Slow crank

Start by measuring the battery's voltage while the engine is running. It must be between 13.75 and 14.75 volts. If that is okay, measure the voltage between a paint-free point on the engine, and the large stud on the starter. Go right on that stud, not the terminal attached to it. Youwill find full battery voltage there. What is important is what that voltage drops to when a helper cranks the engine.

I would also check the braided metal ground cables, could be as many a four. Two on either side could be from hood to firewall and firewall to frame or to engine. Had a friend with 1978 Bronco that had same issue and as he was working on it suddenly would not turn over at all. When I looked at it noticed three of the four braided ground were broken replaced and back to normal.

I know not same year but does not hurt to check.

Mark

Easy way to discover if it is cable related. Take a pair of battery jumper cables, connect one clamp to the negative battery post. Connect the other end to a clean area on the block. Does it make any difference in cranking speed? Now do the same test on the positive side with the jumper between the battery and the starter solenoid and then from the solenoid to the starter.

We're looking for what the 12 volts drops to during engine cranking. The industry-standard is no lower than 9.6 volts. If it is lower than that, we need to figure out if it's because current flow is too high, as in worn bushings in the starter causing it to drag, or current is too low due to a frayed or corroded cable, or a bad connection. We can figure out where to go next once we know what we're starting with.