Staarter Mtr trouble shooting

**jen** · February 13

Cannot find out why 12 vdc 4 (4) commutator starter motor has no power. It will run full speed without load. But with any load will not turn. Repaired broken brush holder and insulated an internal wire to magnet that was bare and rubbing. cleaned commutator segments. Ohm readings while slowly rotating commutator was 1 ohm. When attempting to run with load it will not turn and Bat vdc only pulls down to12vdc. Cannot find an internal wiring schematic. I can buy a rebuild kit ~$35 but WTH?

WeeBee · February 14

Sadly I have very little experience in troubleshooting Starters -What comes in the Rebuild Kit that you haven't already checked or Repaired /Replaced ??

**jen** · February 14

It comes with brushes etc..But the brushes are OK i am concerned a magnet (s) are nad...a resistance check...????

**davefrombc** · February 14

Did you clean the commutator? The second picture shows it in bad need of cleaning off the ends of the segments of the built up carbon that could be shorting some of them out. Thoroughly clean the commutator, both on the brush contact surface, segment ends and between the segments.. There are separators between the segments that soft so be careful to just clean the carbon out and not scrape away any of those insulating strips. Do the springs hold the brushes solidly in contact with the Comm. or is one or two weak? Have you checked the bearings/ bushings in the end bells to make sure they aren't worn allowing the armature to slop a bit?

When I rebuilt my starter the brushes looked good but in fact they were worn considerably shorter than new and that caused then to not have solid contact with the commutator.

Before replacing the brushes my starter had " flat spots".. Spots where the starter would not spin when the button was pressed.. a tap on the starter housing was usually all it took to make it spin again. That was due to the worn brushes.

Your starter is considerably different from mine in the way the brushes are mounted. Mine were spring mounted in the holders while yours look to be solidly connected in them and relying on the holders themselves being spring loaded. Maybe all the starter needs is a thorough cleaning of the commutator. Try that , then if no improvement go from there to either buy a rebuilt unit or try a rebuild kit before biting the bullet for the far more expensive replacement starter.

Edited February 14 by davefrombc

**jen** · February 14

Commutator clean I scraped between the commutator segments but not very aggressively..I was thinking of using a hack saw blade teeth (break end off to expose a pointed tip). Yes the brushes may be worn . before it went completely bad it had "flat spots " like you said. I enjoyed hitting it with a hammer.

I could disassemble the entire thing and look for broken wire between the Magnets. The brushes are fixed so there is no "gain" adjustment.

I will reclean first. I have another identical starter that works and I can compare resistance readings but it is in W Va. I've been traveling and now in FL.

Yes the bronze bushings are worn. New brush kit and bushings is about $34. BTW the JB weld worked great holding the broken brush holder together.

Thanks for info....I was getting frustrated.

**jen** · February 15

Frustrated but still game,, WeeBee...LOL

WeeBee · February 15

Glad to hear it !!

mikeexplorer · February 16

On 2/14/2026 at 12:58 PM, jen said:

Commutator clean I scraped between the commutator segments but not very aggressively..I was thinking of using a hack saw blade teeth (break end off to expose a pointed tip). Yes the brushes may be worn . before it went completely bad it had "flat spots " like you said. I enjoyed hitting it with a hammer.

I could disassemble the entire thing and look for broken wire between the Magnets. The brushes are fixed so there is no "gain" adjustment.

I will reclean first. I have another identical starter that works and I can compare resistance readings but it is in W Va. I've been traveling and now in FL.

Yes the bronze bushings are worn. New brush kit and bushings is about $34. BTW the JB weld worked great holding the broken brush holder together.

Thanks for info....I was getting frustrated.

You would unlikely see the broken wire. What you need to do is take your meter and remove the armature. test ohm reading from one copper point to another and go around the whole thing. The ohm reading should be consistent from one to the next one.

The brush holders are weird, I don't see any springs that would hold the brush against the commutator. If it is an older starter, the brushes could be worn down and the rebuild kit would be the way to go.

**jen** · February 16

I cleaned the commutator better using CRC contact cleaner. I used a cut off hack saw blade ti scrape between segments, Ohm redeadings between segments 0.1 ohms consistant between all segments. segment to ground (starter frame) 10.44 K ohms

mikeexplorer · February 16

5 hours ago, jen said:

I cleaned the commutator better using CRC contact cleaner. I used a cut off hack saw blade ti scrape between segments, Ohm redeadings between segments 0.1 ohms consistant between all segments. segment to ground (starter frame) 10.44 K ohms

Best way to clean commutator is with very fine sandpaper, or a fiberglass brush. I guess your meter's lowest OHM setting is 0.1 ohms. Not a big deal in this case. You have no open windings, which is good. What is NOT good is the frame reading. Segment to frame should read in the megaohms. The armature is ground faulted. Now this is only a 12 volt motor so there is no shock hazard. This most likely happened if the motor was run hard and the windings overheated, or it got wet inside several times.

So have you looked at the permanent magnets in the case? Is any of them cracked? Loose? If the motor was subjected to overheating that will decrease the Gauss factor of the magnets (make them weaker)

You say it spins well unloaded, but does not loaded, does this mean when it is installed or did you try to put a load on the shaft (grab it GENTLY) Did you do this test with the existing wiring of whatever this came out of? Do you have a battery jump box to try? (connect it to the motor and see if it runs under load) You could be losing current in the wiring.

