Re: Diodes Part 2

["ROSS BLAIR" <bigbad69@xxxxxxxxx>]

There are three different types of diodes in a Motorola alternator. The windings have two diodes each, and there is an isolation diode mounted on a red plate at the back of the alternator housing. 

The winding diodes have red or black markings depending upon their polarity. One has the cathode of the diode connected to the case of the diode and the other has the anode connected to the case. I don't recall offhand which is which. These diodes are not very accessable without disassembling the alternator.

The isolation diode is there to prevent the alternator from discharging the battery. There is a low resistance path to ground from the positive battery terminal through the alternator. If the alternator output is less than battery voltage, the current would change direction and discharge the battery. Therefore, you CANNOT short out the isloation diode. The vehicle will run OK, but as soon as you shut it off, the battery will discharge very quickly, possible burning the windings in the alternator.

If you suspect the isolation diode is damaged, you can check it with an ohmeter. In one direction you should read greater than 1,000,000 ohms (1MOhm). Reverse the meter probes on the diode and you should see a resistance in the 1,000's of ohms. Be careful not to touch the metal part of the meter probes. Your body has resistance and will upset the measurement.

When I rebuilt my alternator, I replaced all the diodes, bearings, and brushes. I got all the parts from my guru at the NAPA store.

Ross Blair
69 BBO Javelin 390
Ottawa, Ont.


>You can run one of these Motorola alternators without the isolation diode
>but it is like replacing a fuse with a slug.   The diode mounted in a red
>stamped steel heat sink plate on the back side of the alternator is there to
>protect the internal diodes which are much harder to replace.  Clip your
>jumper cables on backwards and the isolation diode may go, but the circuit
>to the internal diodes should then be open and protected.
      Gary Walker

----- Original Message -----
From: "Todd Tomason" <jayscore@xxxxxxxxxxxxx>
To: <mail@xxxxxxxxxxxx>
Sent: Sunday, December 12, 2004 10:42 PM
Subject: Re: Diodes


> I've never seen these red diodes on an alternator.  I have come across
them on
> eBay and such.  What years did they use them?  Were they only used for
> certain amperage ratings?  I'm assuming we're talking about the Motorola
> alternators.
>
> Todd
>
> On Sunday 12 December 2004 22:33, Gary Walker wrote:
> >    The isolation diode is the big red one on the outside of the
alternator
> > case.   This may be the only one that is damaged.  You can change it
with
> > the alternator in place.  Inspect it by checking continuity in both
> > directions.  Current should flow in one direction only.  If it is found
to
> > be good, and the output is still not over 14 volts, then clean the
armature
> > where the brushes rub,  Check the brushes for length and free movement,
and
> > if that fails then pull the smaller diodes on the inside and check them
for
> > current flow in one direction.  At this point, go ahead and replace the
> > bearings with new sealed bearings from a local bearing house.  They are
> > very inexpensive.  Some parts houses may still have the diodes.
> >          Your AMC friend,
> >           Gary Walker
>
> >            Owner of some old AMC cars and stuff
>
>





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