The cold cranking amps is not the issue. One car I bought new came with a 325 cca battery for a slant-6 cylinder engine and it was still working fine after five years including starting that engine when it was 30 below zero.
The first thing you have to determine is if the starter is cranking too slowly. If it's spinning at a normal speed but the engine is rotating too slowly, that's an issue with the starter drive and / or flex plate.
If the starter motor is spinning too slowly, the next thing is to use a starting system tester with an inductive current probe to measure the current going to the starter. Typically they will draw around 300 amps for just a fraction of a second, then as it gains speed it generates a "back emf", (electromotive force), that opposes battery current so that current will drop to around 200 - 250 amps. Since it's spinning too slowly to generate much back emf, you can expect the current to be higher, ... Closer to that 300 amps. If you find only about 200 amps, that can mean only two things. It's spinning at a normal rate, or one half of the motor isn't working. There's two field coils and two pairs of brushes all acting independently on the armature. Each circuit draws half of the total current. If one brush is not making a good contact with the armature, the initial current will be half of normal, then since little back emf is developed because it's turning so slowly, that current will not decrease very much.
Those current readings can be misleading because you will have around 200 amps while the starter is spinning the engine at the normal speed, and you'll have about the same current when half of the motor isn't working and it's spinning too slowly. What you CAN find though is the current may be way too high. If both halves of the starter are working, real high current is caused by little back emf which means the starter is spinning too slowly. That is due to unusually high compression, hydro-lock, a dry new engine that has no oil on the bearings and cylinder walls, or too many teeth on the starter drive.
For many years people had trouble starting GM and Ford race engines due to the high compression. The common fix was to install Chrysler starters. Those were gear reduction starters so the armature spun much faster and developed tons of back emf. Those starters only draw about 200 amps to crank a high compression big block engine.
If you find starter current is uncommonly low, that will result in slow cranking and that's caused by a bad connection. You'll find that with voltage drop tests. Normally you measure voltage AT a point in the circuit relative to a reference point, in this case, ground. A voltage drop test involves measuring the voltage ACROSS a point in the circuit. That's done at mechanical junctions. The first ones to check, and the easiest to see are the connections between the battery posts and their cable clamps. You put one meter probe on the post and one probe on the clamp, then measure the voltage while a helper cranks the engine. Anything over 0.2 volts is too much. Those two things are the same point in the circuit so the reading should be 0.0 volts, but there is always some resistance in every mechanical connection. That resistance is WAAAAAY too small to measure with an ohm meter, but the voltage drop test will show the RESULTS of that resistance.
Besides that maximum allowable 0.2 volts per mechanical connection, you can also not have more than a total of 0.4 volts per half of the circuit. One half is the positive side. The other half is the negative side. The negative side only has two connections anyway; one at the battery post and one on the engine block. The positive circuit has the battery post / cable clamp, then, depending on which Ford system you're using, if you have the large solenoid on the fender, you have a mechanical connection where the positive battery cable bolts to one of the large copper studs, one where the starter cable bolts to the other stud, one where it bolts to the starter, and two for the contact disc inside the solenoid. You can't get inside the solenoid to measure those two voltage drops, but you can measure from the two studs. If that reading is less than 0.2 volts, you know each one is even less. That's six mechanical connections in the positive half of the circuit. Each one can have no more than 0.2 volts dropped across them, and the total can be no more than 0.4 volts. To measure the total, put one voltmeter probe on the positive battery post, (not the cable clamp), and the other probe on the stud on the starter motor, (not the cable's terminal). During cranking you must not read more than 0.4 volts.
You can also measure the voltage AT the starter during cranking. The minimum acceptable is 9.6 volts. If it is pretty high, as in 11 volts or higher, the starter is not drawing enough current. The brushes would be the first suspect. If the voltage is real low, the current is not able to get through the cables and connections due to too much resistance. The place to start for that is by measuring the battery voltage during cranking. If it is too low, that resistance in the circuit is in the battery. That's called "internal resistance" and that's what is responsible for a 12 volt battery's voltage dropping when high current is drawn from it. High internal resistance is due to the battery being too small for the load, (that's where cold cranking amps comes into play), it is not fully charged, or it's near the end of its life and much of the lead has flaked off the plates.
Friday, March 1st, 2013 AT 8:52 AM