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#1 (permalink) Old 12-15-2008, 12:21 PM
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flickering battery light

I have a 1999 540i, it died on the road, bad alternator/battery, replaced both but the battery indicator still flickers. Alternator is charging (measured 14.25 @ battery terminals w/DMM) battery voltage is 12.5v (engine off). When key is in position 1 the battery indicator is faint (not bright). Additionally when reading the charge voltage on the OBC it indicates 13.6 to 13.8 volts. Any help would be greatly appreciated
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#2 (permalink) Old 12-15-2008, 01:01 PM
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By the sound of it you've got either a battery drain or a bad earth, where it is, is a matter of ilimination.

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#3 (permalink) Old 12-16-2008, 02:37 PM
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Thanks for the info. If this is a ground fault, it may explain the sudden and extreamly fast failure of my charging system (happened during a sever rain storm) this took less than 5 mins (and 2 miles) to completely dischage (everything off). Does anyone have a complete schematic/wiring diagram for the '99 540i (to hunt down the fault)?
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#4 (permalink) Old 12-17-2008, 04:26 AM
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If it happened in a rain storm, underneath somewhere,if you get the car up you will see earth straps, check the earth from batt to body and take the cover of the DME big black box under the hood at the rear, someone on here had theres with loads of water in after a heavy storm, dried it all out and all is well.

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#5 (permalink) Old 12-17-2008, 11:19 AM
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This vehicle has the water cooled alt. the ground is made at the housing to engine interface when I installed the new alt. I cleaned the engine side (surface) with scotch bright (did not use silicone sealent) I did not, however clean the alt side sealing surface (did not want to mess with the O-ring gasket) I will try to add an additional ground strap to see if that improves the problem. I had only checked the ground at the battery (it was good) and I don't know what the "DME" is, but will see if I can find the ground straps and check them. Thanks for the help and for more clues to this freaky problem.
FYI New feature!! started car turned on headlights, they blow out, hit high beams, they blow out. Finally, turned on fog lights, they blow out also. And when I say blow out I mean all bulbs are BURNED OUT! (measured with an DMM). Still points to a bad ground, I think?
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#6 (permalink) Old 12-17-2008, 11:52 AM
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DME is situated at the rear of the bay, big flat black box, mine has star bolts holding the lid on, take the lid off and check for water etc.

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#7 (permalink) Old 12-17-2008, 10:13 PM
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On the 540i there are two (2) black boxes, one on each side of the engine above the valve covers. they have snap on lids and contain point to point wiring. both are dry. the one on the drivers side also contains a diode that attaches to one of the voltage regulator leads (the one that makes the battery light flicker) this had a loose connection but the diode was OK. I think if this became open while running it may have allowed the regulator to overvolt my lights? I am nearly at a dead end, I think I have checked all ground connections, I have measured and remeasured my battery and alt. both seem within spec. disconnected oll wires to the alt and measured the (+) with a diode check it appears fine the same holds true for the one regulator wire. however I am not sure what the second wire does.
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#8 (permalink) Old 12-18-2008, 05:20 AM
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Did you replace with a brand new Alt ?

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#9 (permalink) Old 12-18-2008, 02:28 PM
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Bought a rebuilt unit, and a new battery. found the DME beneath the pass side cabin filter. all was dry, cleaned and tightened two (2) ground wires that exit the DME. still no change. to recapp, voltage at alt @ idle nothing on 14-14.1Vdc however with everything on (driver side seat heater, steering wheel heater, blower on full, radio, parking lights)voltage starts at 14 and continues to drop (stopped @ 13Vdc). If I rev the engine (1000-1500 rpm) voltage returns to 14Vdc. FYI also tested the alt for leaky diodes, with everything disconnected power out terminal measured .495v and OL/open in reverse. At the regulator measured .505v and OL in reverse. No Leaks. still don't know what second regulator wire is for? I think I am near my limit, may need to take it somewhere
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#10 (permalink) Old 12-18-2008, 02:44 PM
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Pull the cluster, just to make sure the batt light bulb is secure in its socket!!! you never know it might be the bulb on its way out!, might sound strange but when you turn the ign switch to pos II is the batt light bright red and doesnt flicker??

