Sooner or later, everyone will have problems starting their 911. 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, it is perhaps one of the most troublesome. Before doing anything drastic like replacing your starter or looking at your fuel injection ECU, you should make sure that your battery is in good condition.
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 the voltage 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 take your battery to your local auto parts store for testing.
While older batteries often exhibited deteriorating performance prior to their failure, I recently had an original OEM BMW battery fail on me in my BMW 5-Series. The car was running perfectly fine—I had just driven about 350 miles the previous day, so it should have been well charged. The next morning, I got in the car, and it started fine. I drove about 3 miles and stopped off to pick something up. I shut off the car and was inside three minutes at the most. When I got to the parking lot, the battery was completely dead. There was not even enough power left to open the power door locks. It did turn out to be a complete battery failure. I was surprised because I’d never had a battery fail like this before—it always seemed to give out slowly. Some of the informal research I’ve done since then seems to indicate that the newer technology used in these batteries tends to lead to this type of occasional failure.
When the car is running, the alternator should be outputting anywhere from about 12.5 volts to about 14.5 volts. If you don’t see any significant change in the voltage after you start up the car, then your alternator could be faulty. If the voltage is high at the battery (around 17 volts or higher), then the alternator’s regulator is most likely faulty and needs to be replaced. Overcharging the battery at these higher levels may cause it to overflow and leak acid all over the inside of your car. See Project 82 for instructions on how to replace the alternator.
Do not ever disconnect the battery ground strap from the battery while the car is running. The battery acts as an electrical capacitor and filter on the entire electrical system, and the car’s electrical components expect it to be there, even if it doesn’t hold a charge. Disconnecting the battery terminals while the engine is running can seriously damage the computers and systems of the car.
Sometimes it may be necessary to reset the computers of the car, for example, to clear error codes. Some books recommend disconnecting both terminals of the battery and touching the unconnected terminals together to empty all of the capacitors and stored electricity in the system. While this works, it can possibly create a quick electrical shock to the system. A trick I learned from Tony Callas of Callas Rennsport is instead to place a resistor across the two terminals or simply use a diagnostic lamp that will act like a resistor. Doing this will slowly dissipate the electricity over a few seconds instead of all at once.
Once you have determined that your battery is fine, you should make sure that your engine 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 an engine ground strap that electrically connects the transmission and engine assembly to the chassis. It’s located on the right side behind the rear wheel (see Figure 1). Check the strap to make sure it’s not corroded or damaged. 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 sandpaper or a wire brush. Doing so will remove any dirt, grime, surface rust, or other corrosion that may interfere with creating a good electrical connection. While you’re at it, clean up the battery terminals as well in a similar manner.
If you have discovered that your battery is weak and needs replacement, then you need to replace it. Follow the steps in the photos of this project. When purchasing a new battery, I recommend the newer-style sealed, or “maintenance free,” types. Sealed batteries may be more prone to damage when you deep cycle them (let them run all the way down), but require less preventative maintenance. Be sure to purchase a battery that has the same group number and CCA rating (Cold Cranking Amps). Check the freshness date on the battery that you are purchasing and avoid any batteries that are more than six months old.
You disconnect the battery by disconnecting the negative or ground lead from the battery. Always disconnect the negative or ground lead first—if you disconnect the positive/hot lead, there is a chance that your tool may touch the metal chassis. This will result in a short circuit, which would be quite dangerous. The worst case scenario would probably be where your wrench hit the chassis and was instantly welded there by the current, and then the battery overheated and exploded, because you couldn’t break the connection. In other words, be sure to disconnect the ground first.
If your car has the original radio in it, be aware that you will need the radio code if you disconnect the battery. This code is typically included with the documentation/owners manual that came with the car. The Porsche dealer can look this code up for you if you don’t have it, but that can be a huge pain, and most dealers will charge you for the service.
Leaving your lights on in your car can seriously damage the battery. Automotive batteries are not typically deep-cycle batteries, which means they do not like to be fully discharged. If you leave your lights on and drain your battery several times, then you will weaken it each time and have to replace it sooner than later. If you need to jump start your battery, then you should refer to the section in your owner’s manual. The procedure is very straightforward and very similar to other cars.
