Amp Gauge vs. Volt Gauge

RickLee · #1 (**permalink**) 02-03-2002, 11:01 AM

Has Shelby American had any direct or indirect experience,knowledge, or dealings with electrical fires related to the use of an amp gauge? The new set of signature gauges I have received from you guys has the amp meter guage instead of a volt gauge. Any special instructions or thoughts?

Specialk · #2 (**permalink**) 02-13-2002, 07:46 PM

I have not heard of any problems with the amp gauge. The Autometer gauge connects the same as the Smiths gauges. Our wiring harnesses are already setup for an ammeter, so there is no problems with installing the gauge. I would assume the Kirkham harness is setup the same?

Kris

65cobra427 · #3 (**permalink**) 02-13-2002, 08:07 PM

FYI, not trying to stir up any controversy, but just for info... An amp meter is wired in series with the load whereas a volt meter is wired in parallel... So I wouldn't try to substitute a volt meter unless it was across a shunt resistor... I'm not a BSEE, but I did spend last nite in a Holiday Inn Express... LOL

Mc spotter

petek · #4 (**permalink**) 02-13-2002, 08:35 PM

If you were to put the voltmeter where the ammeter was suppose to be, the car won't run.

If you were to put the ammeter where the voltmeter was suppose to be, the car won't run ... and you'll create an electrical fire.

petek

Satanpez · #5 (**permalink**) 02-13-2002, 09:33 PM

May I ask why you would cause an electrical fire?

So does all the electricity flow through the ammeter? (I would assume so if it's wired in series to the car)

So would that require big huge cables going to the ammeter?

And are ammeters available for automobiles that don't go inline? (I know that type of ammeter exists, but I'm sure they're expensive)

-steve in nj-

petek · #6 (**permalink**) 02-13-2002, 10:04 PM

The fire would come from the dead short (ammeters are very low resistance) combined with the large gauge wiring. The battery would dump 900+ amps into the wire. A lot of heat would be generated, most likely causing a fire.

Other than ammeters for ac, I don't know of any that don't go in-line. You could use a voltmeter and monitor the voltage drop across an in-line shunt, in effect relocating the meter but not the point of measurement.

RickLee · #7 (**permalink**) 02-13-2002, 10:50 PM

So why would SAI use amp meters vs volt gauge? Is all ok if the harness is made to accept an amp gauge? If not can a shunt resistor eliminate the fire hazzard?

Specialk · #8 (**permalink**) 02-14-2002, 06:04 AM

Shelby uses an ammeter because the 1965 S/C used one. That is why I am almost 100% sure the Kirkham harness is already setup for the ammeter gauge. The wiring to the ammeter gauge is very heavy because of the current load going through it.

Kris

Quote:

Originally posted by RickLee
So why would SAI use amp meters vs volt gauge? Is all ok if the harness is made to accept an amp gauge? If not can a shunt resistor eliminate the fire hazzard?

petek · #9 (**permalink**) 02-14-2002, 08:56 AM

If the harness is already set up to use an ammeter then installing an ammeter will be fine. The wires are probably something on the order of 10 gauge (voltmeter harnesses can be as small as 30 gauge, although most of them seem to be around 18 gauge).

If you were to install a voltmeter into the ammeter harness, the voltmeter will work but the car won't run. Voltmeters are high resistance (20k ohms). You'd be dropping all of the battery voltage across the voltmeter.

You could use a milli-ohm shunt and measure the voltage drop across the shunt to determine amperage. I = V/R For example, let's say you had a 1 m ohm shunt (.001 ohm) and a millivolt meter. You place the voltmeter across the shunt and measure 10 mV. In other words, the shunt is dropping 10 mV. Calculating current, I=V/R, I = 10mV/1mohm = 10A. Note that the voltmeter is going to have to be capable of measuring in the millivolt region. (Ammeters are really millivolt meters with milliohm shunts.)

But what if you wanted to use a "normal" voltmeter with a range say from 0-20 volts. Ah, you could just increase the milliohm shunt's resistance. Let's say you take the shunt's resistance to 1 ohm. Now you get 1V measured for every amp flowing through the shunt. But there is also a big problem ... the shunt is dropping one volt for every amp. Draw 10 amps and you'll drop 10 volts across the shunt. That leaves only 3.8 volts for the rest of the car!

I hope this isn't too much nerdy information. Use an ammeter if your harness is set up for it and you'll be fine.