Greg Mayman wrote in a message to Roy J. Tellason:

 -=> Roy J. Tellason said to Greg Mayman
 -=> about "VEHICLE LED'S" on 09-22-03  04:06.....

 RJT> Last one I remember seeing (taken out of a vehicle -- it was
 RJT> defective) was maybe 4 inches square,  3 inches high,  pretty big for
 RJT> just a couple of diodes being on there.

 GM> Yes, I remember they were big heat sinks.

 RJT> Yes.  The regulator I'm familiar with (Mopar stuff) is grounded,  and
 RJT> only has two other terminals,  one of which goes to the field winding. 

 GM> Some of the older alternators had their fields powered from the
 GM> switched ignition circuit. That way the regulator was looking more
 GM> or less directly at the primary battery, AFTER any isolator diode
 GM> that was charging it from the alternator. So the alternator output
 GM> would be automatically adjusted for the diode drop.

I've thought that this might be the case,  but wasn't sure.  And although I
do have some wiring diagrams,  none of them were handy.

 GM> Unfortunately it doesn't seem to be done that way these days.

I don't know.  The same regulator is being used in my truck (where I'd
probably be applying this) as was used in a number of cars that I've had, 
going back a ways.  Not sure how either of the cars do it,  though.

 RJT> I forget if the other end is grounded or tied to ignition,  and where
 RJT> the other wire from the regulator goes,  but it's pretty simple, 
 RJT> really.  Except for if you get a bad ground,  which I had happen to me
 RJT> one time and which boiled out the battery in my truck,  before I caught
 RJT> it and cleaned things up. 

 GM> The regulator in a car my son owned a couple of years back was 
 GM> stuck on full charge and destroyed one battery before he took my 
 GM> advice and did something about it. He left it so long because "an
 GM> auto electrician might want to charge me $100 to fix it."

Heh.

 GM> I checked the regulator and found a relay with normally closed 
 GM> contacts that connected directly between the ignition circuit and
 GM> the alternator field. I assume the relay was supposed to operate as
 GM> soon as the alternator was putting out enough juice to power the
 GM> field itself and cut out the direct feed.

 GM> But for some reason it wasn't operating.

Before the vehicles I had that had this solid-state stuff I had one that
used a single-coil relay.  It was apparently supposed to operate at a
fairly high rate,  between two sets of contacts or in between.  The damn
thing let my battery keep on running down until I bent the tab that held
the spring for that contact a bit,  forcing the issue somewhat.  I can also
remember earlier stuff yet,  going back to the sixties or thereabouts, 
that had three coils in there.

 GM> I couldn't do anything with the regulator as the PCB part of it 
 GM> was potted and inaccessible, although all the parts were outside 
 GM> the potting compound -- just enough to tease me into wanting to 
 GM> trace the fault and fix it!

I hate it when they do that!  Reminds me of the Ford stuff we had with
their ignition modules...

 GM> OTOH, a replacement from the junk yard cost him only $5. It had 
 GM> already cost him about $25 to replace the battery from the same
 GM> yard!

Heh.

 RJT> The other terminal on the alternator is the output,  and that's where
 RJT> you'd connect the isolator.  #10 wire,  as I recall.  So you'd need to

 GM> I'm not sure about that #10 wire as it is only 0.008 squ.inches. At
 GM> 40A out of the alternator, the current density would be about 5000
 GM> A/squ.inch.

I don't think it's much heavier than that,  though I could find out when I
get my hands on those wiring diagrams.  And it's not like it's carrying
that current continuously.  The ammeter in the truck is actually marked
with a "40" at each end,  and it's *very* seldom I've ever seen
it go the whole way over,  and then only for a fairly short period of time.

 GM> #6 wire is 0.02 squ.inches. At 40A the current density would be
 GM> 2000 A/squ.inch, a better figure considering the the elevated
 GM> temperatures it would get near the engine.

 GM> And if the second battery was any great distance from the 
 GM> alternator, the heavier wire would reduce the voltage drop.

Yeah,  it depends on where I end up putting that second battery.  If I
figure that charging current is probably going to be the highest,  then it
makes sense to put it in the engine compartment.  Loads are probably going
to be minor RV stuff,  lights and such,  but most likely not as much as the
charging current will be.

Maybe if I'm lucky I can find a diesel truck in the junkyard and get the
second battery tray and such from that...

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