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Thread: Battery warning light

  1. #1
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    Battery warning light

    Hello, I am new to the forum but have seen many very clued up responses to a multitude of problems, so I am going to try my luck with a real teaser that is driving me nuts. The car is a 2000 E38 (735i) located in the UK with 225,000 miles on the clock but with me as the only owner for the past 18 years. Showroom condition and running perfectly, apart from a recent glitch that I will describe.

    The alternator ( water cooled ) began to sound noisy and was thus asking to be replaced. 2 days later on my regular journey to work the car stalled and the battery warning light came on. I restarted without problems and continued the journey but warning light stayed on.

    I was concerned that I was draining the battery and so got home in daylight and stuck the car in the garage. Purchased a Bosch reconditioned fully warrantied alternator and fitted it and , at the same time, purchased a new Bosch battery.

    The battery is testing each morning at 12.7v or thereabouts before starting the car.

    When the car is started ( which happens with no problems or dramas ) the battery is showing 13.9 - 14.0v on tick-over - thus receiving a good charge from the new alternator.
    The alternator is also showing the same voltage at its output terminal.

    The OBC is showing a charge rate of 13.7-13.9 on the test No 9.

    Thus everything is doing what it should do.
    Apart from the fact that the battery warning light will not extinguish.

    No fault codes coming up on diagnostics, tick-over running perfectly, no battery drain . . . . . . .


    I am going to remove the binnacle and see if that can be tested in case there is a binnacle problem that is causing the warning light to stay on .

    has anyone had a similar problem ? I have read about replacing alternators, batteries, V belts etc etc but this is all done and 100% OK. The car is fine to drive but it is a very uneasy experience to have the warning light on permanently.

  2. #2
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    I will move it to the E38 forum from 7 series general forum
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  3. #3
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    If the light is on but dimmer than before the alternator replacement I would suspect a faulty diode in the Bosch Alternator.

  4. #4
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    There was someone on here who got 2 or 3 new alternators in a row from their parts store that still turned on the battery light.
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  5. #5
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    Quote Originally Posted by psjr View Post
    There was someone on here who got 2 or 3 new alternators in a row from their parts store that still turned on the battery light.
    Yes I read that as well - with a heavy heart as it is pretty difficult to go back to a dealer and just keep giving back seemingly good alternators with no real proof that there is an internal problem. Also when they come all boxed up literally looking brand new having been rebuilt and resealed by Bosch I really need evidence that this is causing the problem. Whilst not being 100% clued up about the role of the diodes, I assume that they affect the regulator performance in terms of deciding what voltage needs pushing out dependent on the load being drawn by the ancillaries on the car.

    So if the OBC says that there is a constant and reliable 13.7 -13.9 v being received from the alternator and the test from the battery says the same, I scratch my head as to what could be wrong in the alternator.

    My real regret is that when I took off the old regulator that had done 225k miles, albeit getting noisy, I did not check its output as I was certain that it must have been causing the warning light to come on. For all I know that one was still charging correctly and the sudden light up was due to whatever I am searching for now.

    My hope is that there is a short in the binnacle near to the warning light, which is causing the problem.

    Another symptom, which I had not mentioned - the warning light is always bright but every now and then on start up is goes off for 20-30 seconds - that is very rare but it happens and that makes me think a broken/burnt wire is just touching and then breaking connection again.

    The wizard I need is the one that knows what exactly happens to the thin blue and green wires that exit the alternator and head up to the warning light. I can track them up to the RH cable bank on top of the RH cam cover - they then cross to the LH cam cover cable bank and then head around the rear of the engine into the ECU - what next ? can I locate the pin in the multiplug behind the binnacle to see if the voltage is still intact to that point ? at what point is the OBC reading the voltage ( I assume in the ECU ) because that voltage needs to be going to the light to tell it to extinguish , I assume.

    Sorry about the ramblings and ruminations but I have to assume the alternator /regulator is OK while all voltages being emitted are spot on and no battery drain and no starting or running problems being experienced.

    Thanks for any guidance.

  6. #6
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    An update following my battery warning light problem - still no resolution but the binnacle has been out and tested and at the same time all pixels repaired and blackened bulbs replaced. The warning light operation has ben tested by checking that it goes out when the alternator feed voltage exceeds the battery voltage - Pin 3 black multiplug carries the alternator voltage signal. So binnacle has been eliminated from enquiries and restored to new for a very small amount of money. Having spoken to a clued up Bimmer engineer he tells me that he has seen many looms give trouble between the rear of the engine and entering the ECU. the engine movement and vibration can cause damage. So now I am going to run an independent cable from the alternator to the entry point on the ECU - or maybe to the Pin 3 on the back of the binnacle - to see if I can extinguish the warning light. I have also been told I can borrow a slave loom from a Bimmer Indy to test that as well but that is probably an overkill.

    Still interested to hear any opinions - thanks

  7. #7
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    This thread was not exactly swamped with suggestions, probably as it is a very boring topic, but for completeness I am reporting on the outcome which now appears to be pretty common to E38s. after tracking the 2 instrument wires from the alternator ( the large cable goes to battery ), they enter the ECU compartment and enter the DME. When I pulled out the DME to check the connections in the back it was apparent that there had been either some water ingress or condensation or something to cause the centre 40 pin multiplug ( there are 5 multiplug bays ) to corrode. This corrosion has resulted in a couple of pins to have disintegrated completely and it is clear there has been a short across the multiplug.

    Before dismantling the plug I waggled the connections and the warning light went out - hurrah. Now the DME is out and has been sent to ECU technologies for the rear pin connector bays to be unsoldered and replaced with a new connection fitting, which will have new pins etc. I am hoping that there is no sign of damage within the DME but even that can be superficial as long as there has not been a major meltdown. The position of the DME bay is totally stupid as the bonnet water drain drops straight onto the top of the box. This must cause condensation or worse direct water ingress.

    I am very pleased to have solved the problem and await the return of the DME with fingers crossed ( their charge is very reasonable IMO - circa £150 ). So I suggest that anyone with an E38 should open the ECU compartment in the engine bay and examine the connections, pulling each of the connector multiplugs and applying some electrical lubricant to try to avoid the common problem of the pins rusting away .

  8. #8
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    Well done for getting there - but check this:

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