Alternator failure is not a very common occurence. And even though BMW alternators are very reliable, they will not last forever. Fortunately, an alternator in an E36 is quite easy to replace.
The symptoms easiest to spot are:
- Exterior and interior lights flickering simultaneously
- A red battery light on the gauge cluster after starting the engine
It is easy to mistake alternator symptoms with faulty battery symptoms, so testing of the battery should take place. With the vehicle off, attach a volt/ohm meter to the battery terminals. A properly charged battery will read around 12.6 volts, if your battery is substantially lower than this (ex: 10.5 volts), even after a successful charge cycle, (with a known good alternator) it is likely that the battery has an internal short and needs to be replaced. If your battery has sufficient charge to start the vehicle, test the battery while the car is running. It should read around 13.8 volts, but if the reading with the engine running is lower than the reading with the engine off the alternator is not charging the battery. In this case it is not safe to drive the car, it may stop running without any warning.
- New or remanufactured alternator. You need to determine if you have a Valeo alternator or a Bosch alternator, and replace with the according manufacturer.
- Alternator pulley
- New battery - if needed
- Breaker bar and 16mm socket to remove serpentine belt
- 16mm deep socket
- 10mm & 13mm deep sockets
Removing the Alternator
- Firstly, remove the cruise control unit to the right of the intake box. The two 10mm bolts with washers both attach the cruise control unit and the air-box to the vehicle. Be sure to keep track of the two 10mm nuts and washers.
- Remove the air-box. By disconnecting it from the MAF, it should lift out of the engine bay.
- Remove the alternator cooling duct attached via a large hose clamp. Simply loosen and lift.
- Using the 16mm socket on the tensioner pulley, loosen the serpentine belt and simply slip it off the alternator pulley. If your belts have more than 60k miles of service life, I recommend replacement of the serpentine belt.
- Fully disconnect the battery. It's important that the battery is fully disconnected. Remove the negative terminal first, then the positive terminal.
- Next, remove the 2 bolts that attach the alternator to the oil filter housing.
- Cars that have an idler pulley next to the alternator - The top bolt attaching the alternator actually goes through the idler pulley. So in removing the top bolt, you are removing the idler pulley. The lower bolt only attaches the alternator, and once removed the alternator will be free from the oil filter housing.
- Disconnecting the wiring - Looking at the back of the alternator you will notice a rubber boot sealing against the plastic housing. Peeling this back will expose the field and output wire. Disconnect the field wire first via the 10mm nut, then the output wire via the 13mm nut.
- Typically the new alternator doesn't come with a pulley, and if you didn't purchase a new one you will have to remove the pulley from the old alternator and reuse it on the new alternator. A 24mm socket is needed to remove the pulley.
- You need to install the old or new pulley onto the new alternator. Getting the pulley onto the shaft may take some force, so using a hammer and a piece of aluminium or wood may be necessary to drive the pulley on the shaft. Once on, torque the nut to 33 ft/lb.
- Next, reconnect the wiring. The field wire nut should be torqued to 5 ft/lb, and the larger output wire nut torqued to 12 ft/lb.
- Reinstall the two retaining bolts. Be sure to clean the bolts before installation. If you removed an idler pulley before, be sure to install it with the top retaining bolt. Torque both bolts to a maximum of 30 ft/lb, the threads in the oil filter housing can be easily stripped, so take it easy when tightening.
- Next, reattach the serpentine belt. If you only slipped the belt off the alternator pulley and left the rest untouched then simply slip the belt over the alternator pulley. If you fully removed the serpentine belt, no worries, here is a diagram:
- Now, re-install the alternator cooling duct. Simply slip over and tighten screw clamp.
- Next, install the intake box and cruise control unit. No torque is specific to the two 10mm nuts, just make sure they are tight and wont allow the two units to jiggle.
- Re-Connect the battery. You want to start with the positive terminal first, then the negative.
- Only thing left is to start the car and test the charging system. Connect a Volt Ohm Meter to the battery just as above, and you should be getting a reading in-between 13.8-14.2 volts.
Since we had to disconnect the battery for a prolonged time, your car's stereo is probably asking you for an INPUT CODE. This is a 5 digit theft deterrent. The code will either be in the car's original user manual or on a card in the glove box. If you cannot find the code you can call almost any BMW Dealership and request the code. You'll need to provide them the serial code of the radio unit.
There are two ways to get the radio's serial code. One is to remove the radio from the dash, finding the code printed on the back of the radio. If you don't want to go that direction, there is a much simpler way. Hop in the car and turn the key into the "ON" position - do not start the car, this is the key position before that. Next, hold the "m" button on the radio face until a message pops up on the radio screen. You may have to hold the button for longer than 10 seconds, so be patient. It should read "SN" followed by your Radio Serial Number. With this number you can retrieve your input code and be listening to tunes again, with your fresh new alternator installed for miles of driving excitement.
Tony lives in Spokane, Washington and is a 3-D Design Engineer, machinist, and welder. When he’s not at work he is designing and fabricating parts for his M3, and other BMWs alike.