Cooked engine loom question, Looking for cause.

[Barking Mat]

Went out on a breakdown this afternoon, cooked engine loom, well specifically excitation wire to starter motor from dashboard. Symptoms, boat won't start.

Possible causes:

Ignition switch failure causing constant excitation of starter ?

Relay failure, causing constant excitation of starter ?

Or starter motor, stuck on causing constant current drawer on the excitation circuit ?

Anyone ever had this, I'll have to replace some if not all the cables in the loom tomorrow, the ignition barrel, relay and starter, I think it's the ignition barrel, but not 100% sure.

I'd be interested in your thoughts.

Cheers people.

 

[premmington]

Dead short to earth for starter solenoid fly wire.

Or in a marine application - wire not thick enuf when circuit was fitted....

 

[arther dailey]

can a fuse be fitted to that wire , or indeed a timer of some sort only activated when switch is turned for allowing power for xxx seconds for starter exitation?

 

[Barking Mat]

Dead short to earth for starter solenoid fly wire.

Or in a marine application - wire not thick enuf when circuit was fitted....

Yes, a strong possibility.

The wire was quite small, will get heavier section tomorrow.

can a fuse be fitted to that wire , or indeed a timer of some sort only activated when switch is turned for allowing power for xxx seconds for starter exitation?

Fuse, is a damn good idea, electric timer is just another thing to go wrong, but equally a good idea.



All good thoughts, and appreciated folks.

 

[sako243]

Shouldn't need a timer if you size the wire and fuse correctly. The fuse is there to protect the wire and the wire should be sufficiently rated for it's load.

So if you need a 10A current for the load then you could fit say a cable rated for 15A continuous and fuse it at 13A. That way you have a little headroom on the load not blowing the fuse and then the fuse should blow before the wire gets too hot.

 

[Arc Tourist]

Has an instrument gage been changed for one that may not be compatible ?

 

[premmington]

Shouldn't need a timer if you size the wire and fuse correctly. The fuse is there to protect the wire and the wire should be sufficiently rated for it's load.

So if you need a 10A current for the load then you could fit say a cable rated for 15A continuous and fuse it at 13A. That way you have a little headroom on the load not blowing the fuse and then the fuse should blow before the wire gets too hot.

Yes - yes - yes...

Fuse is there to protect the wire - wire size is there to take the current.

But at 12/24v - nobody ever gets the wire cross section right for marine applications - don't know why - but they just don't...

 

[Barking Mat]

Anyone any idea over what current they pull, 1.5 to 2 litre 4 pot diesel engine, same as your average car.

 

[premmington]

Anyone any idea over what current they pull, 1.5 to 2 litre 4 pot diesel engine, same as your average car.

yes...

 

[Barking Mat]

Has an instrument gage been changed for one that may not be compatible ?

No. It's original, the same for same ignition barrel was changed though.

Yes - yes - yes...

Fuse is there to protect the wire - wire size is there to take the current.

But at 12/24v - nobody ever gets the wire cross section right for marine applications - don't know why - but they just don't...

The engine install is original, 12v engine loom.

 

[Barking Mat]

yes...

Give us a clue, kind sir.

Please.

 

[8ob]

I'd be interested in your thoughts.

Cheers people.

SWMBO has told me I have used up my quota of thoughts for this month and I am not allowed any more until October. I am allowed an idea though, make up/bodge a small engine loom together and put the starter and ignition circuit through its paces.

Bob

 

[Barking Mat]

More likely shorted turns in the solenoid coil or possibly continued operation of the starter solenoid whilst jammed.

Bendix or pre-engaged starter? If it had lasted years until now, it is unlikely that the wiring gauge was
at fault all that time.

More complicated than that, always, occasionally had starting issues with this boat, especially when engine is hot, and been turned off for a brief time.

 

[Red'n'Black]

Anyone any idea over what current they pull, 1.5 to 2 litre 4 pot diesel engine, same as your average car.

Last one I measured (2 litre petrol) the starter solenoid drew about 3 amps.

 

[Barking Mat]

I'll measure the cable size, and get back to you tomorrow.

 

[pedrobedro]

Went out on a breakdown this afternoon, cooked engine loom, well specifically excitation wire to starter motor from dashboard.

That wire should only carry current while the key is in the start position, does the engine take a long time to fire ?

 

[Barking Mat]

That wire should only carry current while the key is in the start position, does the engine take a long time to fire ?

No, barely needs pre-heat, not like it's cranking for ages.

New starter motor this year, new battery, 50mm2 cables.

But the excitation cable, barrel ..... Perhaps relay...

 

[Ubique]

I'd be checking the volt drop across the switch contacts (under load), it wouldn't be the first time low voltage has caused a high current to flow and cause insulation to melt - of course, the other option is a fracture in the insulation allowing moisture in to cause corrosion, increasing the resistance of the wire and therefore current draw.

 

[Barking Mat]

I'd be checking the volt drop across the switch contacts (under load), it wouldn't be the first time low voltage has caused a high current to flow and cause insulation to melt - of course, the other option is a fracture in the insulation allowing moisture in to cause corrosion, increasing the resistance of the wire and therefore current draw.

Switch contacts are currently prime suspect, I'm happy to rule out second scenario, bilges and cable runs are dry and correct.

 

[Ubique]

Also bear in mind how starter motor solenoids work, there are actually two sets of windings - pull-in & hold-in. Pull-in takes the most current and has a larger coil, physically & electrically. It is wired from term. 50 to the output stud (so that it has zero current draw once the contacts are closed). the hold-in coil is wired from term. 50 to ground, and will obviously be drawing a lesser current whilst the starter is engaged.

It's not beyond the bounds of possibility that loose or corroded terminals on the solenoid cap battery & motor contacts (or indeed inside the solenoid cap) can cause volt drop and cause the pull-in coil to still be active albeit at 1-2VDC whilst cranking, but passing a much higher current than intended. Typically, this will show itself as a warm/hot starter solenoid after a few cranking attempts.