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Last edited by marti5; 12-18-2008 at 02:50 PM.
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#11 (permalink) Old 12-18-2008, 08:00 PM
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Thanks for the tip, I wish it was that simple. have already verified the the light is bright at switch pos II . additionally I have taken it out for several very short trips (couple of miles, on a dry day, in the light) but if the engine isn't rev'd and I happen to be using any accessories (heat) it starts to shudder. I did take several other measurements, disconnected the batt ( + & - ) and the alt, then measured the resistance of the power cable (to see if damaged) measured 0.0 Ohms (end to end), and checked to see if it had a sneak path to chassis, it measured open. Also measured the regulator output, if I measure from the regulator lead (at the diode) to chassis, I get 13.5Vdc (alt @ power out measures 14Vdc), however if I measure that same wire on the other side of the inline diode (away from the alt) I measure 14.0Vdc same as power out (I guess this is the lead from pin 1?). What is the diode for? What reading should I get from pin 2? any other sugguestions before I cave in and get professional help (for my car)?
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#12 (permalink) Old 12-20-2008, 02:21 PM
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Voltage spike at low beam socket (bad LCM?)

I just found some very interesting test data. I was testing the load response on the charging system (at the low beam socket) when the car was cold (at idle approx. 500RPM) voltage started out at 14.25 (everything off) turned on all accessories voltage started to drop, when down to 11.2 (less than 2 min) engine began to “shudder” then the rpm kicked up to approx 850rpm. Voltage went to 13.8 and was steady. During this testing I turned off and on the headlights (and to my surprise) DMM read OL then >20.5Vdc then to 13.8. Repeated this sequence approx. 15x’s (same results) however after the 15th or so cycle voltage STOPPED peaking, and I cannot repeat the same results (I am starting to think a bad/wet LCM). In addition, I also measured voltage decay (using the OBC test 9) with the engine off. Voltage starts at 12.5 (right after turning engine off) then quickly drops to 11.7 (nothing on). Then starting turning things on (parking lights, radio, interior lights, REST button) voltage dropped to 11.2 and stayed there (longer than 10 min). I am not sure (at this point) if the battery is responding properly, I don’t think the voltage should drop as low as it does, and then hold steady at 11.2 (bad cell?). Also I don’t know what a CEL is. Again all comments/suggestions are (and have been) greatly appreciated!
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#13 (permalink) Old 12-20-2008, 02:36 PM
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CEL ? not sure what you mean, as for voltage 11.2 is V good with all on.

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#14 (permalink) Old 12-21-2008, 04:18 PM
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Not quite sure whats going on with your car but here's what my 94 tds did last week when the battery died.In the last month it gave 3 possible signs that not all was well with the battery by only just managing to turn over and start,one cold start and two on a warm engine,the rest of time it behaved and was fine.Last thursday go out at 11am and it will not turn over just clicking from starter,I took a battery voltage reading of 18.6 !!,this was after sitting all night in the frost and 5 seconds attempting to start-very odd.I checked all earths etc and starter wiring and cranked again,no go and now voltage read 14.7 ??.After holding voltage tester on terminals for five minutes the voltage slowly dropped to 12.22.Why it should drop down slowly I do not know ?,cranked the car for a few seconds and took another reading,this time 17.5 ish volts and again dropping slowly.I fitted a new battery and all is ok,the alternator checks out fine and I can only presume the battery had a dead cell and my thinking is it had voltage but not the amps if that is possible.
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#15 (permalink) Old 12-28-2008, 01:13 PM
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Bad "new alternator"

I the final analysis I replace the alternator and all seems fine. In review, even though the voltages (at the battery and alt.) were reading correctly it appears that the regulator was the culprit. On the 1999 540i one of the two wires coming from the regulator is the battery (charging system) light. If there is a voltage differential (as I had questioned earlier in a different site) quote, “ALSO MEASURED THE REGULATOR OUTPUT, IF I MEASURE FROM THE REGULATOR LEAD (AT THE DIODE) TO CHASSIS, I GET 13.5VDC (ALT @ POWER OUT MEASURES 14VDC), HOWEVER IF I MEASURE THAT SAME WIRE ON THE OTHER SIDE (AWAY FROM THE ALT) OF THE INLINE DIODE I MEASURE 14.0VDC SAME AS POWER OUT”. This is where the light was getting its power (should have tipped me off). Additionally, when I replaced the new alt with the new(er) alt. I lightly sanded (with emery cloth) both sides of the alt to chassis interface (taking extreme care not to damage the o-ring). I also vigorously wire brushed the bolts completely (beneath the head and on both sides of the attached washer). This interface is critical, it is the return ground and even .001 ohms of resistance could drop 20 watts of power at full load! (140 x 140 x .001= 19.6W), I had read that someone had added silicone sealant to this joint, and I think that’s not a good idea. And thanks again for the help!
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