turbolizard 38 Report post Posted May 1, 2010 Broke down today with flat battery that was too discharged to run the car. I put in a fresh battery and got home OK, but voltage is dropping over time and fresh battery is not being charged. Car is e34 540i. Symtoms: 1: Charge warning light in dash not showing at any time (before start or after) 2: Voltage dips as engine is started and then remains at 11.8V with engine running (too low to be charging) 3: Voltage will slowly drop with time as engine runs and draws power from it I wanted to check the alternator is faulty before pulling it out so I.. 1: Checked warning bulb in gauge cluster (tested OK) and swapped with known good bulb in cluster (seems OK) 2: Checked for voltage on pin 12 in diagnostic connector (exciter wire) - has voltage with key on 3: Checked continuity of exciter wire to alternator from pin 12 (OK) 4: Checked for voltage at alternator exciter on back of alternator (has voltage with key on) 5: Jumper to alternator exciter circuit from battery positive (alternator did not charge) 6: Grounded exciter circuit to check light in gauge cluster (light did not come on) I thought after step 5 that the alternator was faulty, but then step 6 indicates a problem in the exciter line doesn't it (but all checks on that line seem OK). I would be surprised to have both problems at the same time. What do I do, or check, next? Pulling the alternator looks like a major pain. Quote Share this post Link to post Share on other sites
turbolizard 38 Report post Posted May 2, 2010 Following on... Today I did some more diagnosis and found one of the problems - I DID have a bad bulb in the instrument cluster (don't ask me how it tested OK yesterday and swapped with another - I don't know!). After replacing that bulb I can get the light to come on by grounding the exciter circuit at the alternator, so the exciter circuit looks OK - I get 330mA to ground. If disconnect the alternator from the exciter circuit by pulling out the unloader relay and then ground the exciter circuit at Pin 4 on the unloader relay, the dash indication lights up and I get 250mA flowing - seems about right. The thing that seems a bit strange is that the current to ground seems higher with the alternator in the circuit - does that mean there is an additional current contribution through the alternator circuit? I think there is something odd within the alternator that was keeping the voltage high on the exciter side so that the light did not illuminate at any time. Possibly some fried diodes I guess. In any case I am convinced enough that it is worth pulling the alternator from its hiddy hole and that the problem is not within the exciter circuit. PLEASE tell me quickly if I am wrong. Quote Share this post Link to post Share on other sites
bezzal 1 Report post Posted May 2, 2010 Is there any current being drawn from the battery when everything is shut down? I have had a short circut in an alternator, that gave similar symptoms. Changed the alternator and was all good. FYI was in a Pajero not a BMW. Following on... Today I did some more diagnosis and found one of the problems - I DID have a bad bulb in the instrument cluster (don't ask me how it tested OK yesterday and swapped with another - I don't know!). After replacing that bulb I can get the light to come on by grounding the exciter circuit at the alternator, so the exciter circuit looks OK - I get 330mA to ground. If disconnect the alternator from the exciter circuit by pulling out the unloader relay and then ground the exciter circuit at Pin 4 on the unloader relay, the dash indication lights up and I get 250mA flowing - seems about right. The thing that seems a bit strange is that the current to ground seems higher with the alternator in the circuit - does that mean there is an additional current contribution through the alternator circuit? I think there is something odd within the alternator that was keeping the voltage high on the exciter side so that the light did not illuminate at any time. Possibly some fried diodes I guess. In any case I am convinced enough that it is worth pulling the alternator from its hiddy hole and that the problem is not within the exciter circuit. PLEASE tell me quickly if I am wrong. Quote Share this post Link to post Share on other sites
jochen 4 Report post Posted May 2, 2010 Note that Glenn has a policy of always fitting a brand new battery when replacing the alternator The theory is that a failing battery can load the alternator too much and thus cause the alternator to fail prematurely Given the low cost of batteries compared to the alternator, it would be a wise move. You then get a car that starts reliably again Quote Share this post Link to post Share on other sites
hotwire 352 Report post Posted May 2, 2010 Following on... Today I did some more diagnosis and found one of the problems - I DID have a bad bulb in the instrument cluster (don't ask me how it tested OK yesterday and swapped with another - I don't know!). After replacing that bulb I can get the light to come on by grounding the exciter circuit at the alternator, so the exciter circuit looks OK - I get 330mA to ground. If disconnect the alternator from the exciter circuit by pulling out the unloader relay and then ground the exciter circuit at Pin 4 on the unloader relay, the dash indication lights up and I get 250mA flowing - seems about right. The thing that seems a bit strange is that the current to ground seems higher with the alternator in the circuit - does that mean there is an additional current contribution through the alternator circuit? I think there is something odd within the alternator that was keeping the voltage high on the exciter side so that the light did not illuminate at any time. Possibly some fried diodes I guess. In any case I am convinced enough that it is worth pulling the alternator from its hiddy hole and that the problem is not within the exciter circuit. PLEASE tell me quickly if I am wrong. You are complicating the diagonsis somewhat. If no alt light on dash with ign on - earth the exciter wire at the alt (as you have done) if it illuminates when earthed - fault is in alternator - (assumming alt has other supplies to it present). This wire leads through the alt field circuit to earth - bringing the light on. When alt is charging - voltage from field circuit opposes ign supply to bulb - extinguishing the light, No output from alt - no voltage - light stays on. Equally - as you have - open circuit in field circuit - no light at all Quote Share this post Link to post Share on other sites
turbolizard 38 Report post Posted May 4, 2010 Thanks for the help. It was certainly enough to convince me it was worth pulling the alternator out. What a PITA that was. Anyway, alt is now out and being looked at by auto electrician. Quote Share this post Link to post Share on other sites
kiwi535 538 Report post Posted May 4, 2010 Thanks for the help. It was certainly enough to convince me it was worth pulling the alternator out. What a PITA that was. Anyway, alt is now out and being looked at by auto electrician.hope it all is solved easily now Quote Share this post Link to post Share on other sites
