Hi, my Denso starter (900SS 1994) started playing up a bit, first intermittently working then stopped completely, so I decided to strip it down to clean it up and replace the brushes. While dismantled I did the "3 part test" to check it over, (resistance between cummutator connections, 180 degress across the commutator and resistance commutator to the rotor). I read that the resistance between connections should be maybe 0.5 to 1.5 Ohms (just the resistance of a wire coil), but mine seems to be zero, a dead short, and every commutator pair is the same. I dont believe every wire coil could be shorted, surely that's impossible. Something's very weird - does anyone know what's going on or have I misunderstood something about the wiring? Cheers Pete
How good is your meter.... What range are you on? 0-10 Ohms would be good.... or at least your lowest range. What is it across the leads? circa 1 to 1.6 Ohm... depends on lots of design criteria. Looks good (very clean) to me.
Thanks for the message. The meter is a good one, auto ranging and it goes to the most sensitive range and says zero ohms. I even double checked with another one and it's the same story. So between commutator segments is a dead short, that's every segment to every other segment, all zero resistance. Strangely I found about 8 or 9 MegaOhm between the commutator and the armature/rotor, and that's just with a normal meter, at higher voltages I suppose it may be worse. Its got me stumped especially because it seemed to be working, then went intermittent, then nothing, so I assumed it just needed a cleanup which as you can see I did (the commutator was quite dirty from the brushes) Pete
Clamp it in a vice and bench test it for operation then at least you can rule out problems in the starter button, relay, solenoid, cables etc.
I'll try again but I did that before and just got a big spark when touching the battery but no rotation (without solenoid in the circuit, just battery, jumper cables and starter)
Possibly a breakdown of the internal insulation within the armature winding? I'd expect some sign of overheating though which doesn't seem evident.
Ohms is a measurement of resistance . Mohms is a measurement of insulation resistance . If you were looking for a resistance of 0.5 to 1.5 ohms and your meter is set on continuity and you have a reading of 0 that's an open circuit . If you were looking for an insulation resistance reading of 0.5 to 1.5 Mohms and your meter is set on Mohms and you have a reading of 0 that's a dead short . I wasn't to sure from your question what your measuring .
Not sure what multimeter you have, but you need to skip it..... Closed circuit will NOT read 0... unless it is at the wrong end of the display and flashing!!
I think the meter you are using is not capable of reading resistance down to the level that you need to measure commutator coil windings, which as you say should be in the region of 0.5-1.5 ohms. Your meter is displaying Zero ohms, as that's all it can do as the resistance you are measuring is so small. You need to use a meter that is capable of reading in 0.1 ohm increments. ( I made exactly the same wrong assumption recently when quickly doing a check on the coil windings on an e-bike, used my multi-meter which was to hand and jumped to the wrong conclusion that the coils had a dead short as the meter was reading zero ohms, tried the same measurements later, using a more accurate meter and realised that the resistance was actually circa 0.4 ohms )
If you budget can run to it get a scope meter.... different tool though.... I carry about 6 different electrical meters on the van.
Sorry but thats not true, a reading of zero ohms is a short circuit. And MOhms is simply a scale for much greater resistance, not specifically insulation resistance although it's often used for insulation testing. But anyway, I have about 8meg between the commutator and the rotor, I'm not certain but perhaps this should be open coircuit and the fact that Im seeing anything other than an open circuit probably means the insulation is breaking down somewhere. Or would the starter still work with 8megs from commutator to rotor (ie earth), who knows, but Im beginning to doubt it.
Nice meters by the way! I have two, a cheaper conventional type and a 'clamp meter' which also measures Amps. My starter shows the same with both so I dont THINK the meter is the issure in my case, but do they measure down to sufficient accurancy? I'll check - thanks for the suggestion. Cheers for all the replies though, much appreciated. A 'fluke meter' ? Very nice but Im not made of money you know
12V through 8 Mohms gives a leakage current of only 0.0000015A. I can't see that having much effect on the starter.
To measure insulation resistance you need a Megger... It uses 500 or 1000V to test resistance, not 0.6V or 3V. It allows a whole load of tests other meters just don't do, like PI.
