[Fot] Single wire alternator

[Michael Porter]

Bill Babcock wrote:
> I can't tell you how many times I've seen someone turn off their kill switch
> and nothing happens. I suspect that's one of the most common race car wiring
> errors. By the way, your solution isn't a great one--you'll have battery
> voltage to your alternator all the time. What you really need is a double pole
> kill switch with the alternator wire connecting to the smaller switch and then
> to either the batter or engine side.
>
>
>   

My story about this is more commercial than hobby.  When Detroit Diesel 
first started to put electronic controls on their commercial engines, 
their engine computers ran on 12V and the injector drivers on 24V.  So, 
when the 12V kill switch was turned off, the engine quit.  Then, with 
the Series 50, they converted to their DDEC III computer, which ran 
entirely on 24V.  Now, no one thought about it, because there was a 24V 
kill switch, too.  Shut them both off, the engine should shut down in an 
emergency. 

But, they didn't.  The MBTA in Boston went nuts on a new order of buses, 
because they'd trained all the emergency personnel, fire, cops, 
ambulance, their own people, to shut down the engines in an emergency 
with the kill switches.  It was a simple circuit problem--because of the 
way the harnesses were wired--with separate 12V and 24V circuits and 
regulators, the 24V side formed a loop with the alternator charging 
circuit which kept the computer voltage at 24V.  Hence, nothing stopped 
when the 24V switch was thrown to off.  When I heard my boss explaining 
to the MBTA on the phone about how our engineers said it would take 
$600/bus and a year to design a fix, I went to the catalogs and found a 
Cole-Hersee switch with a field shut-off for about four bucks more than 
we were paying for the 12V switch from them we were using, and designed 
a jumper harness to the 24V reg circuit that cost us $1.80 to make. 

That Cole-Hersee switch is probably still in the catalogs (ten years 
ago, it was about $40 in bulk) and was made for marine use 
(silver-plated contacts).  That's the way to fix any problem with 
persistent running.  Shut off the battery positively, shut off the field 
and/or alternator output positively (in the case of single-wire units) 
and the electrons stop, too.


Cheers.




-- 


Michael Porter
Roswell, NM


Never let anyone drive you crazy when you know it's within walking distance....