This article describes how to use a simple Touch-type Ammeter to measure charging or cranking amps in cars, trucks, boats, motorcycles, tractors, etc.

Sears L-clamp or "trough" type DC Ammeter

On 2021-05-28 by ron fancher

I found an old amp gauge on a Ford tractor i am restoring, the gauge has no wires attached it just has a loop on the back that the charging wire passes through.

I would like to know more about this design if possible, it works fine and i took it off and put it on another vehicle and it worked fine there too. thanks ron

This Q&A were posted originally

at OLD ELECTRICAL WIRING TYPES

On 2021-05-28 by (mod) - Procedure for using a One-Wire Sears DC Ammeter to Measure Starter or Charger Current

@ron fancher,

Sure, that amp meter about which you asked works by responding to the electrical field of generated by 60 Cycle current running through a wire. You'll see that same method used in clamp-on amp meters they're in the hands of most electricians.

Instruction for using the Sears DC Ammeter P/N 171.2176 to measure Charging Current (+/- 0-75 DCA) or starter current (+/- 0 - 400 DCA)

The ammeter I show here was sold by Sears until probably sometime in the 1980s. My meter was made in April 1982. It's operation is easy and simple.

The meter is simply held with the larger or smaller L-shaped trough on the back so that the automobile's DC cable to the starter (for starting current) or to the battery (for charging current) runs in the trough.

To measure starting current or Amps the meter is held in place where you can read it and your accomplice cranks the starter to start the engine while you red the current draw on the lower black gauge that measures +/- 0 to 4 - multiply that number x 100 to obtain starting or cranking amps.

To measure charging current the Ammeter is held in place, again where you can read it, along a battery cable to the generator or alternator through the voltage regulator to the battery while the engine is running and you can read charging Amps.

Watch out: the most important most-dangerous hazard to avoid is getting your necktie or clothing caught in spinning motor parts or to get your arm or hand chopped by moving engine parts. Be sure that you and your clothing and any other tools or implements nearby are kept clear of moving parts.

Similar devices have been used for many decades to measure DC amps typically when checking the current drawn by an automotive starter motor. These photos show how to use this simple "clamp-on" Sears DC AMP meter that I've had for 40 years.

...

Above I'm measuring charging current by running the negative battery cable under the smaller L-shaped "clamp" on the back of the DC ammeter.

The black wire shown by my arrows is of course connected to the negative battery terminal and to the Jeep's charging system.

The engine is running and our vingage Sears DC Amperes gauge (above) shows a charging current or amps of just under 24 DCA .

...

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