Get any service manual, Chiltons, Motors, whatever. Follow the test
procedures. You will need a voltmeter. My Chilton's covers 89 to 91 Dodge
trucks ONLY. It's mostly copies of the factory manual and is well worth the
money. (Part no. 8166.) Got it at Wal*Mart.

Al

Hi Ed,

To answer your question, yes -- if the ammeter is open, you will get no
charging current into the battery.  However, it's likely the ammeter itself
isn't open.  Assuming you don't have a factory 100A alternator setup, I'd
start by checking the connections on the firewall where two large wires pass
thru, from the alternator and to the battery.  Even a few ohms of resistance
will severely limit the current supplied by the alternator.

At the connector assembly, check connections for the larger two wires that
pass thru there.  One comes from the alternator (12ga black), and one goes
to the battery (12ga red).  Unplug the connector on the engine bay side and
inspect the terminals -- there should be little or no oxidation and no
evidence of heating.  If the connections have produced any significant heat,
the plastic connector housing tends to melt.
http://www.madelectrical.com/electricaltech/amp-gauges.shtml shows this.

If any pins are burned, they should be replaced -- the dealer will have
replacements.  If you don't care about keeping it all original, you can
bypass the connector assembly.
http://www.madelectrical.com/electricaltech/amp-gauges2.shtml shows one
method.  FWIW, when equipped with a 100A alternator, larger wires were fed
thru the firewall using separate individual connections.

If your connections on the firewall are OK, pull the instrument cluster and
check the connections on the ammeter itself.  These are ring terminals, held
onto the ammeter with nuts.  I've seen them loosen and cause a similar
current limiting (and heating/melting) issue.

Bryan