This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
One of the nice things about the configuration of the 911 engine is the relative ease with which you can replace the alternator. The alternator is located directly in front of the fan: a great location because of the amount of cooling air it receives. The replacement and repair process is straightforward, and should take you about an afternoon to complete.
The first thing that you need to do is to make sure that your alternator is indeed the cause of the problems with your charging system. Sometimes bizarre electrical problems can be caused by a number of faults other than the alternator. It's important to troubleshoot the system prior to replacing your alternator.
The first thing to check is the fan belt. Is it tight and amply turning the fan? If not, then retighten it, according to the procedures outlined in Pelican Technical Article: Fan Belt Replacement. Modern fan belts rarely break, but they get brittle and glazed with age, and can slip on their pulleys. Replace it with a new one.
The next item to check is the voltage at the battery. This should read a little more than 12 volts with the engine off. When the car is running, the voltage should read at in the range of 13.5 to 14.5 volts with the engine at 2000 rpm. If your battery appears to be leaking, then your voltage regulator has probably failed. The battery will usually only leak acid if it has been overcharged at a much higher voltage. If the voltage measured at the battery is more than 16 or 17 volts, then the regulator is probably bad. If your battery has boiled over and has acid overflowing out the top, make sure that you clean up any spilled acid immediately. Dousing the area with a water and baking soda solution should help considerably to neutralize the acid, and prevent it from eating away at the metal.
The following guidelines are useful for troubleshooting your alternator:
- Alternator indicator lamp is on with the ignition key off.
- Alternator has failed, and diodes have shorted out. Replace alternator and in the meantime, disconnect battery to prevent it from becoming completely drained.
- Alternator indicator lamp does not light when ignition key is turned on.
- Alternator bulb has burned out.
- Regulator has failed
- DF wire is disconnected or broken from alternator
- Alternator has failed, and internal windings are open
- Indicator lamp remains on after engine is started and running above 2000 rpm
- Regulator has failed
- Battery lead to alternator has been disconnected
- Alternator is not firmly grounded
- Alternator internal bushings are dirty or worn
- Alternator is bad
- Indicator lamp is dim after starting, and gets brighter as engine rpm increases.
- Battery lead to alternator loose or bad
- Ground connections are bad
- Battery is bad
- Alternator is bad due to open diode failure
- Indicator lamp is dim after startup, but eventually gets dimmer and goes out when engine RPM increase.
- Low charged battery
- Poor connections to battery
- Indicator lamp is dim after startup, and gets dimmer as rpm increases, but never goes out.
- Regulator has failed
- Alternator is failing
An important item to check on your car is the transmission ground strap. The engine is electrically isolated from the chassis by rubber motor mounts. If the transmission ground strap is missing or disconnected, then you might have a whole bunch of problems, including electrical system malfunctions and difficultly turning over the starter. See Project 87 for the exact location of the transmission ground strap.
All 911s up to 1981 had external regulators and from 1982-on, the alternators had built-in ones. If you've determined the regulator to be working properly, then you should probably remove the alternator for testing and inspection. Before starting any work, make sure that you disconnect the battery. The positive battery terminal is directly connected to the alternator, and it can be dangerous to work on if it's live.
The first step is to remove the fan belt. Refer to Project 3 for detailed instructions on the fan belt removal. Removing the fan housing itself is a very easy process. The entire assembly is affixed to the engine case by a metal band that is tightened around the fan. Loosen this band by removing the one or two large cap screws that secures it. There should also be two nuts that secure the coil, and a few fiberglass shroud screws that need to be removed. Once the assembly is loose, the fan can be pulled away from the engine far enough to access the nuts and electrical connections in the rear. Be careful not to pull too hard, because the wires that are connected to the alternator are not very long.
