Staarter Mtr trouble shooting

[jen]

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]

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]

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

[davefrombc]

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]

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]

Frustrated but still game,,  WeeBee...LOL

[WeeBee]

Glad to hear it !!

[mikeexplorer]

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]

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]

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]

 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]

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]

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]

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]

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]

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

 

[jen]

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]

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]

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]

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]

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]

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

[mikeexplorer]

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]

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]

Rinse with cleaner....

[davefrombc]

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]

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]

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

[WeeBee]

Looks good from here !

[jen]

Well its clean!