Monday, May 14, 2018

Runaway Starter

Starter motors are not designed for continuous service - they are not provided with any cooling capabilities.  This is because the design expectation is that they will see only intermittent service.  They are high-current, high-power devices however, especially the ones that are expected to crank diesel high-compression engines.  Now hold that thought...

A recent discussion on The Retirement Project bears review here.  It seems that when TJ went to start his engine, the engine start push button stuck, leaving the starter motor running continuously after the diesel had started.  Because of the lack of cooling, a runaway starter motor is a serious problem - aside from destroying the starter, a fire could result.

I recall an incident where a young woman pulled into a gas station where I was refueling my car, and proceeded to fuel hers.  Her starter motor was running, after she shut down her engine (presumably it had been running since she started her car...).  Running, but running poorly - the heat buildup had caused the armature to swell and it was dragging on the field poles, creating more demand for electricity and even more heat.  I opened her hood and found the battery lead to be red hot, smoking, with all the insulation burned off.  Remember, this was at a gas station, where this car (and mine!) were actively taking on fuel.  I shut off the fuel feeds to both cars and then used a tire iron to break the red-hot wire (easy - copper is soft when it is red hot).  The cause?  Welded contacts in the starter solenoid.

One time when driving (I was 17 at the time) my father's 1959 Oldsmobile, the same thing happened to me.  Again, the tool of choice was a tire iron, and I used it to try to pry the battery connection off of the battery.  And failed.  Instead, the post and part of the battery plates came out of the top of the battery, complete with plenty of sparks and acid.  In retrospect, it is lucky I didn't get to experience a hydrogen explosion.  Again:  the cause was welded starter solenoid contacts.

I guess it is not surprising that this happens - these contacts carry prodigious current - 75 - 100 amps in a car engine and more for a diesel, and are connected to a very inductive load.  When they are asked to open, the magnetic field in the starter collapses, boosting the voltage at the contacts, keeping the current flowing for an instant even tho the contacts are open: an arc occurs.  Most of the time, the contacts continue to open, extinguishing the arc.  But if the contacts are already damaged from arcing, the arc gets a head start because the contacts are already hot...

In TJ's case above, the cause was not welded contacts in the starter solenoid, but rather a stuck starter button.  But in my experience, this is much rarer than welded contacts in the starter solenoid.  Regardless of the cause however, the remedy to a runaway starter is the same:  Disconnect the battery.

Easier said than done.

All cars, and almost all boats have a hard-wired connection between the battery and the starter.  In boats, the usual case is that that "Off-1-Both-2" battery switch is only carrying the house loads - the starter is hard-wired.

I believe that this is a pretty serious safety hazard.

Blue Seas M-Series Mini Selector Battery Switch
Those big battery switches can easily carry the starter current load.  Even the smallest ones have tremendous current carrying capabilities. Here's a mini Blue Seas one:
  • Cranking Rating: 10 sec. 1,500 Amps
  • Intermittent Rating: 5 min. 500 Amps
  • Continuous Rating: 300 Amps
For comparison, cranking Eolian's starter (Perkins 4-236 diesel) draws 200 amps - I know this because everything on Eolian goes thru the shunt for our Link2000 monitor. And everything also goes thru the battery switch on the power panel. Including the starter.  I can't take credit for this - Downeast (Eolian's manufacturer) built her this way.

If the starter on your boat does not go thru a battery switch (or THE battery switch), I'd sure try to find a way to make it so...



Jason said...

Another fun read. Thanks Bob. Feeling thankful that I'm all good on Madrisa in this department.

Robert Salnick said...

Thanks Jason!

Rick Bailey said...

Good info, Bob. I didn't realize many boats weren't set up like mine. Having the starter circuit bypass the battery switch seems like such an obviously bad idea. And obviously as well, no fuse in the circuit!?

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