Cranks slow with the key, fast at the relay
this is what was described above.
It must be done while cranking. Cranking puts a load on the contacts and if there is resistance it will raise the voltage reading above the 0.5V or so.
Think of it this way...
lets say you have a wire with 15 strands and you measure resistance of that wire with an ohm meter (no load) you would get 0 Ohms (best case, or what ever your meter reads when its leads are touching)
now take the same wire and cut all but 1 of the 15 strands and measure resistance your meter will read the same as in the first measurement.
The wire with all its strands can handle (just a numbers for illustration) 15 Amps, the wire with only 1 strand left can handle 1 amp.
If you try to power that 15 amp load with a wire only able to handle 1 amp the wire will heat up and resistance will be increased and voltage will be decreased so the voltage drop of the wire will be 11 volts and you only have 1 volt to power the load
In starter motor world 12-13V would be great, 10-11 is typical, 8-9 is marginal, 6-7 probably will result in the dreaded click of death. so you can see how 2 volts lost can make or brake the starter system.
AGAIN this is assuming there is not some other system in the car drawing a load while cranking... lights, electric fans, high watt amplifier, shorted field cir in the alternator, other high current draws.
this is what was described above.
It must be done while cranking. Cranking puts a load on the contacts and if there is resistance it will raise the voltage reading above the 0.5V or so.
Think of it this way...
lets say you have a wire with 15 strands and you measure resistance of that wire with an ohm meter (no load) you would get 0 Ohms (best case, or what ever your meter reads when its leads are touching)
now take the same wire and cut all but 1 of the 15 strands and measure resistance your meter will read the same as in the first measurement.
The wire with all its strands can handle (just a numbers for illustration) 15 Amps, the wire with only 1 strand left can handle 1 amp.
If you try to power that 15 amp load with a wire only able to handle 1 amp the wire will heat up and resistance will be increased and voltage will be decreased so the voltage drop of the wire will be 11 volts and you only have 1 volt to power the load
In starter motor world 12-13V would be great, 10-11 is typical, 8-9 is marginal, 6-7 probably will result in the dreaded click of death. so you can see how 2 volts lost can make or brake the starter system.
AGAIN this is assuming there is not some other system in the car drawing a load while cranking... lights, electric fans, high watt amplifier, shorted field cir in the alternator, other high current draws.