No power to starter solenoid

My experience has been a failure shows up in multiple places, and bypassing one of those places won't solve anything. When ours would not crank, bypassing the starter relay would get the starter to crank the engine, but the engine would not start or run. After sitting a few minutes, the starter system worked normally and the engine would run. At the same time, when the engine would not crank with the key, it still would crank and run normally with the remote start on the key fob. Multiple systems were shutting down intermittently.

If you could find the starter relay control circuit inside the TIPM, and ground that terminal on the relay, the computer is going to see that 0 volts there when it shouldn't be. Knowing there's a defect, it will shut down or disable some circuits.

All three things you listed can be seen on the scanner. You can watch the items change state as you turn various switches on and off or shift gears. There's really no other way to know what's working and what isn't.

Your four points about the four relay terminals is exactly correct. The problem now is the circuits for terminals 85 and 86 get their power and ground through computerized switching circuitry, and the switches are inputs to those computers. Instead of just a neutral safety switch interrupting the ground circuit, any number of things can be inputs to the computer as it decides whether or not to allow terminal 85 to be grounded. Besides not being in "park", for example, the ground circuit could be interrupted by a door not being closed, the anti-theft system being activated, battery voltage is too low, etc.

Same with the positive side. An aftermarket remote start system might operate, but a factory system won't work if the Check Engine light is on for any reason. That inhibit function might be an input to the "starter relay control" circuit in the Engine Computer. That, along with all the other inputs, must all give the go-ahead for the circuit to work where that had been done with just the ignition switch.

As for what to look for on your scanner, we didn't know what many of the abbreviations stood for. We looked for the things related to starting and watched for those that didn't change state when we turned a switch. For example, "transmission gear". Under "Body Computer", it would list the gear the transmission was in, but not by looking at the transmission range sensor. The scanner was displaying what the Body Computer was seeing, whether that was from a switch or digital data from a different computer. In this case the transmission range sensor takes the place of the neutral safety switch. It's an input to the Transmission Computer. When it's the Transmission Computer's turn to broadcast its data onto the data buss, all the other computers can see it and use it or ignore it. The Body Computer sees that data and picks off the range sensor data, then, when it sees the transmission is in "park", it allows the TIPM to activate the circuitry involved with running the starter relay. All we need is to see on the scanner the final step in that sequence to know everything leading up to that point has to be working. No need to waste time testing the range sensor or its wiring, and things like that.

We spent so many hours on our truck, and developed so much confusion, but it finally came down to everything pointed to the TIPM. That's when we finally broke down and found a good used one to substitute as a test. Being so intermittent, you can never be sure it's solved, but so far the problem hasn't come back.

Hello

I seem to have confused terminals 85+86.
Terminal 86 is the 12V activation from the PCM, dark/green/orange wire from connector C3 pin 38 at the PCM. The TIPM is switching a ground at terminal 85 when cranking.
The voltage from the PCM at pin 38 is not enough.
I would suggest to check directly at the PCM, pin 38, when cranking/key on if 12V is given there. See image below
If it is then we know that the PCM is outputting the correct voltage.

Next check if 12V is given at the TIPM, connector C1 pin 41. Image 2 below
If 12 v is given at this terminal then this proves that the wire is intact.
If the voltage at the TIPM pin 41 is only 4V then this shows that the wire has a high resistance and needs replacing.

If 12V is given at pin 41 and only 4V given at terminal 86 then this shows that the TIPM is bad

Cheers, Boris

Hello,

The PCM connector will have to be connected to the PCM as the PCM will not power up. You will need to back probe the connector at pin 38.

I suggest you use a thin metal pin to gain access and then use the voltmeter and check voltage at the pin while cranking.

Do not pierce the wire, back probe into the connector.
See image below.

Hope this helps.

Cheers, Boris

Hello,

I sounds like that the pin you used to back probe the wire at pin 38 did not make proper contact. You had a reading of 4V+ at the starter solenoid terminal 86 before. Have you tried to bridge terminals 30+87 at the starter solenoid socket? With ignition on this should start the motor. Ensure that transmission is in park/neutral before trying this

Code P0750 will not cause a starting issue, it will put the transmission in limp home mode.
Did disconnecting, cleaning and then reconnecting the transmission connector fix P0750?

A good discussion regarding P0750 can be found here:
https://www.ramforumz.com/threads/limp-mode-with-p0750.174265/

Testing for P0750 is done at PCM connector C4(black/green), pin 10, and transmission connector pin 2. Test wire pin to pin for possible open circuit or high resistance, more than 5 ohm.

This DTC is strictly an electrical fault and cannot be caused by any internal transmission failure other than an open in the Transmission Solenoid/TRS Assembly. If the Transmission Solenoid/TRS assembly is in need of replacement †do not replace the Valve Body.

Cheers, Boris

Hello,

Theory of operation to set P0750

The Transmission Control System uses six electronically controlled solenoids that allow hydraulic fluid to be applied to various friction elements (clutches), which enables the gear requested. The continuity of each solenoid circuit is periodically tested. Each inactive solenoid is turned on for a few milliseconds, then off. Each active solenoid is turned
off for a few milliseconds, then on. This pulsing of voltage to the solenoid causes an inductive spike which can be sensed by the Transmission Control System. If an inductive spike is not sensed by the Transmission Control System during the
continuity check, it is tested again. If the test fails three consecutive times, the appropriate Diagnostic Trouble Code (DTC) is set. If the solenoid test is run in response to a gear ratio or pressure switch error, one failure will result in setting the appropriate DTC.

Possible causes:

Related TCM Power Input DTCs Present.
(T20) L/R Solenoid Control Circuit Shorted to other Circuits.
(T20) L/R Solenoid Control Circuit Open.
(T20) L/R Solenoid Control Circuit Shorted to Ground.
Transmission Solenoid/TRS Assembly.
Powertrain Control Module (PCM).

Next test at the transmission connector would be to check if any of the other pins has continuity (short circuit) to pin 2. Also test the pins for a possible short to ground or positive.
Test pin 2 to pins 12, 17, 19, 20 and 21.
If no short to any other pin can be seen, then you most likely have a failed transmission solenoid pack.

This will still not cause the non-start.

Cheers, Boris