DeTomaso Mailing List: June 2000, Message #106

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