My dad's car (65 coronet, 440 RB, 727) starts to crank then doesn't. He already replaced the battery and starter and starter relay. I'm thinking it's the positive battery cable but not sure how to test. We are able to turn the engine from the crank bolt, so it doesn't seem like any mechanical binding to me.

Check grounds. Tight cables,clean connections.

Visual inspection is the best way I know. Look for corrosion progressing where you can actually see the strands of wire. Bend and feel it for signs of hard or soft spots.

You have probably done this but I'll mention anyhow. Clean all connection points. 80 grit emery cloth works well on the connectors themselves, wire brush battery terminal cleaner for the battery connections and a flat file on the block at the ground mounting boss. Cleaning the ground bolt hole with a tap is a bonus.

If its the cables, the heaviest offering at the part store (4 gauge?) will get you going without breaking the bank... though homemade or custom made would get a better/heavier cable.

cranking slows to a crawl or stops completely? And that is after how many seconds of cranking? when did this start (after any changes?). Clean each end of each large cable including the ground path. Might pull and ground the coil secondary wire (dist end) & see if normal cranking is (now) restored. Holler back & more later

Sorry for post hi-jack but I have a similar problem in that my car starts great when the engine is cold but when I drive for 15 mins to work or get gas and shut off engine, the starter is super slow to crank and won't fire the engine. Once everything cools off it starts up again.

New battery, starter, added a remote solenoid, new Chrysler starter relay (which has no ground post for some reason?) still same result. Battery cable is 1 gauge although it is about 12 years old. Negative cable is 1/0 gauge and grounds are all solid. Was hoping it is just my old positive cable? Anyone run into or have more insight to this problem?

Thanks

Use a voltmeter set on DC volts. (analog, one that has a needle that sweeps back and forth, works best) Set on a low scale if possible, 1-3 volts. Place leads on both ends of the cable, attempt to crank the engine. The voltage LOST in the cable to resistance will read on the voltmeter (if needle tries to sweep to negative voltage, swap leads to other end of cable and try again). You should never have more than half a volt. This will work on either cable, or connection you place it on and try to operate, batt post to cable end, cable end to ground, cable end to starter terminal, etc.
Jess

If its old and the car isnt a show poodle toss it and the neg cable.Go to NAPA etc and get 1 gauge cables.The ones on my 65 looked good years ago.But had a hard time cranking.I replaced then with 1 gauge and big difference,Then I installed a min starter and that really made a huge difference,Even with 12 to 1 domes my 440 cranks like a /6.Mini and cables have been in close to 20 years and thats with headers.Rocky

Yes the simple voltage drop test that the shop forman at a dealer I worked for years ago did not know how to do so he asked me and I had to show him. Its the best way to check electrical items. Do what this man says. Ron

The culprit was the battery disconnect switch. It's been on there for about 2 years. :|

Glad you found the problem.