E28 Charging System: Difference between revisions
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Add E28 charging system Tech FAQ from forum t=155731 |
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== The regulator == | == The regulator == | ||
It is two screws on the back of the alternator. Often rebuildable with new brushes; once in a while they just smoke. Calling it a five-minute job on an E28 overstates it, but it is quick. Brushes are cheap. A spare is a good idea on a long trip. | It is two screws on the back of the alternator. Often rebuildable with new brushes; once in a while they just smoke. Calling it a five-minute job on an E28 overstates it, but it is quick. Brushes are cheap. A spare is a good idea on a long trip. | ||
The battery light is how the alternator first gets field current: a 2 W cluster bulb is only about 0.17 A. Contrast that with 3–4 A once it is running. That is why a worn regulator or a lazy excite can sit there after start until you blip the throttle — residual magnetism and a bit more rpm are enough to start generating. Wiring a ~12 Ω resistor in parallel with the bulb would let it excite at cranking speed, but then the indicator would not work. | |||
Some later clusters put a diode between the alternator and the light, and a resistor so a burned-out bulb no longer means no charging. Early cars with a simple bulb in the D+ line will not charge if that bulb is dead. | |||
== Voltages == | == Voltages == | ||
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|- | |- | ||
| Over ~15 V | | Over ~15 V | ||
| Too high. Bright headlights, short bulb and battery life. An older BMW manual says 14.5 V max; a tenth or two above that is usually fine, half a volt is not. | | Too high. Bright headlights, short bulb and battery life. An older BMW big-blue manual says 14.5 V max; a tenth or two above that is usually fine, half a volt is not. | ||
|} | |} | ||
Modern flooded batteries are happier around 13.7–14.2 V. 15 V will cook electrolyte out of most of them. 1980s batteries were a different chemistry and lived with a slightly lower setpoint; some later cars never quite reach what a new battery wants. Aftermarket adjustable regulators exist, but they have been seen to spike — if you try one, watch it on a meter. One long-term report of a Regitar VRB195 (marked 15 V) ran a steady 13.8–14.0 V. | |||
If running voltage at the battery is higher than sitting voltage, the alternator is working. Low running voltage is more often a regulator or a '''wiring''' problem than a dead alternator. | If running voltage at the battery is higher than sitting voltage, the alternator is working. Low running voltage is more often a regulator or a '''wiring''' problem than a dead alternator. | ||
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A phantom draw (something on that should be off) is a different problem. This page is only the charging system. | A phantom draw (something on that should be off) is a different problem. This page is only the charging system. | ||
== | == Ground straps == | ||
Alternators with isolator / rubber bushings and a '''missing ground strap''' will not charge, even with a known-good alternator and a working battery light. Measure running voltage from B+ to the '''alternator case itself''', not to the battery negative or the body. An M5 with isolator bushings and no strap was "not charging the entire time I owned it" until the strap went back on. | |||
== Cluster contacts == | |||
A no-charge / no-light car with a good bulb and a good alternator has been fixed by pulling the cluster and cleaning / tightening lamp-holder contacts, terminals, and sockets. The light has to actually light to excite the field. | |||
== Several warning lights at once == | |||
ABS, Brake, Park Brake, and Charge all coming on together after start, then going out on the first throttle blip, is a common complaint. On a stock cluster the charge lamp going to ground (D+ not generating) is the only "official" reason for Charge. The others look like independent circuits on the ETM. | |||
In practice: | |||
* Many of those lamps also go through the SI-board processor, so a low-voltage or SI-board fault can light more than one. | |||
* ABS (and possibly brake-lining) modules are voltage-sensitive — they will light the lamp below about 13 V even if the switch itself is fine. Park brake is a dumb switch. | |||
* When it happens, '''measure at the alternator right then'''. More than once the alt simply was not charging, and the other lamps were along for the ride. | |||
* Worn regulator brushes have cured the "all lights until I blip the throttle" version. | |||
== From the discussion == | |||
Additions above are from the [https://forums.mye28.com/viewtopic.php?t=155731 FAQ thread]: [[Mike W.]], [[RossDinan6]], [[austin8753]], [[///M]], [[Chicagotrader92]], and [[tig]]. | |||
[[Category:Tech FAQ]] | [[Category:Tech FAQ]] | ||
