BMW 318/325/M3/525/535/M5 1989-1993 Repair Information




Several precautions must be observed with alternator equipped vehicles to avoid damaging the unit. They are as follows.

If the battery is removed for any reason, make sure it is reconnected with the correct polarity. Reversing the battery connections will result in damage to the electrical system.
When utilizing a booster battery as a starting aid, always connect it as follows: positive to positive, and negative (booster battery) to a good ground on the engine.
Never use a fast charger as a booster to start vehicles.
When servicing the battery with a charger, always disconnect the battery cables.
Do not short across or ground any of the terminals on the alternator.
Turn off the ignition switch and then disconnect the battery terminals when performing any service on the electrical system or charging the battery.
Disconnect the battery ground cable if arc welding is to be done on any part of the vehicle.


When faced with the possibility of a charging system problem, there are certain tests that can help find the cause. Before embarking on a tear down of the entire charging system, check the obvious. Many charging system problems are caused by bad connections that are either loose or dirty. Many times just going over all the battery, alternator and cable connections can solve a problem.

Check the battery for the correct electrolyte fluid level. Check that the battery cable connections are not corroded and are tight. Clean and tighten the connections if necessary. Refer to Routine Maintenance for battery procedures.

Check the alternator belt for tightness and proper tensioning. If the belt is slipping, the alternator will not be able to put out a full charge. Check the belt for signs of slippage and glazing. Do not use belt preparations to cure slippage or glazing; replace the belt with new if necessary.

If the alternator light does not illuminate when the ignition key is turned to the on position, check the indicator bulb. Disconnect the D+/61 wire from the alternator, it will be marked on the alternator body. Ground this wire with the ignition key turned to the on position. If the alternator light does not come on, check the indicator bulb and related wiring. If the bulb does light, there is most likely an internal problem with the alternator.

If the alternator light comes on with the ignition switch in the off position, there is an internal problem with the alternator. The diodes are damaged and the alternator should be rebuilt, replaced or have the diode plate replaced.

If the alternator lights comes on or glows with the engine running, check the alternator charging voltage and amperage. With the engine running at 1500 rpm, the voltage should be 13.5-14.2 volts. The amperage measured with an inductive meter clipped over the battery cable or a non-inductive meter connected as instructed by the manufacturer, should be at least 15 amps. If the amperage is low, switch on the high beams and other heavy current users. The amperage should go up with each device turned on. If the amperage stays the same, the alternator is not charging and should be replaced.

If the alternator light is not illuminated while the engine is running but the alternator still does not charge the battery or doesn't charge it enough, check the connections at the alternator and battery. Check the condition of the battery, alternator belt and wiring. If no obvious cause is found, check the alternator charging voltage and amperage as above. Replace the alternator if the voltage or amperage is incorrect.

If no problems with the charging system or alternator can be found, the cause of the problem will most likely be the battery. Replace the battery as necessary.


The fan belt tension is adjusted by moving the alternator on the slack adjuster bracket. The belt tension is adjusted to a deflection of approximately 1 / 2 inch under moderate thumb pressure in the middle of its longest span. On many engines, the position of the top of the alternator is adjusted via a bolt that is geared to the bracket. This bolt is turned to position the alternator and determine tension, and then is locked in position with a lock bolt. On engines with a serpentine belt, the self tensioner will automatically adjust the tension.


See Figures 1, 2, 3 and 4

  1. Disconnect the negative battery cable. Remove the air cleaner assembly. Remove the fan and cowl on the M5, 525i with M50 engine and 325i with M50 engine.
  3. Disconnect the wires from the rear of the alternator, marking them for installation. Note that there is a ground wire on some vehicles. On the E30 325i and M3, it may be easier to remove the alternator mounting bolts, turn it, and then remove the wires.
  5. Loosen the hose clamp on the alternator cooling duct, if equipped. Remove the cooling duct.
  7. Loosen the lock bolt, turn the tensioning bolt so as to eliminate belt tension and then remove the belt. Remove the bolts and remove the alternator.

Click image to see an enlarged view

Fig. Fig. 1: Alternator mounting showing the adjuster bolts 318i shown

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Fig. Fig. 2: Loosening the serpentine belt on M50 engine

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Fig. Fig. 3: The tab on the tensioner must be fit into the slot on M50 engines

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Fig. Fig. 4: Removing the fan by holding the pulley with tool 11 5 030 or equivalent and turning the left hand threads of the fan nut with tool 11 5 040 or equivalent

To install:
  1. Install the alternator in position and install the retaining bolts.
  3. The tensioning bolt on the front of the alternator must be turned so as to tension the belt, using a torque wrench, until the torque is approximately 5 ft. lbs. (7 Nm). Then, hold the adjustment with one wrench while tightening the locknut at the rear of the unit. Make sure, if the unit has a ground wire on the alternator, it has been reconnected.
  5. Replace the fan and cowl. Install the air cleaner and connect the hoses. Install the alternator cooling duct. Connect the wires to the alternator.