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.
At one time or another, everyone will have problems starting their 911. It's almost inevitable with older cars of any make, and Porsches certainly are not exempt. What do you do when the car won't crank? There are a few steps and procedures that you can follow to track down the problem. It is important to note that the subject of this project deals solely with the starter motor and it's electrical system. If the car is cranking, and won't fire up because of a fuel or ignition system problem, then you'll have to check some of the other troubleshooting projects in this book.
The first place to look for trouble in your starting system is your battery. The battery is perhaps the most important electrical component on the car, and due to its design and nature, is perhaps one of the most troublesome. Before doing anything drastic like replacing your starter, you should make sure that your battery is in good condition.
Is the battery more than five years old? Have you accidentally left the lights on and drained it down considerably? Are your lights dim? Does the battery sit for long periods of time when you don't drive the car? Do you need to charge it up every two months or so? If you answered yes to any of the previous questions, then you should take a long, hard look at the condition of your battery. Lead-acid batteries are not meant to be drained completely. If your battery has been drained, then most likely it has been damaged. The exception is a deep-cycle lead acid battery like the ones used in marine applications. These can be drained significantly, and still return to function properly.
Begin by checking the voltage on the battery posts using a voltmeter. Place the meter's probes on the posts of the battery, not the clamps. This will give the most accurate indication of the voltage in the battery. A normal battery should read a voltage slightly above 12 volts with the car sitting still, and no electrical devices on (the small trunk light in the front trunk shouldn't make a difference in this reading). A typical reading would be in the 12.6 volt range when the battery is fully charged. If the reading is 12 volts or less, then the battery needs charging or needs to be replaced with a new one. To be certain, you can usually take your battery to your local auto parts store for testing.
When the car is running, the alternator should be outputting anywhere from about 12.5 volts to about 14 volts. If you don't see any significant change in the voltage after you start up the car, then your alternator or voltage regulator could be faulty. If the voltage is high at the battery (around 17 volts or higher), then the regulator is most likely faulty and needs to be replaced. Overcharging the battery at these higher levels will cause it to overflow and leak acid all over the inside of your car.
Once you have determined that your battery is fine, you should make sure that your transmission ground strap is properly installed. The engine and transmission are mounted to the chassis using rubber mounts. While great for the suspension, the rubber mounts make lousy electrical conductors. To compensate for this, there is a transmission ground strap that electrically connects the transmission and engine assembly to the chassis. To accurately assess the condition of the ground strap, you need to crawl underneath the car after it's been jacked up, and take a look at the bottom of the transmission. If the strap is corroded or damaged, it might be best to install a new one. Make sure that you clean both ends of the strap and the areas that it mounts to on the chassis. With all electrical connections, it's a good idea to clean the area that you are mounting to with rubbing alcohol, and also to sand the area lightly with some fine grit sand paper. Doing so will remove any dirt, grime, surface rust, or other corrosion that may interfere with creating a good electrical connection.
Another problem area for starting is the starter, of course. The starter is a somewhat complex device for what would seem to be a simple task. There is a solenoid on the starter that both actuates the small gear that turns the flywheel and switches on the main starter motor. It is important to throw in a note of caution here. The starter motor is connected directly at all times to the positive terminal of the battery. If you accidentally touch the terminals of the starter with a metal object that is grounded, you will quickly generate a lot of sparks, heat, and enough current to fry your alternator and a large chunk of your electrical system. I have heard of two separate occasions where a person was working on their car, and their watch touched the terminals of the starter and the chassis ground. This literally caused the watch to become welded to the chassis of the car! The lesson: exercise caution in this area. Don't wear any jewelry when working on the car, and always disconnect the negative terminal from the battery.
Another potential problem is the starter teeth on the flywheel or ring gear. If the starter seems to engage and spin up with a high-pitched whirring sound, then it is likely that the starter is not fully engaging the flywheel. This is especially prevalent with intermittent problems where sometimes the starter will work fine and then other times it will spin freely. The fix for this is to inspect the flywheel teeth, and to replace the flywheel or ring gear when the engine is out of the car.
