That means way too little current is going to the starter, AND the battery cables and connections related to the lights do not have a bad connection. The cables to look at are the large negative cable that bolts to the engine block and the large positive cable that bolts to the large terminal on the starter. It's possible the starter has one defective winding out of the two but that will usually still cause high current and the lights to dim.
Grab an inexpensive digital voltmeter and measure the battery voltage. It should be near 12.6 volts. Measure it a second time while a helper tries to crank the engine. It should not drop below 9.6 volts. Move the voltmeter's negative lead from the battery terminal to the engine block and measure a again while your helper cranks the engine. Again, 9.6 volts is the minimum. If you found no problem up to there, move the positive probe to the large copper stud on the starter, not the terminal end that is bolted to it. 9.6 is the minimum acceptable. If the voltages are all high enough all the way up to the starter, it has to be defective. From your description, I think you're going to find one of those voltages is way too low. That's the place to look for a bad connection or some of the copper strands in the cable are broken or corroded under the insulation.
May 7, 2011 at 8:13 AM
