MGA! Tech Talk – Wiper Motor Armature Repair

Tech Talk
with Bob Shafto, Technical Editor

Wiper Motor Armature Repair

While investigating an MGA wiper motor failure, I found the armature windings were fried. There were a few wires disconnected from the commutator, but the main cause was wires shorting to ground because of rubbing against sharp corners on the armature.

The first step of disassembly is to unsolder and remove all the windings. Clean the soot left by burnt insolation being careful not to damage the insulation on the end face or the sleeves on each end of the shaft. Clean the commutators by spinning the armature in a drill or lathe while using a folded piece of 400 grit sandpaper on the commutator. When the commutator is clean, use a toothpick or pointed end of a small tie wrap / zip tie to clean the grooves between commutator segments.

Next, inspect the insulating pieces and replace if necessary. Insulation is to prevent the wire from contacting metal (ground) especially around sharp corners. Insulating material (fish paper) comes in a varity of sizes and shapes. It should be flexible and able to withstand 230 degrees fahrenheit. The ends are .050” to .060” thick and the slot paper is .010” thick, 1 1/2″ long 1 1/8” wide.

For the ends (Figure 2), make a hole in the center to fit snugly over the insulator sleeve and glue it to the armature face. Then use an X-Acto knife or similar, to cut the gaps between segments using the segments as a guide.

The sleeves are usually not damaged, but if needed, can be formed by wrapping and gluing insulation paper around the shaft ½” tall and ½” outside diameter.

To replace the insulator on the commutator end, use the output shaft end as a templet, cut it in half (slot to center hole) then glue the pieces in place from opposite sides of the shaft. (Figure 3)

Place insulator paper into the armature segment slots (Figure 5). The insulation paper should extend above the slot to protect the sharp edge as the windings are placed.

Trim the slot insulation paper to the armature outer diameter after all the windings are done. Once the armature is clean and insulated, you can begin wrapping the coils. (Figure 5)

Begin by numbering the segment openings (holes) clockwise (Figure 6). You will need a total of about 135’ of 24ga coated transformer wire (1 lb. spool is enough to wrap 6 armatures). The wire is 0.020” diameter, 0.02567 ohms per ft. Each coil takes 26 wraps and there are 10 coils. Each winding resistance should be 0.29 to 0.35 ohms.

Wrap the armature in 5 parallel layers. (Figure 7) Scrape the varnish off the ends with an X-Acto knife and wrap around commutator tab but don’t solder ends until all windings are done.

Notice that commutator 1 is between slots 1 and 2. Wrap bare wire end around commutator terminal 1 then run the wire down hole 1, across the bottom and up hole 5, across the top and down hole 1 again (this is one wrap). Do this 25 more times keeping each winding snug.

The ends of the paper will fold over the corner on the ends to protect the sharp edges. The 26th wrap will go down hole 1 and up hole 5 then down hole 1 and up hole 10. Cut this end at the top of the commutator, clean off varnish and wrap around commutator tab.

Use the diagram (Figure 6) and chart (Figure7) to wrap the rest of the coils. When all the windings are complete solder the ends to the commutator tabs.

Make a final inspection to be sure all windings and insulation paper are recessed and do not protrude out of the slots.

While the windings and layout will differ, the methodology applies to any armature.

Optional
For added electrical and mechanical protection, coat the new windings with Insulating Varnish 4228 or equivalent. (Figure 8)