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Monday, May 27, 2024

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Doug Vetter, ATP/CFI

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BMW E36 Alternator DIY

(Image: New alternator installed)
While BMW alternators are quite reliable they don't last forever.
Fortunately, when they do fail, replacement is easily within the reach of the DIYer.


This DIY outlines the labor required to replace the alternator on an E36 BMW equipped with an M52 6 cylinder engine. The procedure and principles involved may be applied to several other BMW models including the E46.


The most obvious symptoms of a failing alternator are:

  1. All interior and exterior lights flickering simultaneously.
  2. Red battery annunicator in gauge cluster illuminated after engine start.

Most other electrical symptoms can be traced to a failing (or failed) battery. Since battery failure is far more likely than alternator failure, some basic troubleshooting is in order.

To confirm the state of the battery attach a volt ohm meter (VOM) to the battery terminals with the engine off. A fully charged battery will read approximately 12.6 volts. If the voltage is a few tenths lower than that, it's possible that the battery is simply discharged and might respond to a charge cycle. If the voltage is considerably lower, i.e. 10.5, particularly after a charge cycle, it is very likely that the battery has an internal short and must be replaced.

If the battery ultimately takes a charge sufficient to start the vehicle, it's important to perform another voltage check at the battery terminals with the engine running. This reading should be approximately 13.8 volts. The exact voltage is subject to temperature conditions and the state of the battery, but if the voltage with the engine running is not higher than the voltage with the engine off, the alternator is not charging the battery.

In this case, the alternator may be at fault and the vehicle should not be driven until the cause is determined because the vehicle may stop running without warning when the battery discharges below a point sufficient to power the DME.




Alternators provided by BMW are remanufactured in Germany. They do not include a pulley but do include a new wave washer and 24 mm pulley retaining nut. This means you'll need to transfer the existing pulley to the new alternator or buy a new pulley. I recommend the purchase of a new pulley just in case it is not possible to remove the pulley from the old alternator with tools on hand. Given the cost of the pulley, it makes no sense to buy special tools such as a bearing splitter to remove the pulley.

Be very wary of remanufactured Bosch alternators sourced elsewhere as they may contain cheaper parts. Also be aware of the significance of the terms "remanufactured" and "overhauled". Typically, "remanufactured" means replacement of all wear items (bearings, slip rings, etc.) whether they need it or not, while "overhauled" means that only the parts worn beyond the minimum specification are replaced and everything else is reworked but not replaced. This can reduce the cost of the overhaul and thus the price of the unit but they are not the same thing. Make sure you ask the vendor exactly what you're buying or you may wind up doing this job again soon.

Part Highlights

Removal / Disassembly Procedure

  1. Remove the cruise control unit. Remove the electrical connector from the cruise control unit by twisting the outer body of the connector counterclockwise 180 degrees and then pull to remove it. There are two (2) 10 mm nuts with captive washers that serve double duty -- they fasten both the cruise control unit and the airbox to the vehicle. Remove those nuts and then carefully fold the cruise control unit back over the shock tower to a point near the fuse box. It will stay there for the remainder of the procedure. No further disassembly is required.
  2. Remove the airbox by disconnecting the captive clips connecting it's outlet tube to the mass airflow sensor (MAF). Being careful not to misplace the large o-ring that mates the outlet tube and MAF, pull the box forward and then upward and out of the vehicle. Set it aside somewhere safe.
  3. Remove the alternator cooling duct. The duct is fastened to the alternator with a large hose clamp and to the forward air duct with a press / interference fit. Simply loosen the hose clamp and then pull the duct clear.
  4. Remove the accessory belt from the alternator pulley. A 16 mm socket connected to a long breaker bar will maximize the leverage required on the tensioner pulley. Simply slipping the belt off the alternator pulley is sufficient. The entire belt need not be removed, but this would certainly be a good time to replace the belts if they have been in service more than 60K miles.
  5. Disconnect the battery. The battery MUST be completely disconnected from the vehicle's wiring BEFORE removing the alternator. Using a 13 mm shallow socket remove the negative terminal followed by the positive. Put each in a spare glove or other insulation to prevent either wire from contacting anything conductive.
  6. Remove the two bolts that fasten the alternator to the oil filter housing. On vehicles equipped with an idler pulley next to the alternator, the top bolt that fastens the alternator is shared with the idler pulley. Therefore, removing the top bolt also removes the idler pulley. As you remove the pulley, take note that a small nub on the rear of the pulley keys with a notch on the alternator to prevent the pulley base from rotating. The lower bolt is dedicated to fastening the alternator. Once both bolts are removed, the alternator will be free from the oil housing but will remain connected to the vehicle by its wiring.
  7. Disconnect the wiring from the rear of the alternator. The wiring is protected by a rubber boot that mates with the plastic housing on the rear of the alternator. Tilt the front of the alternator down enough to gain access to the rear of the unit and then pry the boot away from the housing. Disconnect the field wire by using a deep 10mm socket to remove the retaining nut. Disconnect the output wire from the unit using a 13mm deep socket. The BMW remanufactured unit came with new nuts and wave washers designed to prevent the nuts from loosening. If the replacement part you obtained lacks these parts, be careful removing the retaining nuts. Do not lose them.
  8. Remove the pulley from the old alternator. Assuming the replacement alternator came without a pulley installed, the pulley from the old unit will need to be transferred to the new alternator. The alternator pulley retaining nut is 24mm and must be loosened with a relatively thin wall socket to prevent interference with the inner diameter of the pulley. With the nut and wave washer removed, the pulley should pull off the shaft by hand. If the pulley refuses to budge, obtain a bearing splitter or other device with sufficient leverage to remove it from the shaft.

Installation / Reassembly Procedure

  1. Install the pulley on the new alternator. Use a hammer and a 22mm socket as a drift to press the pulley onto the shaft of the new alternator and then install a new wave washer and retaining nut. Then install the new wave washer and nut provided with the new alternator. Torque the nut to 45 Nm or simply apply an air wrench for a few seconds to ensure the nut is sufficiently tightened.
  2. Install the alternator wiring. The smaller nut that secures the field wire should be torqued to 7 Nm (5 ft*lbs) while the larger nut should be torqued to 15 Nm (11 ft*lbs). Make sure the boot is properly reinstalled to protect the wiring.
  3. Install the alternator using the two retaining bolts. Clean the bolts with some brake cleaner and then torque the bolts by feel, or to a maximum of 40Nm or 30 ft*lbs. Over-torquing the fasteners may pull threads out of the oil filter housing and increase the cost of the repair considerably. Take it easy.
  4. Install the accessory belt. Make sure that the belt is aligned in all pulleys before releasing the belt tensioner.
  5. Install the alternator cooling duct. I recommend positioning the hose clamp such that it can be easily reached by a screwdriver when everything is reassembled because this duct must be removed for several other jobs.
  6. Install airbox and cruise control unit.
  7. Reconnect the battery. Connect the positive terminal first followed by the negative.
  8. Start the engine and test the charging system. Connect a VOM to the battery and confirm the alternator is charging the battery. If this is the case the voltage will typically be between 13.8 and 14.2 volts.

Procedure Highlights


Replacement of an alternator is a simple task well within the reach of a DIYer. In fact, it's a good first project.

Normally I'll try to buy the least expensive parts I can find provided I know the parts are of equal qualty. The problem with alternators is that it's impossible to know the quality of the part you're getting without cracking it open. Since I had no plan to do that and had no time to screw around, I bought from a reputable source.

While I tend to buy a lot of parts online now I bought this at my local dealer in order to simplify the core credit process.