]

Thank you all, I appreciate the responses! -Ryan

 

[Bmarler]

There could obviously be a few reasons for the extra relay. Some have been identified already.
A couple more ideas might be, multiple stations for starting the engine, or multiple power sources that separate the controls from the starting duty.
Without the schematic you’d need to trace it out to see why.

 

[Larry M]

Another reason for the relay is the neutral switch on the transmission. It could not take full current in the starting circuit. This diagram is close but it shows the neutral switch in the circuit which all of our boats should (will) have.

 

[GoneDiving]

A couple more ideas might be, multiple stations for starting the engine, or multiple power sources that separate the controls from the starting duty

Thanks for that. I have been looking for way of standardising to 12vdc at the helm while keeping 24vdc start. A relay at the engine with 12v switching and 24v switched is problem solved. Thanks again.

 

[C lectric]

Pilot relays like you ask about also protect the ignition switch on the dash. The starter mounted coil/solenoid may draw a much heavier current than the ign. switch can handle so the pilot relay is used to allow the ign. sw. a reasonable life.

 

[Mischief Managed]

I don't think there are any inboard or I/O applications that don't use a starter relay in the circuit. There's just too much voltage drop from the battery to the key (or start button) and back to starter solenoid and too much current for the key or the start button. it's just physics. You never want the starter solenoid to bounce in and out, that can make a mess of the ring gear on the flywheel. The glow plug relay gets power from the solenoid post solely for convenience and short wire run.