DeTomaso Mailing List: June 2000, Message #116

Excellent reply, Bill. This maintenence is something we should all do
--   I still get teased by Carbabe about all of the Pantera fires.
  Al.

SOBill@aol.com wrote:
> 
> Tom,
> 
> Here is how Ampmeters work.  The meter movement itself will deflect to full
> scale with (typically) 0.050 volts across it's terminals.  In order to
> measure current with such a meter. a very low resistance is connected across
> the meter terminals.  The size of the resistor is selected such that with the
> maximum design current flowing, 0.050 volts will be generated across the
> resistor.  For a 75A full scale reading and a 0.050 volt meter movement, a
> resistor of 0.0006666 ohms is required.  This resistor is basically a piece
> of carefully trimmed metal called a 'shunt'.  The shunt is packaged inside
> the case of most auto ampmeters OR appears as a short jumper from one meter
> terminal to the other.
> 
> If the ampmeter shunt is good, when 75Amps are flowing, the shunt will
> disipated ~3.75 watts.  This will not get hot.  If the shunt were to change
> value to 0.006666 ohms (i.e. 10x), the meter needle will be totally pegged
> (or the meter will be blown out and likely read zero with no movement) and
> the shunt will disipate ~37.5W....getting hot now.
> 
> So.  The first test is does the ampmeter MOVE more or less normally?  If yes,
> the shunt and meter are probably O.K.  If the needle is pegged, the shunt is
> likely bad.  If the needle reads Zero all the time, the shut is probably bad
> and the meter movement is burned open.  The Ampmeter will have to be
> replaced.  You can eliminate the Ampmeter from the problem (and the system)
> completely by installing a heavy, solid copper wire between the terminals.
> The car will operate normally except you will have no Ampmeter readings.
> 
> If the meter moves more or less normally (i.e. shows charging and discharging
> with engine speed and is in it's normal range) the problem is most likely
> corrosion on the wires and terminals at the rear of the Ampmeter.  All the
> current in the system (except the starter) flows thru those connections.
> 
> So.  Disconnect the battery.  Pull the dash forward.  Remove the wires from
> ONE ampmeter terminal.  Wire brush all the wire terminals and the meter
> terminal until they are clean metal.  Reconnect all the wires.  Repeat for
> the other terminal.
> 
> The ampmeter is good and the terminals are all clean and tight, there is no
> source of resistance to cause overheating.  But, 0.16 ohms of resistance in a
> 12 Volt circuit will flow 75Amps and dissipate 900W. Things will get HOT in a
> hurry.  So pay real attention to clean, tight connections at the Ampmeter.
> 
> Sorry to be so long winded, but you easily replace the Ampmeter, clean the
> connections in the process and never know that the connections were the
> problem to begin with.
> 
> SOB