SL/R230: Battery Control Module Normal Operation
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Senior Member
Joined: Jan 2015
Posts: 390
Likes: 115
From: Seattle, WA
SL500 & GL450
Battery Control Module Normal Operation
I've a failing starter Battery and had to have a replacement special ordered since nobody in the area had stock on that odd V4 group battery size. After returning to the car today in a untrafficed area, the car would not start. I'd alway been under the impression that in an emergency, the BCM would divert power from the consumer battery to assist an underpowered starter battery, but this was not the case today. Since I was pretty confident that I'd not see another motorist any time soon, & AAA said it would likely be 2 hours or more to get one of their contactor tow trucks to me, I decided to run jumper cables from my consumer battery to the starter battery. The car started just fine, but I now have concerns that the BCM may be malfunctioning?
Does anyone know for certain if the BCM is indeed supposed to divert power from the consumer battery to assist in a no start scenario? The car exhibits no charging issues & I hooked up a multimeter to the starter battery terminals to confirm the alternator was producing a proper charge.
Does anyone know for certain if the BCM is indeed supposed to divert power from the consumer battery to assist in a no start scenario? The car exhibits no charging issues & I hooked up a multimeter to the starter battery terminals to confirm the alternator was producing a proper charge.
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Joined: Jan 2006
Posts: 1,982
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From: Atlanta, unfortunately
'09 S600, (2) R129 300SLs, '03 SL500, '03 SL55
While the starter battery can assist a weak consumer battery, the BNS module is not programmed for the reverse scenario. However, the following procedure can be used for a strong consumer battery to start the engine with a weak starter battery.
Disconnect the ground cable to the consumer battery. Turn the ignition key to position 2. For c. 30 seconds the BNS will put the starter battery electrically in parallel with the cables to the consumer battery by energizing the "battery isolation" relay. Reconnect the ground for the consumer battery and start the engine.
Disconnect the ground cable to the consumer battery. Turn the ignition key to position 2. For c. 30 seconds the BNS will put the starter battery electrically in parallel with the cables to the consumer battery by energizing the "battery isolation" relay. Reconnect the ground for the consumer battery and start the engine.
Thread Starter
Senior Member
Joined: Jan 2015
Posts: 390
Likes: 115
From: Seattle, WA
SL500 & GL450
While the starter battery can assist a weak consumer battery, the BNS module is not programmed for the reverse scenario. However, the following procedure can be used for a strong consumer battery to start the engine with a weak starter battery.
Disconnect the ground cable to the consumer battery. Turn the ignition key to position 2. For c. 30 seconds the BNS will put the starter battery electrically in parallel with the cables to the consumer battery by energizing the "battery isolation" relay. Reconnect the ground for the consumer battery and start the engine.
Disconnect the ground cable to the consumer battery. Turn the ignition key to position 2. For c. 30 seconds the BNS will put the starter battery electrically in parallel with the cables to the consumer battery by energizing the "battery isolation" relay. Reconnect the ground for the consumer battery and start the engine.
