Starter voltage test

[70runner]

OE starter in my 70RR was doing the single <solenoid> click dance a few weeks ago, so I pulled it to disassemble and inspect. Since I had it apart, replaced solenoid, cleaned/checked armature, brushes were good, clutch good, new battery stud and contact assembly (old ones had some divots). Reassembled and bench tested with VOM at battery stud, video below. Used portable battery jump start, recently charged. Starter appears to engage normally, though voltage drops from 12.5v to 8v during test. Does this voltage drop seem excessive?

 

[RJRENTON]

OE starter in my 70RR was doing the single <solenoid> click dance a few weeks ago, so I pulled it to disassemble and inspect. Since I had it apart, replaced solenoid, cleaned/checked armature, brushes were good, clutch good, new battery stud and contact assembly (old ones had some divots). Reassembled and bench tested with VOM at battery stud, video below. Used portable battery jump start, recently charged. Starter appears to engage normally, though voltage drops from 12.5v to 8v during test. Does this voltage drop seem excessive?

IMO.....NO. The reason being that the starter had no load imposed, as if it were cranking the engine. You sre basically seeing the results of the combination of field coil voltage drop, armature voltage drop, brush voltage drop and solenoid coil voltage drop. The total amps consumed will be low, so the voltage losses will be low......Ohm's law applies. E (voltage) = I (amps) x R (total resistance) In actual operation, the largest voltage drop will be the combination of armature, field coil and brush drop. In order to develop the torque required to turn ithe engine, overcome internal friction, etc, the starter will consume 300-400 amps which will result in 3-4 volt drop, including cable connections. Cold engines with high compression and thick oil will pull more amps. This is why the mini starters have less losses (voltage losses and consume less cranking amps) are built with PERMANENT MAGNET FIELD COILS for less internal losses which result in higher torque output. The basic motor design is a series wound construction to develop maximum torque at ZERO armature RPMs called starting or breakaway torque. Only an electric motor develops maximum or stall torque at zero RPM......
BOB RENTON

 

[RemCharger]

Starter appears to engage normally, though voltage drops from 12.5v to 8v during test. Does this voltage drop seem excessive?

Yes it does. It shouldn't drop more than about 10.5 as installed in the car. If your results dont meet your preconcieved notions, recheck your variables.
#1. Test with a good properly tested automotive battery.

 

[pnora]

Using a jump pack is fine for basic operation testing. It will not be good for a proper draw test. You need a good fully charged battery for those tests.

 

[70runner]

Appreciate the replies. Can Autozone/Oreillys do a proper draw test?

 

[pnora]

Appreciate the replies. Can Autozone/Oreillys do a proper draw test?

On the car maybe. Bench test I doubt it.