Beginner question about starter solenoid

[leecea]

I was trying to diagnose my intermittent starting problem (Yanmar 3GM30F). I read that voltage drop to the solenoid was a common problem.

I came to the boat with the idea that I'd measure voltage across the solenoid terminals while someone started the engine.

Step 1 was to check the voltage at rest (no key in the panel) and I don't understand the result



There are two large terminals - one goes to the starter motor and one has the large red battery wire and a small red wire. The small terminal has a black and white wire.

I expected to see 12.7V across the two large terminals and no voltage between the battery terminal and the small terminal. I expected to see voltage there only when the key was turned, because I thought that's what energized the relay.

But I see 12.7V between the battery wire and the small terminal even with the key off.

I also noticed that a previous owner seems to have wired a small extra relay into the system (which I heard is one possible solution).

Can someone help me understand what voltages I should see across different solenoid terminals when everything is off and when the key is turned? Does it even make sense to be trying this test?

Thanks.

[Jammer]

+12v is applied to the small terminal when the switch is turned to engage the starter. If the switch isn't turned the voltage on the small terminal should be at ground, so yes, you'll see around 12.7 volts between the battery + and the small terminal, or around zero volts between the battery - and the small terminal.

[leecea]

Thanks. That is what I saw. I thought I understood how this worked but now I'm confused. I'll do some more research.

[krid2000]

You seem to have a three-post solenoid there. The solenoid is grounded through its case and through wherever it's mounted, which closes the electrical control circuit & energizes the coil when you apply +12V to the small control post.

That coil then closes the main contacts. The solenoid normally has no internal connection between the small control post and any of the large load posts, i.e. it doesn't care what it switches on or off.

HTH
Dirk

[Hv2sail]

I'm no expert but if there is a relay in the starter circuit the previous owner may have been trying to compensate for a long wire run. I had this same problem on a previous boat I owned with a yanmar engine. I ended up not adding a relay but upgraded the ignition wiring with a larger gage. worked flawlessly. You could always jump start the starter solenoid to see if that part of the circuit is working. Good luck!

[leecea]

Thanks everyone.

Yes, I'd planned to run new 10AWG wire as recommended in quite a few posts and may still do that.

I agree that the PO may have solved the problem with the extra solenoid. It looks like a normal auto solenoid. I wonder if they wear out and I should start by replacing that.

I still hope I can get a baseline by measuring the voltage when the key is turned, now that I know which terminals to use. I'm hoping I get a low voltage reading and then see a better reading after I make my changes.

[Madehn]

You don’t say what the intermittent issue is but the first step is to remove ever single wire contact and clean thoroughly. This includes at the key switch and all relays and solenoid. Do the big wire connections at the solenoid as well. Be careful one of those big wires goes straight to battery positive and can carry a lot of juice if you brush the bare wire against the engine ( think welding arc type current). Next make sure battery connections are clean and tight. Finally and probably most important is make sure ground connection at both engine and battery are clean and tight. Spray all connections down with corrosionX when done. Many times this procedure will fix intermittent starting issues. Good luck

[TrentePieds]

leecea:

Let's get you over the hump once and for all by running through the function of the solenoid.

It is no more than a simple switch dimensioned internally to take very heavy current, say up to 600 amperes. No switch in your instrument panel could handle that amount of “draw”. Normally, this switch in the “Off” position.

To save you having to muck about in the engine compartment to “make” the switch (close it) so current will pass through it, we activate it with an electromagnet, normally just referred to as a “coil”. This switch and coil built together into a single casing is what is called a “solenoid”. The coil is in an electrical circuit separate entirely from the circuit that carries the heavy current for the starter motor. The circuit in the electromagnet is constructed so it only requires a WIMPY current, maybe 3 amperes, to generate a magnetic force strong enuff to overcome the spring tension of the heavy-duty switch and thereby close it.

This separate, low “ampage”, circuit gets its current from a wimpy switch in the instrument panel via the little terminal on the solenoid — in your picture the little flat terminal. From that terminal the current flows through the electromagnet to the casing of the solenoid, thence through its mounting bolts to the engine bloc and thence through the fat black wire attached to the engine block back to the negative terminal on the battery.

The switch in the instrument panel that sends current to the “coil” in the solenoid is a “normally open” switch, either a push-button type, or more commonly part of an integrated multifunction switch we often refer to as the “start switch”. You'll already know that the “start” position on such a multifunction switch is spring loaded so the moment you stop twisting it and holding it in the "start" position, it goes back to the “run” position.

VOLTAGE has nothing, or almost nothing, to do with your problem and measuring it is a waste of time and will confuse you. You can get a reading of 12.5V from a “drained” battery! If you want to know the “state of charge” (SOC) of your batterie(s), measure the gravity of their electrolyte, cell by cell, with a hydrometer you can buy at NAPA Autoparts or Bumper-to-Bumper for ten bux. If you've got sucked into installing AGM batteries you are skunked! You cannot measure the gravity of the electrolyte in such batteries, and therefore you cannot conveniently know the SOC them!

If you hear the solenoid rattle when you try to start the engine, but the engine doesn't turn over, you can be almost sure that your battery/ies is/are too “low” to do the job. Charge them using a reliable, quality charger. Sometimes you can't save your batteries. They just won't hold a charge. That happens when you've abused them by drawing them down too low, say down to 12.5V !! Then just get new ones. Lead/acid batteries are cheap and shouild last at least five years. The Alternator in your boat is VERY unlikely to have enuff oomph to recharge a drained or weak battery to the point where it will start your engine reliably.

One last thing: Unless your wire connections THROUGHOUT the circuits (including those on the fat black wire from the engine block to the battery) are clean and shiny when you assemble them, the starter motor may not turn because it cannot overcome the electrical resistance in the dirty connections to draw enuff “ampage”. It will then seem as if your batteries are low, when they are really not. So the first thing to do, if you have done it already, is clean your terminals.

Good luck! On second thought — DON'T rely on luck ;-)!

TrentePieds

[Madehn]

Great explanation of a starter solenoid.!!!