Re: Key--off current draw from 12SI alternator

In article
<062d91c5-b347-42bb-84d2-0a92011c2990@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Ed Treijs <ed.toronto@xxxxxxxxx> wrote:

On Jan 11, 6:51 pm, aarcuda69062 <nonel...@xxxxxxxxxxxxx> wrote:

Alas, the advice was a little late,

Sorry, I was replacing stepper motors in a 2003 Silverado instrument
cluster. (gotta eat)

as I decided to "do something" and
the project just grew. But I do have more questions now.

3) I have an overhauled/rebuilt 10SI with unknown components in it.
Can the diode trio and rectifier bridge be swapped between a fairly
high amperage 10SI and my 12SI?

Doubt it.  There are many different sizes and amp ratings of rectifier
bridges.

I took both the 12SI and the 10SI apart. Once I got going, I decided
to treat it as an educational experience. If I break something, I will
learn from that. If I put something together that works, I learn from
that too.

I got puzzling readings from the 12SI bridge: it seemed to have no
conductivity either direction. I'd put it down to error or tool misuse
(I'm using a digital multimeter that has a diode check setting) but
the 10SI bridge checked out as I'd expect it (conductivity in one
direction). Additionally, the 10SI diode trio checked out fine, but
the 12SI trio again seemed to have no continuity either way!

That alternator shouldn't have worked at all if the entire rectifier
bridge was open.

You do realize that the diode test function on your DVM tests bias
voltage correct? You should be reading something like .7 at one
polarity and 0.0 when the test lead polarity is reversed.

The big old honkin Fox valley meters that were sold 30 years ago were
great for testing alternator internals, the ohmeter was powered by nine
volts which was more than enough to bias the PN junction to test the
diodes. I think the old Simpson 'tombstone" meters did as well.

Anyway, I didn't understand how the rectifier bridge was assembled,
and in learning how it's assembled I destroyed the 12SI bridge. The
10SI bridge fits, although the heat sink cooling fins aren't ideally
shaped for the 12SI end frame vents.

They don't come apart, I used to bend the straps up from the studs to
totally isolate each individual diode from the rest of the assembly.
IIRC, I got bit in the ass by a bridge frame that was shorted between
positive and negative sides, time to re-invent the test.

I presume that the SI-series rectifier bridges come preassembled on
the heat sinks, because the diodes appear to be bonded to the sinks (I
broke most of the diodes from the 12SI heat sink).

Yes, that is how they come.

IOWs, you either have a short that is feeding voltage to the voltage
regulator (alternator) or you have a problem inside the alternator which
is feeding voltage back out to the ignition switch and (according to the
wiring diagram), the heater blower switch.

Aside: contrary to the 1978 factory service manual, my car does have a
blower off position. My '79 Firebird did indeed have a blower that ran
all the time, but my non-A/C '78 definitely has an off position.

Obviously, I should check if there's unwanted voltage on the tan/black
wire when the key is off. Would be annoying if I did all this teardown
stuff when it really was a wiring problem!

Yup.

I put the 12SI back together with the 10SI bridge and diode trio. The
10SI did not have a resistor between the +12 brush connector and
ground--this is labelled as "Resistor (some models)" in my factory
manual, probably R5 in the circuit diagram GM provides. The 12SI does
have this resistor: it's about 48 ohms. I wonder why "some models"--
what causes it to be or not to be present? I might guess that the
resistor's presence is matched either with the regulator or the diode
trio, but which? Should I keep it?

That resistor was needed on some applications with a warning light, its
purpose was to stop the warning light from glowing dimly at idle speeds.
With a no warning light application it wasn't needed, its presence or
lack there-of makes absolutely no difference in a vehicle with gauges
and no light.

Anyway, if the alternator works at all, I will have to get a proper
rectifier bridge, and also get the bearings replaced. 12SI stator is
noticeably heftier than 10SI stator; couldn't see significant
differences in the rotor.

Stator is bigger because the output rating is higher. At low amperage,
the 10SI bridge will probably work okay, at higher amperage it's gonna
smoke.

So to summarize:
Didn't check tan (field) wire--need to check
12SI bridge and diode trio check out open; bridge is wrecked anyway
10SI bridge and diode trio check out fine--in use
12SI had resistor, and I kept it
Haven't reinstalled alternator to test
Certainly have learned a whole bunch about alternator innards, for
what use it is

Check the tan field wire for voltage with the key off BEFORE you
re-assemble with the new rectifier bridge, if there is no voltage there,
I'd recommend replacing the voltage regulator unless you don't mind
taking it all apart again.
.

Relevant Pages

  • Re: Key--off current draw from 12SI alternator
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  • Re: Key--off current draw from 12SI alternator
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