Once you have access to the rear of the fan, remove the plastic air flow guide that surrounds the alternator. You should now have enough access to disconnect and remove the wires from the rear of the alternator. Make sure that you mark them, and/or take a picture of the entire assembly so that you know where each wire goes when it's time to reconnect them. You can destroy your alternator, and portions of your car's electrical system if you hook the connections up improperly. Also make sure that the alternator was properly grounded to the engine, as this can cause it to malfunction. The ground strap is a thick copper cable that connects the housing of the alternator to the engine case. Disconnect this ground strap before attempting to remove the alternator.
After the alternator is disconnected electrically, you should be able to remove it along with the fan assembly from the car. The fan is pressed onto the alternator (ultimately held on with the pulley nut) and the alternator is bolted into the fan housing. If you are planning on replacing just the brushes on the alternator, then you can remove them now without removing the alternator from the housing. On a side note, almost every Porsche parts diagram has the brushes mislabeled as a 'support.' There must have been something lost in the translation from German!
Remove the brushes by unscrewing the small assembly from the back of the alternator, and inspect them to see if they are significantly worn. If so, then replace them (they typically cost about $50). If you think that you need to replace your entire alternator, then gently tap it out of the housing with a soft hammer. The fan is pressed onto the shaft of the alternator, and needs to be gently removed. Don't bang on the alternator shaft too heavily, as this may damage the bearings inside.
The installation of the new alternator is simply the reverse of the removal process. Make sure that you reconnect all of the wires to their proper terminals when you are done.
In 1975, Porsche implemented a five-blade fan instead of the normal 11-blade one on all the 911 motors. The purpose of this fan was to provide less cooling to the engine, forcing it to run hotter. This was accomplished solely for the task of meeting emissions requirements. The hotter the engine, the better the gases burned, and fewer emissions were produced. However, this didn't help the longevity of the engine. These engines overheated easier, and generally lasted significantly less than their predecessors. One upgrade that should be performed on every 2.7 liter motor is the replacement of the original five blade fan with an eleven blade one. The two fans are interchangeable, and can be switched when replacing or removing the alternator. The only other part that may need to be replaced is the pulley half - it must be matched specifically to the size of the fan. Also, different sized fans and pulleys used different length belts: make sure that you get the one that's appropriate for your setup.
The alternator is attached to the fan and the fan housing. The housing is strapped to the engine case using a long metal strip that needs to be loosened and disconnected before the housing can be removed. Remove the coil and the few select screws that attach the fiberglass engine shroud to the fan assembly. The long metal strap does not need to be completely disconnected from the case: leave it attached to the case at the bottom.
Once the housing is disconnected from the engine, remove the rear cream colored plastic air guide that surrounds the rear of the alternator. This piece is held on with nuts that are attached to studs located on the alternator. Be careful not to crack this piece when you are reinstalling it. Using too much torque on the nuts can easily damage the air guide upon reinstallation.
Carefully label and disconnect the connections to the alternator. Remove the large, thick ground strap as well. Be aware that the wires are quite old, are subject to heat from the engine, and may be more brittle than you would think. Try to avoid moving them around or bending them too much. Inspect the connectors carefully, and replace any that look damaged or rusted. Double check the wires upon reinstallation. It's very important not to confuse and mix up the terminals to which the wires are connected.
The alternator should slide out of the housing. It may need some gentle coaxing, but be careful not to damage the studs that mount it to the housing. Some light taps with a soft hammer should force it loose from the housing. Try to tap evenly around all of the studs: it's easy for the alternator to get jammed inside the fan housing.
The fan is pressed onto the alternator shaft, and requires a bit of effort to remove. Don't use too much force in prying the fan off, or the bearings inside the alternator may become damaged. If you have any doubts, take the fan to your local machine shop and have them use a press to remove it. Now would be a good time to upgrade to the 11 blade fan if yours doesn't have one already installed.
Some of the 911 alternators have brushes in the rear that can be easily removed and replaced. Remove the brushes and inspect them carefully if your alternator is not working well. If the contacts are as short in length as they are in the photo (shown by arrow), then they are worn and need to be replaced. If they are long, then you probably have some other internal problem with the alternator that is causing it to malfunction.