Strange Voltage behaviour on Starter

[Delfini]

I have a 24v domestic set of batteries which get charged from shore power or alternator and a small'ish 12v engine battery which only gets a charge when the engine is running via its separate alternator - since I took ownership of the boat last year the starter has been temperamental - sometimes it goes - sometimes it doesn’t - I have a remote starter button in the cockpit which takes its 12v power from the battery via the starter +ive terminal through various thermal and switching circuit breakers and eventually drives a relay in the engine bay which fires the solenoid - its been working OK for 3 months or so after I rewired the engine control box and replaced the starter motor and solenoid late last year - last week after doing a bit more work on the engine accessories and having changed the oil and filters etc it packed up again and given I need to move the boat this weekend I resorted to wire a starter button directly across the solenoid and postpone further investigation in the long and torturous cabling runs to and from the cockpit - the button worked OK

On Monday the engine didn’t start and after significant cursing I found myself measuring the terminal voltage across the starter - it was 6V - after a bit of mucking about with various switches (including the main isolator) and various pulling and pushing of wires it came back up to 12v and the engine started

Today I went to the boat to check all was well and the again engine didn’t show any signs of starting - I measured the starter terminal voltage and it was 1.5v - the voltage monitor on the chart table showed 2v - I checked the terminal voltage on the main 12v battery and it was 12.8V - crazy - I then mucked about with various switches and relay connections and the solenoid wiring and watched the voltage as it rose to just over 12v - again the engine started

I have persuaded myself there is something wrong with the main double pole isolating switch (which I have recently started to use when I leave the boat) since thats the only thing between the battery and the starter motor but I cant really believe any dodgy contacts or high impedance in the switching unit would result in the low voltage measurements I am seeing at the starter motor

Does anyone have experience - is it possible that the heavy duty (250A) dual pole switch could be giving me the run around - or not ?

Thanks for your feedback

 

[lw395]

Yes, dirty or burned switches can do that.
As can any other dodgy connection.

 

[VicS]

I have

I have persuaded myself there is something wrong with the main double pole isolating switch (which I have recently started to use when I leave the boat) since thats the only thing between the battery and the starter motor but I cant really believe any dodgy contacts or high impedance in the switching unit would result in the low voltage measurements I am seeing at the starter motor

Does anyone have experience - is it possible that the heavy duty (250A) dual pole switch could be giving me the run around - or not ?

Thanks for your feedback

If you suspect the isolator switch you could try temporarily bypassing it or to scientifically test it clip a voltmeter across it while you try to start the engine. Ideally the reading across the closed ( ON ) isolator will be zero but in practice you will almost certainly read a small fraction of a volt. If the switch is faulty you will see a goodly part of 12 volts and the engine will not crank.

Check other connections and sections of the battery to starter wiring in the same way. Don't forget the negative . Might also be the solenoid ... which check with the voltmeter across the main terminals

 

[Graham376]

Agree with the above suggestions but query the "through various thermal and switching circuit breakers and eventually drives a relay in the engine bay which fires the solenoid". Sounds over complicated with several points for potential failure.

 

[Refueler]

Agree with the above suggestions but query the "through various thermal and switching circuit breakers and eventually drives a relay in the engine bay which fires the solenoid". Sounds over complicated with several points for potential failure.

That was my thought as well ... but declined to post because of 'people watching what I post' !

Lets be honest - the Solenoid is in effect the relay ... why that needs a relay to engage the low power side ?

I'm a great believer that the more items in the chain - the more likely to have a failure. And trying to find the point of failure leads to this poor guys situation.

 

[RichardS]

Agree with the above suggestions but query the "through various thermal and switching circuit breakers and eventually drives a relay in the engine bay which fires the solenoid". Sounds over complicated with several points for potential failure.

I also agree with that. Surely all that's needed is a low-current starter switch, a starter solenoid with a low-current and a high-current side and a high-current starter motor. Of course, these days the solenoid and starter motor are usually combined into one unit. Keeping it simple has its attractions.

Richard

 

[vyv_cox]

My Yanmar GM30, in common with many others of the same make, was often reluctant to start in a similar way. I traced the wiring loom and found four bullet connectors along the way, two of which were wrapped and difficult to access. I ran a new, single wire between the starter switch and solenoid and have never had the problem since. About 5 years ago.

I believe that later engines have a relay to protect the switch from arcing but mine does not.

 

[Refueler]

I also agree with that. Surely all that's needed is a low-current starter switch, a starter solenoid with a low-current and a high-current side and a high-current starter motor. Of course, these days the solenoid and starter motor are usually combined into one unit. Keeping it simple has its attractions.

Richard

Which I found out myself just only these last weeks. If I had OP's set-up - I think I would still be looking at a dead starter situation.

I realise that my 'small boat' setup is most likely nothing like the serious job on OP's ... but main power switch to turn off all power to boat ... a simple toggle switch (I was going to fit an ignition key switch in its place) to allow the Press to Make starter button to energise the solenoid. Brmm Brmm .. away she goes.

But I'm sure someone will toddle along and tell me I'm wrong again ...

 

[Refueler]

My Yanmar GM30, in common with many others of the same make, was often reluctant to start in a similar way. I traced the wiring loom and found four bullet connectors along the way, two of which were wrapped and difficult to access. I ran a new, single wire between the starter switch and solenoid and have never had the problem since. About 5 years ago.

I believe that later engines have a relay to protect the switch from arcing but mine does not.

Where I need to have a split connection - I've done away with crap so called marine quality connectors and substituted far better ones from the RC world. RC world has connectors that are designed to handle KW's ... high ampage ... without breakdown.
I realise that some may poo poo the use of - but they are reliable, easy to fit to cables and above all work.
Of course best connector is no connector at all !!

 

[Graham376]

I also agree with that. Surely all that's needed is a low-current starter switch, a starter solenoid with a low-current and a high-current side and a high-current starter motor. Of course, these days the solenoid and starter motor are usually combined into one unit. Keeping it simple has its attractions.
Richard

Assuming there is is a 12v instrument panel which becomes live when the 12v battery isolator is switched on, I would take the feed to starter button from the ignition switch or, if there isn't one, to any live + on the panel and then take a wire from the other side of the button direct to starter solenoid. Absolutely no need for thermal breakers or extra solenoid in that simple circuit.