How to Rebuild an Old Auto Generator

by Michelle Schaefer
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The venerable generator provided the electrical power for all types of vehicles during the early years of the automobile. The generator used a spinning armature to generate DC electrical power and in practice was both rugged and reliable. With the advent of more electrical accessories like power windows, the generator could not meet the additional demands and was replaced by the more powerful alternator. Many collector cars on the road today still use a generator, and from time to time a rebuild is required. Rebuilding a generator at home is both an easy and rewarding experience.

Step 1

Disconnect the battery ground cable from the battery terminal. Remove the wire leads from the generator, making note of which terminal they were attached. Squeeze the generator belt with your hand and loosen the pulley nut with an open end wrench. Squeezing the belt prevents the pulley from turning. Remove the pulley from the generator. Remove the generator hold down bolts and lift the generator off the car.

Step 2

Remove the commutator end frame by first removing the through bolts. The through bolts are either fastened with a hex-head bolt or by a slotted screw. Tap the end frame with a rubber mallet to loosen it from the field frame. Remove the drive end frame and armature assembly from the field frame. The armature assembly is the part with all the copper windings around a central shaft and will slide out from the field frame.

Step 3

Clean the commutator end frames, commutator, and field frame with a clean cloth. Do not use solvent of any kind on the generator parts. Inspect the end bearings for excessive wear or corrosion. Replace the end bearings if they appear pitted or worn.

Step 4

Examine the commutator for high bars, high mica, pitted bars or excessive wear. Turn the commutator on a lathe if any of these conditions are present. Clean the commutator with a fine grade of sandpaper if it appears to be in good condition. Wipe off any sandpaper residue or dirt with a clean cloth.

Step 5

Remove the brushes from the field frame and discard. Clean any corrosion off of the brush frames and install new brushes. Ensure the preformed angle of the brush matches the commutator contour. Ensure the brush frame springs are in good condition and maintain tension on the brush. Replace any springs that are worn or broken.

Step 6

Test the armature for ground faults. Place one lead of the growler probe on the armature core and the other lead on the commutator bar. If the growler lamp lights up, the commutator is grounded and must be replaced.

Step 7

Place the armature in the V-notch of the growler. Turn the growler on and touch the armature with a metal hack-saw blade. The hack-saw blade will become magnetically attracted to the armature and will vibrate if the armature is good. Rotate the armature and check each winding in the same way.

Step 8

Test the field coil for open circuits. Place on lead of the growler probe on the field terminal and the other lead on the field coil lead to the armature terminal. If the lamp does not light then the field coils have an open circuit and must be replaced.

Step 9

Test the armature terminal for ground faults. Place one probe of the growler to an armature terminal and place the other probe on the generator frame. If the lamp lights then the terminal insulation through the generator frame has broken down and must be replaced.

Step 10

Test the brush holders for ground faults. Place one probe of the growler on the insulated brush holder and the other probe to the generator frame. If the growler lamp lights then the brush holder is grounded and the insulation must be replaced.

Step 11

Solder any loose wires and replace any bad ground insulation as required.

Step 12

Install the armature into the field frame. Install the end frame, being careful not to damage the brushes on the commutator. Install the drive end frame and install the through bolts. Install the drive pulley and fasten the pulley nut hand tight.

Step 13

Install the generator onto the engine and install the hold down bolts hand tight. Place the generator belt over the engine pulley and generator pulley. Tighten the generator belt using a pry-bar wedged against the generator frame and engine. Tighten the hold down bolts while holding tension on the generator with the pry-bar. Squeeze the generator belt with your hand and tighten the pulley nut with an open end wrench.

Step 14

Attach the wire leads to their original mounting studs on the generator. Connect the battery ground lead to the battery. Polarize the generator by momentarily contacting the "BAT" terminal of the voltage regulator to the "GEN" or "ARM" terminal using a jumper wire. Apply 8 to 10 drops of light oil to the bearing oil cups located on each end of the generator.

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