3.6L ESS Cold Start

[Jebiruph]

(Disclaimer: Theories regarding the 3.6L ESS starting process are educated guesses, here's mine.)

After months of people suggesting that the PCR separates the batteries before a cold start, I finally tested this several weeks ago. I found that the PCR is activated for a very brief instant before a cold start. I've concluded that this brief activation of the PCR is a test of the Aux battery, to determine if it can power the system electronics. If it fails, the starting process is terminated, preventing someone from driving off with a bad Aux battery.

To help analyze this, I added the Keyless Ignition Node Module and the Radio-Freq Hub Module to my Starting Relay Diagram.

The starting process is a sequence of events, the final event is the PCM activating the Starter Solenoid Relay to crank the starter. The process starts with the pressing of the Start/Stop switch, which is relayed from the Keyless Ignition Node Module to the Radio-Freq Hub Module, that then signals the Body Control Module to initiate a start.

Sometime between the pressing of the Start switch and the starter cranking, The PCM briefly activates the PCR, separating the batteries. Both the Keyless Ignition Node and Radio-Freq Hub modules are then powered only from the Aux battery. If the Aux battery cannot provide enough power for the modules, the status of the start switch is lost and the starting process terminates.

This also explains why it will start with just the Aux battery, but not just the Main battery. With just the Aux battery connected, the status of the Start Switch is maintained while the PCR separates the batteries, and the starting process continues when the PCR reconnects the batteries. With just the Main battery connected, the starting process terminates as soon as the PCR separates the batteries and the KIN and RFH modules lose power.

Additional 3.6L ESS system information is available here.
3.6L ESS Dual Battery Consolidated Information
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-dual-battery-consolidated-information.2

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[Rahneld]

...the final event is the PCM activating the Starter Solenoid Relay to crank the starter.

Thanks for testing this Jerry.

What is your best conclusion on what energizes the Starter Solenoid Relay, the ESS/Aux battery alone, the main battery alone, or both batteries?

What I **may** be taking away from your testing and writeup here is that it's not so much that

• A dead ESS battery lacks the ability to energize the Starter Solenoid Relay in isolation, to then put both batteries back in parallel and fire this Starter Solenoid, but rather,
• That tests are effected on the ESS/Aux battery in isolation, earlier on in the engine cranking (cold or ESS) sequence, that if prove the ESS/Aux battery lacking in power, allow nothing to proceed further.

Thanks.

 

[Jebiruph]

There is a difference between what powers or triggers a relay and what it is switching. The Starter Solenoid Relay is powered by the Aux battery, but is switching power from the Main battery. A bad Aux battery will cause the starting sequence to terminate prior to the Starter Solenoid Relay getting energized.

 

[Rahneld]

There is a difference between what powers or triggers a relay and what it is switching.

Agreed. In fact the need for relays to exist often happens where two distinct electrical systems (here the ESS/Aux battery and the Main--or both batteries) need to synchronize a process.

Jerry I'm sure knows this, but a classic example for those of us who are less familiar is the thermostat on your home heat, and the heat itself. The thermostat often runs on low voltage wiring, and when a temperature threshold is reached, closes a circuit that triggers another circuit running higher voltage to:

• In the case of natural or oil gas heat ignite a flame
• In the case of electric heat activate what's usually a 220V circuit that makes the heat through electrical resistance.

The low voltage wiring is safer and doesn't require an electrician to install.

The Starter Solenoid Relay is powered by the Aux battery

Ok, thanks.

but is switching power from the Main battery.

I think what you're saying is that the ESS/Aux battery, is "what powers or triggers a relay" (from your words above) and the Main battery, (in this case in combination with the ESS/Aux battery is "switching" the starter solenoid.)

A bad Aux battery will cause the starting sequence to terminate prior to the Starter Solenoid Relay getting energized.

And I think what you're saying here is 'it isn't so much that the 3.6JL doesn't crank because the ESS/Aux battery doesn't have the power to operate the relay (although this may very well be the fact) but that tests against the ESS/Aux battery that occur earlier on in the engine cranking sequence, upon detecting an ESS/Aux battery lacking in power, stop the sequence there and it never even gets to the point where the ESS/Aux battery is given a chance to even try operating this relay.'

Yes?

 

[Jebiruph]

And I think what you're saying here is 'it isn't so much that the 3.6JL doesn't crank because the ESS/Aux battery doesn't have the power to operate the relay (although this may very well be the fact) but that tests against the ESS/Aux battery that occur earlier on in the engine cranking sequence, upon detecting an ESS/Aux battery lacking in power, stop the sequence there and it never even gets to the point where the ESS/Aux battery is given a chance to even try operating this relay.'

Yes?

Let's step back a bit. There are two different starts, cold starts and auto stopped starts. For a cold start, the process starts with the batteries connected, both batteries power everything, including all starting related relays. With the batteries connected, the status of the Aux battery is unknown. The PCR is blipped, just enough to test the Aux battery, then the PCR reconnects the batteries. If the Aux didin't fail the test, the starting process continues with both batteries powering everything, including all starting related relays.

For a start from an auto stop when the batteries are separated, who powers what makes a difference.

 

[Rahneld]

Let's step back a bit. There are two different starts, cold starts and auto stopped starts. For a cold start, the process starts with the batteries connected, both batteries power everything, including all starting related relays. With the batteries connected, the status of the Aux battery is unknown. The PCR is blipped, just enough to test the Aux battery, then the PCR reconnects the batteries. If the Aux didin't fail the test, the starting process continues with both batteries powering everything, including all starting related relays.

For a start from an auto stop when the batteries are separated, who powers what makes a difference.

Thanks Jerry but the auto stop scenario kinda leaves me handing with the "who powers what" phrasing above...and I have a strong suspicion that prior to the 3.6L contemplating an autostop (i.e. when all other tests pass, like engine warm, not in manual mode for automatics, etc.)

• A test similar to the one you describe is effected to see if there's enough current in the ESS/Aux battery to engage the auto stop, and if so, during it,
• I suspect the ESS battery is monitored frequently (continually?) such that it dropping below a certain power threshold ends the autostart before the owner/operater does.

I admit though that I base this on anecdotal findings and owner's manual snipetts rather than on definitive multimeter, etc. testing like you have. : - )

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