Have you thought about it? Limit this scenario to when the car is starting. That situation is crucial.
The battery is the only power source. The device that demands most, if not all of the power from the battery, is the starter. Cars have their electrical systems designed such that the path of least resistance from the battery is to the starter. What do big audio power amps and starters have in common? Big thick, juicy, low resistance electrical cables, correct?

Now, the basic rule of electricity is that an electrical charge will ALWAYS follow the path of least resistance.

You say that the body's grounding does not have much resistance. When you consider that the body's ground has to "compete" with the battery cables ground, guess who wins, and guess where all the electricity goes when you run the starter? The effect is more easily visualized when you turn the headlamps on and then start the car. The headlamps dim. The voltage has dropped going to the headlights because the starter demanded everything from the battery, and like I said before, the electricity took the path of least resistance.

Now, multiply that effect to the starter getting all the power , and all other electrical devices being starved of it. TPS, CKPS etc. All ECU controlled functions, while the starter is going, are running at reduced capacity, if at all. By ensuring a clean connection from engine to tranny, you are giving all the elctrical devices a more even chance of sharing the electricity. And that will help the car to start.

Just to add, when the OP duplicated the battery cable to starter connection with the extra ground cable, that improved performance.....of the starter. All the other components were still starving, and more so than before. That starter was drunk with all the power.

Also, if you roll start the car, it runs fine. The battery is no longer supplying electricity. The ALT has taken over. Not only that, but the ALT has it easy compared to the battery. It doesn't have to run the starter, the component that is by far the most electrically demanding of the vehicle. When you buy a battery that advertises so many hundreds of cold cranking amps, it is addressing one simple issue: having enough power to drive the starter. Nothing else.
Now, because the ALT has it easy, it can more easily distribute the electricity to all components of the vehicle, and it will do that while charging the battery, too. In fact, if you combine all the other electrical components demand for electricy, be it your car stereo and sub, sunroof, rear window defroster, headlamps etc, their combined need for electrical charge is still much much less than the starter.

You can PM me if you want to discuss this further. Otherwise, I'm out.

 

Go away man. You're wrong. Get over it. Try to learn rather than just liking the sound of your voice.

 

adeste - I highly suggest you take a systematic approach to fixing this. (Something I always implore people to do TBH.)

You should look at: Resistances, function checks, and the battery. Many problems are masked by issues that have been ignored/overlooked/etc. Eliminating these will be the first and best step towards making good progress. Refusing to address these possibilities can skew future results and send you down the wrong trouble-shooting path.

Resistances: The most common is terminal and wires corrosion. A lot o people just ignore it, others do not notice it, and even some who look cannot see it as it will creep up the wire under the casing. Make sure your terminals are not corroded , the wire is clear of the same, and that any connection is no packed with greese/grime. (Caveat – dialectic is not to be applied before the connection is made as it is, in laymen's terms, a insulator.) Also, if you 'clean' the wire/connector in a neutralizing solution and there is any visible pitting or gaps, replace it.

Function checks: Although unlikely in your situation, ensure that your issue does not pertain to a dying starter motor, bad relay, or shot solenoid, etc. (I know you did not mention these problems but some people exclude them as they do not understand what they are or how they work. I mention them to rectify this situation if it should apply. Either way, might as well check so you know for your PMCS records.)

Battery: Although many dead battery assumptions can be attributed to poor conductivity, obviously a faulty battery remains a likely issue. People forget that batteries are maintenance items that need to be routinely checked/replaced. Check your resting voltage, check your voltage during charge, and check the discharge rate. (Some places will check your battery for free. I would watch them to ensure they are giving you accurate information.) Once you have ensued good connections an easy way to TS the battery is swapping for a known good. [If this fixes your issue please do not rule out items that may have caused your battery to die.]

After you do these things report back; let us know if you found the problem or what symptoms still exist.

.
.

*I will try to make another post tonight explaining why you might have seen the results you did and how it applies to electrical theory. For now, I have to get some sleep as I work tonight. Hope all goes well! Good luck and God bless!

 

Through the stock negative battery cable that connects to the head (or is it the block, I can't remember).

I've always wondered why people used a ground wire between the engine and block. Running a negative cable from the battery to the starter bolt would give the starter more juice. You would essentially have two parallel circuits, allowing more current for the engines sensors. The positive power for the engine management has nothing to do with the starter as, it is a wholly separate circuit getting its power right at the battery terminal.

Negative cable comes from the battery, bolts to the body under the battery tray, and terminates at the front head. The starter gets its negative voltage from the engine block. not the other way around. I suspect the transmission mounted CKPS is the culprit that is getting low voltage, due to the voltage drop at the mating surfaces when under load by the starter.

 

I pulled the transmission back out. Sanded the mating surfaces down and cleaned the crank shaft position sensor and the car started right up