Testing a Model T Generator Armature Using a Growler

Figure 1 – Growler equipped with all three options

The generic term “Growler” describes a specialized tool used to test generator and motor armatures. Any generator or motor repair shop will have one of these tools. All Growlers have the basic “Growl” test feature and some have two additional test features depending upon the manufacturer. All three capabilities are desirable to comprehensively test an armature.

1. Basic “Growl” test function is the capability to place the armature windings adjacent to high strength magnetic field and check for shorted windings.

2. A Growler may have a set of two probes with a low wattage light bulb in series to make continuity test for low resistance shorts.

3. A Growler may have a single probe with two tips at a fixed distance apart. It can be placed on two adjoining commutator segments with the growler operating. This can be used to measure the current in a particular winding. An Ammeter will indicate the amount of current in the winding.

The “Growl” Test

We all know that by moving a loop of wire through a magnetic field a current is produced in the wire. The growler has a transformer with a set of open circuit magnetic laminations (a cradle) where you can place the armature.

See Figure 2.

A 60 Hertz alternating current power source is applied to the transformer core via an off/on switch providing an alternating magnetic field in the transformer laminations. When power is applied the laminations in the transformer vibrate with a loud humming sound; hence the name “Growler”. This magnetic field is impressed upon the armature windings when the armature is present in the cradle. In a good armature there is no closed circuit (loop) in any of the windings i.e. both ends of each winding are terminated to two adjacent commutator bars that are insulated from each other by Mica. Hence there is no current in the windings and no magnetic field from the windings is produced. In this condition when the hacksaw blade is placed parallel to the top of the laminations while rotating, the armature the blade will not vibrate. The armature must be rotated in the transformer cradle without moving the hacksaw blade to check each armature winding.

Figure 2. Growler Testing an Armature

If there is as an intra or inter winding short in the armature windings, an alternating magnetic field will be created in the laminations associated with the closed-circuit loop winding causing the hacksaw blade to vibrate rapidly and noisily.

Winding Ground Short Test

If the growler is equipped with this test capability there will be a set of two probes that are used to make continuity tests. See Figure 1 and 3. These probes have a low wattage test lamp wired in series which will light when the tips of the probes are connected to each other. By placing the Black probe on the armature shaft (ground) and the Red probe on any of the commutator segments (remember they are all connected together in series) if there is a winding short to ground the lamp will light indicating a short is present. If the lamp does not light there are no windings shorted to ground. Caution: The two probes are connected directly to 120Volt AC power at all times when the growler is plugged into a wall outlet. If the growler does not have this continuity test capability this same test can be conducted using a Volt/Ohm meter.

Figure 3 Ground Short Test Using the Growler

Winding Current Test

This test – in Figure 4 below – measures the relative current flowing in each individual armature winding. The amount of current flowing in each of the windings should be approximately the same. It is important to measure the current in each winding with the probes connected AND the armature in the same position on the electromagnetic cradle. The electromagnet will create different Ammeter readings because the magnetic strength differs based on position of the armature windings to the electromagnetic cradle. The two fixed probes are placed touching two adjacent commutator segments (one winding) with the growler turned on and the armature mounted in the electromagnetic cradle. There is an Ammeter wired in series with the probe tips. The magnetic field produced by the growler will create a current in the armature winding under test. The amount of current flowing in each the winding will be shown on the Ammeter scale.

Figure 4 Winding Current Test

The Growler is a specialized tool that all generator and motor repair shops will have. The tests described above take very little time and most repair shops will complete them for a small charge. There is one armature fault the Growler tests above will not detect. It is referred to as a “flying short” This is a condition where winding short is only present when the armature is spinning and the high centrifugal forces upon the winding creates the short. This situation is not very common fault, but does occur.

Copyright Ron Patterson 12-5-2018

Troubleshooting the Model T Ford Charging System by Ron Patterson and Bob Cascisa

This week we are blessed to have a very instructive article written by Ron Patterson and Bob Cascisa.

Over the years I have had the opportunity to help many with Model T electrical system problems. I discovered that many Model T Fords were incorrectly wired. In some cases those cars were being driven and the owner thought everything was working correctly. I quickly ceased believing anything anyone told about their car and developed some simple tests that will validate if the wiring is correct so one could rely on the dash Ammeter to provide reliable data when looking for problems.

I suggest you first read the article “Model T Ford Battery Charging System” to fully understand how the charging system operates. This article can be found

Here in the Model T Ford Fix website (modeltfordfix.com).

The basic function of the Model T Ford Charging System is to maintain the Battery at a proper level of charge to provide sufficient electrical power for the starting motor, ignition and headlamps while providing an Ammeter indication the system is working properly. The Ammeter indicates the net power in the electrical system. If it indicates a Charge, the generator is providing sufficient power to meet the electrical demands (Ignition and lights) and to keep the Battery charged. If it indicates a Discharge, the Generator is not providing enough power to carry the load and keep the Battery charged. A dead or weak Battery will result by driving the car with the Ammeter continually showing a discharge condition.

When working on Model T wiring it is important to be sure you are using a correct wiring schematic. Many commonly available wiring schematics are incorrect, particularly in the charge/discharge circuit wiring. Additionally, many reproduction dash Ammeters do not have their polarity clearly marked + or – on the terminals and as a result is commonly wired incorrectly. These wiring errors can allow the Ammeter to show only discharge current and no charge current or vice versa. I recommend you use the wiring schematic in Figure 1. This wiring schematic is electrically correct for the starter-generator equipped Model T.

Here is an easy way for anyone with limited electrical knowledge to functionally check the charging system and correct any trouble.

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Repairing a Model T Ford Front Spring

A 1916 “Wide Track” touring shows its unique clipped leaf front spring. Car belonged to Julius Neunhoffer of Kerrville, TX when photographed in 2015.

The front springs used on the Model T Ford evolved over the 18 model years. In this issue we will look at the evolution of changes and see what it takes to rebuild a worn out original for 100 more years of use.

Continue reading “Repairing a Model T Ford Front Spring”

Suspended Animation – Storing your Model T Ford

With fall weather approaching many Model T enthusiasts face the last ride of the season. If you live in an area with snowy winter weather the Model T will probably be off the road until spring time brings more favorable driving conditions. This leads us to the subject at hand which is an in depth look at the best practices for keeping the old Model T in shape for that first ride of the next driving season.

Above we see Henry Ford’s personal car in the winter of 1914, a custom built 1914 Couplet. We see snow on the ground next to the driveway – straw hat season is over. Needless to say he would have driven the car to work that day with a bowler hat!

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The Model T Ford Battery Charging System by Ron Patterson

The Model T Ford battery charging capability is a subsystem of the Ford FA Starting and Lighting System designed by Mr. Fred Allison at the Ford Motor Company electrical engineering department and introduced in late 1918 for use on Model T Ford cars and trucks.

Many people have trouble with the charging system on their Model T and need help repairing it. Here is an electrical (not physical) schematic diagram and technical description of how it works.

Above is a Model T Ford generator rebuilt by Ron Patterson equipped with a Fun Projects voltage regulator in place of the factory cutout.

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