When 911s have trouble turning over the starter motor, it is often because there isn't enough current to fully trigger the solenoid on the starter. This can be caused by a number of reasons. The most common reason is old wiring in the car. As the car ages, the wiring has a tendency to lose some of its electrical conductibility. This can be caused by the wires getting bent or crimped, or it can also be triggered by the constant heating and cooling of the wires. This tempering of the metal within the wires can directly affect their conductivity. With age often comes corrosion, and as we can see simply by looking at the Statue of Liberty, copper corrodes quite easily, leaving a light green layer that doesn't conduct very well.
The solution is to track down the problem in the wiring and fix it. Very often, tracing back the electrical connections from the starter, and carefully cleaning all the contacts will improve the situation significantly. While this can be a time consuming process, it's really the only way to track down these electrical gremlins. Chances are if you are having wiring conductivity problems with your starting system, then it's probably affecting other electrical systems as well.
The primary method of tracking down bad connections is to test them with an electrical multi-tester. Test the resistance across lengths of wires and connections in your car and look for any that are significantly higher than others. Chances are with a little cleaning of both the wires and the contacts the system will improve.
The electrical portion of the ignition switch is another source of trouble. This small part often wears out and fails after many years. Some symptoms of this problem is an ignition switch that requires a lot of force to start the car. Another symptom is headlamps that flicker on and off when you wiggle the key. For the replacement of this switch, see Pelican Technical Article: Replacing your Ignition Switch.
If you cannot find the problem causing your starting woes, there is a potential solution called a hot start relay kit. While this solution will work by bypassing some of the faulty wiring, it is basically a Band-Aid put on a much bigger problem. The hot start relay kit takes the power from the starter cable that is connected to the battery and uses it to turn over the solenoid. The relay is powered by the electricity that travels through the âfaulty' wiring. Since the hot start relay only requires a few milliamps of electrical current to operate, it fixes the problem of starting the car. However, as mentioned previously, if you have electrical starting problems you probably have additional electrical problems elsewhere. These are best tracked down and fixed, instead of glossed over.
Finally, if you have determined that the problem lies with your starter, you should replace it. The starter is relatively easy to replace: simply disconnect the electrical terminals and unbolt it from the transmission. See Pelican Technical Article: Engine Removal, Engine Removal, for a few more details on the removal of the starter.
You can replace your unit with a genuine Bosch rebuilt unit, or for about the same cost, you can opt for a quality aftermarket unit. The aftermarket starters are higher torque starters, weigh less, use newer technology, and are generally a really good bet for placement in your car. In most cases, the cost between the OEM and aftermarket starters is negligible. Compare the two before you make a decision. If you want to go with a completely stock 911, then stick with the Bosch unit. Otherwise upgrade to the aftermarket starter.
Another popular option for upgrading your starter is to install a 1.5 HP Bosch unit. The early 911s shipped with the Bosch 17X starter which is rated at only 3/4 HP. The later style Bosch 68X unit is a bolt-in replacement and offers much better cranking than the original 17X starter. New Bosch units are not available, but rebuilt ones can be found if you have a good used core.
The infamous transmission ground strap is one of the easiest parts on the car to overlook, yet can cause so many electrical troubles. Since the transmission and engine mounts are insulated by rubber, the ground strap is the only significant ground to the engine. If the ground strap is disconnected or missing, then the current that turns the starter must travel through the engine harness or other small points of contact. Needless to say, this situation usually doesn't provide enough current to start the car. The white arrow points to the location where the transmission ground strap attaches to the transmission. The yellow arrow shows the chassis mounting point.
While upgrading to a high torque starter shouldn't be your first step in troubleshooting a difficult starting 911, it can be a worthy upgrade to a starter motor that has genuinely worn out. The aftermarket starter motors offer higher horsepower, and are also significantly lighter than their factory counterparts.