Car battery goes flat overnight

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Yorkshire Engineer
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Joined: Tue Dec 20, 2016 5:14 pm

Car battery goes flat overnight

Post by Yorkshire Engineer »

Car battery goes flat overnight. No obvious hot spot to explain where the power went. Amp meter in battery lead shows a constant drain of about 1 Amp this damages modern lead acid batteries. You may need to replace battery soon.
Other symptom: Windows may occasionally close unexpectedly whilst driving.
Cause: Power Windows sub-system is not going to sleep after 5 seconds of inactivity.
Test: remove power window fuse in fuse box by driver's right knee.
Probably root cause: 'All windows up' switch attached to passenger side door key lock is still active when key is in the near vertical position at which it can be removed.
Solution: remove lock and adjust operating angles of this switch.
Background: Each door has its own windows control computer that goes to sleep when there is no activity. The 'all windows up' signal is a low priority command that is ignored once window is up but if it persists prevents computer from going to sleep resulting in a power drain. A 1 Amp drain that persists overnight causes permanent damage to modern lead-acid starter batteries. Professional battery test will show battery capacity has dropped to about 8 AmpHrs from the 40 or so of a new battery.
Fault can be intermittent depending on how you turn and remove door key.
Applies to cars with central locking and power windows.
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planetc
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Re: Car battery goes flat overnight

Post by planetc »

Probable cause is a poor secondary contact on the drivers door switch. In all the years I worked with them I only saw one that wasn't and it had damaged wiring.
"No the temperature gauge doesn't work........
we've driven 150 miles today........
the heater went cold last Thursday........
they check the level when it's serviced don't they?"
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