**jen** · February 16

The brushes make strong contact as you ca see here when in relaxed position. I lightly sand papered the commutator and used a one inch eraser to polish. I probably need to flush the armature windinsgs with brake cleaner. I tested starter under load with a jumper / booster attached to bat. The voltage did not pull down. under load or when not under load. With n load starter spins full speed.. If I hold even lightly with pliers it will not rotate.

mikeexplorer · February 17

What does this starter go to? (type of machine) Depending on the battery size you probably wouldn't see a large drop in voltage. I do see in your picture that the brushes do have a spring mechanism in the arm. So they would be contacting properly. There is a high resistance somewhere. The only thing I can think of is to assemble the motor, attach the meter to the leads with clamps if possible (if not strip a section of wire and wrap it around the meter leads so you don't have to hold it. ) Set meter to low ohm reading.

Rotate the shaft SLOWLY I mean one full rotation should take you 30-60 seconds. see if you get jumps in ohm readings. It will jump around a bit but when done slowly, you will not get an inductive EMF backlash voltage reading. (from the magnets) It is possible the brushes are not maintaining contact as it rotates. I would feel the shaft with the commutator and feel for any bumps, that would deflect the brush for a moment and lose contact.

Another test is with the starter assembled, connect one lead to either connection, the other to the frame, set meter to 100k range and slowly rotate the shaft. You were getting a static reading of about 10K which isn't good. If it changes a lot that could mean the power is bypassing the windings and the armature is no good.

What is the highest DC amp range your meter can do?

I would re-check the windings. Put a meter lead on one commutator point, the other lead to the one next to it. do this all around the armature. Ohm readings should be the same.

At 0.1 ohms, that starter could pull 120 amps from the battery under full load.

**davefrombc** · February 18

It looks like the brushes at the top in the photo don't have much spring load when you compare them to the lower pair.. lower pair closed up top pair hanging wide open. Both sets of brushes should be fairly tight together if the spring loading is good.

**jen** · February 18

Yes the connected to the one brus holds it like that. I will remove the four (4) magnets and wash to check wiring and increase Ohms to ground readings taht were 10.44 Kohms .

mikeexplorer · February 18

20 hours ago, jen said:

Yes the connected to the one brus holds it like that. I will remove the four (4) magnets and wash to check wiring and increase Ohms to ground readings taht were 10.44 Kohms .

Magnets are probably glued in, I would not try to remove them, probably would crack.

**davefrombc** · February 18

If that one brush holder set does not have adequate spring pressure on the brushes, that is where your starter problem is.

**jen** · February 19

I am going to wrap the commutator in fine sand paper and turn with the sand side against the brushes to seat them. The brushe appear to seat. but the sand paper ill show up gaps. The magnets are held in with screws...But haven't got the loose as yet.

mikeexplorer · February 20

On 2/18/2026 at 5:35 PM, davefrombc said:

If that one brush holder set does not have adequate spring pressure on the brushes, that is where your starter problem is.

That could be the case, That would cause resistance under load. She would have to see if the spring is weak.

**Gwbarm** · February 20

Spins with no power is normally a commuter problem i have ran into this on several occasions something seems to short internally causing this. I have replaced the commuter on starters and its good to go never could really figure out the problem with it, and on other occasions i have replaced it with no change, so i just chaulked it up as weak magnets. As starters have gotten cheaper over the years i really dont mess with it if it doesnt start i replace it.

mikeexplorer · February 20

3 hours ago, Gwbarm said:

Spins with no power is normally a commuter problem i have ran into this on several occasions something seems to short internally causing this. I have replaced the commuter on starters and its good to go never could really figure out the problem with it, and on other occasions i have replaced it with no change, so i just chaulked it up as weak magnets. As starters have gotten cheaper over the years i really dont mess with it if it doesnt start i replace it.

She mentioned she reads about 10K ohms from the commutator to the frame. Normally this should be infinite to high megaohm. If this was an AC motor it would be a shock hazard. One possibility is as the motor is put under load, the leak current gets higher bypassing some of the windings. I am betting at one or more times this starter was overheated and the insulation has broken down. The 10K reading would be enough for me to junk it.

**jen** · February 20

Iam going towash the housing /wondings to see If I cam increase tge resostance. I will compare it to a good one have . Replacement $300+.

**Gwbarm** · February 20

Mike, that sounds like a good diagnostic theory i would go along with that.

mikeexplorer · February 21

12 hours ago, jen said:

Iam going towash the housing /wondings to see If I cam increase tge resostance. I will compare it to a good one have . Replacement $300+.

It won't be the housing. Take the commutator and measure resistance from the shaft to any of the contacts, That is where your resistance will be.,

**jen** · February 21

Yes That was also swirling around in my mind but did not mention that. I cannot get the magnets apart so I think I will just douche them with cleaner.

**jen** · February 21

Rinse with cleaner....

**davefrombc** · February 21

Do you have another identical starter? if so. Try swapping the end bells with the brushes and see if the bad one works and the good one goes bad. Just from the photos I suspect the problem is in the spring pressure (actually lack of it) on the one set of brushes.

**jen** · February 21

OK good Idea. I have an Identical starter that works,. Its in WVa ...I'll be in WVa late next week . I am waiting for the winter storm to pass this weekend. I am going to finish clean this one and try it again before I leave. .

**jen** · February 21

I flushed it clean with Brake cleaner spray. It looks better.

WeeBee · February 21

Looks good from here !

**jen** · February 21

Well its clean!

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