1993 E31--Which of the 2 Batteries Starts the Engine?

**Denis G** · 05-09-2022, 10:08 PM

850Ci sat for a day...not enough juice to turn the engine over.
All lights/locks/accessories work fine.
I had switches put on both batteries...I need to know which one should be shut off to preserve it for starting the engine.

Thanks!

**ben325** · 05-10-2022, 02:05 AM

Both batteries are used in parallel to start the engine (and for accessories). You may not disconnect one of them, because the batteries must have the same charging level.

**rjjablo** · 05-10-2022, 07:19 AM

Have you replaced Capacitors in General Module?

Have you replaced both fusbile links in Trunk?

**Denis G** · 05-10-2022, 09:28 AM

Originally Posted by ben325

Both batteries are used in parallel to start the engine (and for accessories). You may not disconnect one of them, because the batteries must have the same charging level.

Thanks....that's what I was afraid of

- - - Updated - - -

Originally Posted by rjjablo

Have you replaced Capacitors in General Module?

Have you replaced both fusbile links in Trunk?

Yes, everything checked or replaced.

**rjjablo** · 05-10-2022, 09:36 AM

Start Chasing Battery Drains

Verify main ground from RF Motor Mount to body, this caught me hard once

**Denis G** · 05-10-2022, 09:45 AM

Originally Posted by rjjablo

Start Chasing Battery Drains

Verify main ground from RF Motor Mount to body, this caught me hard once

Will do...thanks for the advice!!!

**Auraraptor** · 05-10-2022, 10:45 AM

As alluded to, E31 do not have a dedicated starting battery, rather both are in parallel.

I would also start w the main ground and verify both batteries are good. E31 can be picky w borderline batteries

Then stepwise find what module is stuck on / causing the drain.

**Denis G** · 05-10-2022, 10:59 AM

Originally Posted by Auraraptor

As alluded to, E31 do not have a dedicated starting battery, rather both are in parallel.

I would also start w the main ground and verify both batteries are good. E31 can be picky w borderline batteries

Then stepwise find what module is stuck on / causing the drain.

Batteries were tested and good, I will double check main ground and had not considered that a module may be stuck on...I'll check that as well, good idea...thanks!!

**ben325** · 05-10-2022, 12:32 PM

I have had à battery drain issue à few weeks ago: The wiper pressure control relay was always driven (the relay remain hot). I didn't find the cause, but may be linked to the fact that the batteries were disconnected. The relay stopped only after I used the windshield washer function, and my issue was solved.

The fisrt thing to do is to check that your car is going to the sleeping mode. You can check it this way: turn on the spot lightnear the rear mirror. It may switch off automatically after 16 minutes. If not, your car do not go into sleeping mode...

**Denis G** · 05-10-2022, 12:48 PM

Originally Posted by ben325

I have had à battery drain issue à few weeks ago: The wiper pressure control relay was always driven (the relay remain hot). I didn't find the cause, but may be linked to the fact that the batteries were disconnected. The relay stopped only after I used the windshield washer function, and my issue was solved.

The fisrt thing to do is to check that your car is going to the sleeping mode. You can check it this way: turn on the spot lightnear the rear mirror. It may switch off automatically after 16 minutes. If not, your car do not go into sleeping mode...

Valuable insight...I had used the windshield wipers the day before this happened...first time I had used them in weeks...I'll try the washer function as well...thanks!!

**shogun** · 05-14-2022, 12:13 AM

Premature Battery Discharge
Model: BMW E31 850 CSi with EWS 11 produced from January 1995 up to September 1995.
Complaint: When the vehicle has been parked for a few days, the vehicle may not start, due to the vehicles battery not maintaining an adequate charge level. The vehicle owner may also experience a humming noise from the EML actuator motors after the engine is switched off.
Remedy: Install the Isolation Relay Kit (PN 99 00 0 002 354) between the circuit of DME1 and its main relay, and the circuit of DME2 and its main relay.

from here http://www.bba-reman.com/gb/content....s_and_problems

Hints and Tips http://www.bba-reman.com/gb/content/hints_and_tips/ BMW Technical Help
Faults on BMW engine management
Faults on BMW ABS systems
Faults on BMW wiring harnesses
Faults on BMW sensors
Faults on BMW electrical systems auxiliary equipment
Faults on BMW starter motors
Faults on BMW alternators
Faults on BMW engine electrical systems

**Denis G** · 05-14-2022, 11:55 AM

Originally Posted by shogun

Premature Battery Discharge
Model: BMW E31 850 CSi with EWS 11 produced from January 1995 up to September 1995.
Complaint: When the vehicle has been parked for a few days, the vehicle may not start, due to the vehicles battery not maintaining an adequate charge level. The vehicle owner may also experience a humming noise from the EML actuator motors after the engine is switched off.
Remedy: Install the Isolation Relay Kit (PN 99 00 0 002 354) between the circuit of DME1 and its main relay, and the circuit of DME2 and its main relay.

from here http://www.bba-reman.com/gb/content....s_and_problems

Hints and Tips http://www.bba-reman.com/gb/content/hints_and_tips/ BMW Technical Help
Faults on BMW engine management
Faults on BMW ABS systems
Faults on BMW wiring harnesses
Faults on BMW sensors
Faults on BMW electrical systems auxiliary equipment
Faults on BMW starter motors
Faults on BMW alternators
Faults on BMW engine electrical systems

Thank you....I will share this directly with my mechanic.