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E34 (1989 - 1995)

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Old 04-25-2010, 08:24 AM
robertobaggio20 robertobaggio20 is offline
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Bad alternator alerts ?

Hi everyone,

My friend ran into a problem with his alternator that i believe is relevant for all of us here.

A little history may be useful before i get to the point. Basically, the alternator failed, and stopped charging the battery. The car ran fine on the battery's charge for a period of time, after which it completely died when the charge became fully depleted. Fortunately, when this happened, the car's door was not locked if not he might not have been able to get in and check it out. The car had to be towed to the wshop for the alternator to be replaced.

This is a real doozy, as its one of those things that happens without warning and can leave you totally stranded somewhere at a super inconvenient time. Towing services may then have a field day with you, as well as workshops which understand that you're unlikely to argue with quotes when you can't easily drive off to the next workshop. Furthermore, an alternator failure will likely damage the car's battery which will could cause inconvenient problems and increase costs down the line. Our batteries are not deep-cycle batteries which can tolerate total discharges. One full premature discharge (i.e. a discharge before the batteries natural death date) will create permanent damage to its reserve charge capacity. I've known of two new maintenance-free batteries which were discharged in full within the 1st year of operations due to light being left on etc. (the cars were jump started and were taken on a long drive and used daily, but the batteries failed again 1 week later, this time without any help from lights being left on foolishly.)

There's an excellent article on how an alternator should function at this link :


Basically, it says that the voltage across a battery's terminals when the engine is off should be around 12.6-12.8 volts, and should rise to between 13.7-14.5 volts once the engine has started. The extra 1 volt you get when the car is running is the charging voltage from the alternator. A higher charge is needed to overcome the battery's internal resistance to an external current. If the voltage while the engine is running is not at those levels, your alternator is probably bad (could be other problems too). Basically, when the alternator is not functioning, the voltage reading when the engine is one should be about the same as when it is off.

It occured to me that it should be possible to rig up a system to alert the driver when the voltage consistently falls below 13.7volts, such as when it falls to 13.2 volts or lower. This alert will enable the driver to test his charging voltage immediately, note any problems, and arrrange his car to be taken to the workshop for a closer inspection. Being stranded unexpectedly is thus totally avoided. This system could be centred around some kind of a fuse, or voltage regulator, or something. It should produce a light on the instrument panel, a chime, noise, or something.

The point of this post is this : how can such an alert mechanism be created cheaply? Any ideas?


Last edited by robertobaggio20; 04-25-2010 at 08:27 AM.
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Old 04-25-2010, 08:41 AM
Tourburgring Tourburgring is offline
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When I was a (much) younger man, quality cars came fitted with Ammeters and/or Volmeters in their dashboards.

The Ammeter gave you an instant indication of power draw (positive or negative) when the engine was running. Switching on the headlights would cause a dip in the Ammeter reading. You could therefore see instantly if your Generator/Alternator was charging sufficiently to keep the battery charged. If it was always in the negative then the battery is being drained.

The Voltmeter gave a readout of the battery voltage and would indicate if the battery was not receiving a proper charge, although it would drop so slowly that you usually only realised there was a problem when it was too late.

I see no practical reason why either of these gauges couldn't be installed in an E34.
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Old 04-25-2010, 08:18 PM
robertobaggio20 robertobaggio20 is offline
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Tourburging, that's a good idea. In fact, its the solution.

The only thing is that these meters are clearly not in the right style for the e34. They would look odd on the dashboard. Is there a way to rig up a system that creates a chime or alarm when the charging current falls below a certain level, and can be hidden away? Or some other way to achieve the same purpose? And cheaply of course.
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Old 04-26-2010, 05:41 AM
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TTCummins TTCummins is offline
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I'm not sure about the circumstances that surrounded the failure of youre friends alternator, but I've seen it 100 times in bosch alternators. I would say out of those 100 times I've seen a BMW not charging 86-92 of the times it has been related to the voltage regulator. It takes two screws to pop out typically and when you do you'll have two copper sticks that are supposed to push out into the commutator on the alternator shaft. This is how the current is transmitted from a spinning shaft to a fixed wire. These two sticks push into the round commutator on the shaft and as long as there is spring life and length available on the sticks you'll get charging as you are supposed to. The problem arises when after 100K miles or so that those sticks wear down to the point where they loose contact or have intermittent contact with the commutator (commutator wears as well but not nearly as bad as the sticks). You swap out the 15 dollar voltage regulator and get yourself another 100K miles out of your charging system, and you don't have to spend the big bucks for a rebuild or a replacement.
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Old 04-29-2010, 11:02 PM
M5ish M5ish is offline
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2x! I believe that the copper sticks are called brushes which comes spring loaded iirc.
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