Join Date: Mar 2002
Location: Austin, TX, USA
Re: Is It An Amp Gauge Or A Volt Meter?
Larry, as you are discovering there are two types of amp meters. All the ones I am familiar with on GM cars work as Jim described - they measure the tiny voltage drop accross the charging wire between the battery junction block and the horn relay. The wire acts as an external "shunt". But, as in your case, some route system voltage through the amp meter itself because the "shunt" is internal to the meter. I've only seen that type used on farm equipment and a few older cars.
Here is a good way to tell which is which. Use a volt-ohm meter starting on the highest scale (if it is an analog volt-ohm meter). Place it accross the meter terminals. If you see the meter defelect and some resistance, the meter is an external shunt type. If you see little or no deflection, set the meter to a lower scale (if it is an analog volt-ohm meter). You'll probably be reading near 0 ohms resistance between the terminals if it is an internal shunt type amp meter.
The external type meters are better IMHO because they don't lengthen the charge wire. Also, they dont' route hot power through the firewall via a heavy guage wire (fire hazzard). On the other hand, the external shunt type depend on the charge wire having a known amount of resistance. If you were to upgrade your charging system to say a CS130 alternator, you'd probably want to up-size the charge wire. As a result, the external shunt ampmeter will likely read too low, though it would still give you a visual indication of charge / discharge.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI