Engine Cranking Question

[Rahneld]

Hi:

If answered--please show me where, and apologies.

Research tells me that while the engine starter's connected to the main battery, it won't draw power from it to crank the engine unless it gets the "go-ahead" to do so from the Starter Relay: which is connected to the Aux/ESS Battery? (Correct?)

Question: If so, why do you imagine that the Starter Relay wasn't connected to the main battery when designing the ESS system? This question may have a solid answer that my ignorance questions.

The ESS battery is there, when energized, to permit ESS events, running the vehicle electronics when the engine is stopped at, say, a traffic light, right?

If it lacks juice, certainly, as designed, an ESS shouldn't occur, but cold start JL failure too???....N1<-->N2 jumpering notwithstanding?

Thank you.

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[Jebiruph]

On a properly functioning system, both batteries are connected together and both power everything, except during an auto stop when the batteries are separated. As to how it is wired, the starter solenoid relay switches main battery power to the starter solenoid to activate the starter, but the relay is powered by the aux battery, so both batteries are involved, but it's only relevant for starting from an auto stop.

 

[Rahneld]

Thank you sir. I'd like to ask this. If I was designing the electrical system I might have put the starter relay on the main or both batteries so lack of (adequate) juice in the ESS battery while on the road wouldn't cripple the JL from cold cranking.

Do you disagree with this premise (I could be wrong, or legitimate reasons could exist to make this a bad idea that I don't understand why it is as such.)

Yes--both "batteries power everything" as you say....except the starter relay. : - (

Would allowing the starter relay to run on the main or both batteries have been so bad? A dead ESS battery, to my way of thinking, should involve nothing more than the ESS system not engaging and a warning message being displayed, not crippling a vehicle---deference to your N1 - N2 jumper comments (which I read and loved) notwithstanding.

Thank you.

 

[Jebiruph]

Thank you sir. I'd like to ask this. If I was designing the electrical system I might have put the starter relay on the main or both batteries so lack of (adequate) juice in the ESS battery while on the road wouldn't cripple the JL from cold cranking.

Do you disagree with this premise (I could be wrong, or legitimate reasons could exist to make this a bad idea that I don't understand why it is as such.)

Yes--both "batteries power everything" as you say....except the starter relay. : - (

Would allowing the starter relay to run on the main or both batteries have been so bad? A dead ESS battery, to my way of thinking, should involve nothing more than the ESS system not engaging and a warning message being displayed, not crippling a vehicle---deference to your N1 - N2 jumper comments (which I read and loved) notwithstanding.

Thank you.

Both batteries power everything, including the starter and all relays. The power control relay that connects the batteries is designed so that even when the batteries are dead, both batteries are still connected to everything. The issues seems to be with a bad or partially charged aux battery that can cause unpredictable electronics, so yes that is probably an unintended system design issue. I think the aux battery was designed to just be a helper for the main battery, like Gilligan helps the Skipper. But then like Gilligan, it causes way more trouble than it should.

 

[Rahneld]

Both batteries power everything, including the starter and all relays. The power control relay that connects the batteries is designed so that even when the batteries are dead, both batteries are still connected to everything. The issues seems to be with a bad or partially charged aux battery that can cause unpredictable electronics, so yes that is probably an unintended system design issue. I think the aux battery was designed to just be a helper for the main battery, like Gilligan helps the Skipper. But then like Gilligan, it causes way more trouble than it should.

...I thought the batteries were temporarily isolated upon cold crank and the the starter relay depends upon the ESS battery at these times.

...and if I was right---and I guess I'm not, I wasn't sure if this was cold, ESS crank, or both.

I thought this from situations where cold cranks against only the ESS battery were successful, while cold cranks against only the main battery were not.

Thanks.

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