Problem with starting

[PMan_S13]

Ok, right off the bat I am pretty sure this is a starter problem. But I have never heard anything like this before...

Ok, when I try to start the motor ('91 240sx) it will crank over a couple times and then it will make a loud clunk, like the whole motor has stopped moving abruptly. the timing chain is not broken so it can't be hitting valves or anything.. and also the lights dim out completely when it "thumps".. I tested the starter for voltage (on the power) and resistance (on the ground)..

with the ignition off- 11-13 volts - .04-.06 ohms

with ignition on- 11-13 volts- .06-1.1 ohms

while starting- 10-12 volts- 1.1-1.5 ohms

when it thumps - ~6.5 volts- ~13.0 ohms

I think my starter is not disengaging from the motor, but I am only guessing

[U12 2NR]

if the starter wasn't disengaging, you would hear it spinning along with the motor even grinding pergaps, but it doesn't stop the motor from spinning.

[PMan_S13]

could the starter just be seizing on me then? it is a very audible thump and it will not crank when it does it

[NISTECH]

you have a loose ground connection along your negative cable. posibly at the battery or on the block