electrics/ battery

[willberforce]

Hi all, I have a 88 325I. I drive it 25 miles and work for 7.5 hours when I get to the car it only just turns over and starts? and yet sometimes it will be ok for 2 weeks. battery and alternator have been changed both bosch items. the only thing constantly on is the clock. any ideas? is it worth paying for an auto electrician to check it over? thanks for any help.

[Billwill]

Check the battery connections are clean and tight, the negative lead going onto the chassis is clean and tight and the grounding strap that grounds the engine to the chassis is clean and tight at both ends.

Could be the Black/Yellow wire coming from the ignition switch has a bad connection...either at the starter solenoid itself or from a failing ignition switch. You can short out this Black/Yellow wire to the large terminal on the solenoid that carries the thick cable coming from the battery positive/alternator....this will prove if you have a bad ignition switch or bad solenoid. I have fitted a hidden push switch that takes +12 volts direct from the battery to this Black/Yellow wire to use in an emergency when my ignition switch goes a bit funny.

You need to check the voltage across the battery when you are having a problem to see if the battery still has all its charge or if something is draining the battery. With ignition OFF you should get about 12.5 volts across the battery. The voltage should not drop below about 10 volts while turning the engine over and while at idle you should get about 13.5 volts.

[Brianmoooore]

All the above, then disconnect the battery negative lead and connect a milliammeter between the lead and the battery pole, with everything off to see what the static current drain on the battery is.
Open a window first, in case you have an immobiliser that locks the doors when then battery is connected.