The Carrera has a few power-saving stand-by modes that it will enter after certain periods of time. If the ignition key is removed, accessories such as the trunk lamp, interior lamp, radio, etc., will be switched off automatically after approximately two hours. If the car is locked, then these will switch off after 10 minutes. If the car is not started or unlocked for more than 5 days (7 days on the 2005 and later models), then the remote control standby function is switched off, and you will need to unlock the car using the key.
After you disconnect your battery, the DME computer will lose some of its history memory used for adapting the fuel injection system. As a result, the idle may fluctuate, and the fuel injection mixture may be slightly off as the car relearns its settings.
Probably the best way to protect your battery from drain and damage is to install a trickle-charger/battery maintainer on it. This charger plugs into the wall when you are not using the car, and constantly monitors the battery, charging it as needed when the voltage runs down. Every car battery has internal electrical leakage that will cause it to become fully discharged over time if not properly maintained. A trickle charger can keep your battery fresh year-round, even if you don’t drive the car for months at a time. Beware of cheap chargers though, as they can accidentally overcharge your battery, causing more harm than good. The trickle charger I like to use in all of my Porsches is the Battery Tender, available for $40–$60 at PelicanParts.com.
The infamous drivetrain ground strap is one of the easiest parts on the car to overlook, yet it can cause so many electrical troubles. Since the transmission and engine are insulated by rubber mounts, 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. When I was working my project Boxster for one of my other books, I forgot to install the ground strap. When I went to start the car, it wouldn’t start, and instead, the current ran through the right side stainless steel brake lines and completely cooked the line. That was a new experience for me—don’t forget your ground strap!
Always disconnect the black colored negative or ground connection first from the battery post (green arrow). If you are not planning on removing the battery, then this connection is all you need to disconnect—there is no need to disconnect the positive/hot lead to the battery. When you disconnect the ground from the battery, make sure that you place or tape the ground lead aside—you don’t want it accidentally falling on the terminal of the battery while you’re working and effectively connecting up the battery again. If you are replacing the battery, be sure to properly hook up the vent hose once you have installed it in place (blue arrow). The yellow arrow shows the wire for the Battery Tender harness. You can route it through the fire wall (see Figure 3 of Pelican Technical Article: Installing Litronic Headlamps on the Porsche 911 Carrera), or you can simply zip-tie it to the plastic mesh located on the right side.
If you are removing the battery, then simply loosen and remove the hold-down clamp that attaches the battery to the chassis (yellow arrow). Stand inside the trunk and lift the battery out of the car from there (they are quite heavy). The inset in the upper left-hand corner shows the permanent attachment of the Battery Tender cable to the negative lead. A handy device I like to install on all my cars is a battery cutoff switch (lower inset). Installation of this switch on the battery ground allows you to remove the green knob and shut off all power to the car. An added tip—connect a small inline fuse from one end to the other, and a small amount of current will continue to flow, keeping your radio and DME from being cleared out when the battery is disconnected.
Whenever I pick up a new car, I almost always install one of these within the first few weeks. The Battery Tender is a necessary tool if you’re not planning on driving your car everyday. It plugs into the wall and trickle-charges the battery so that it won’t run down. Although the kit comes with alligator clips for temporary installations, I prefer to hard-wire the charger into the battery, and simply leave the charging unit in the bottom of the front trunk.
Comments: Kind of an idiot question here. I've got a deltran battery tender with alligator clamps for my 997. What's the proper way to connect the clamps to the battery? Currently, I have the positive clamp connected to the positive battery post, and the negative clamp connected to a nearby bolt/ground on the chassis in the battery compartment. Should the negative clamp instead be connected to the negative terminal of the battery? Does it make any difference?
February 3, 2015
Followup from the Pelican Staff: There are NO idiot questions!! This is actually a good question. If you leave the wires connected all the time it does not matter. If you connect the alligator leads to the battery each time you connect the charger it is safer to connect the Red clip to B+ first and then connect the black clip to a ground connection away from the battery in case of arcing, you don't want to ignite any gasses coming from the battery case. - Casey at Pelican Parts
Comments: Would like to replace a dead battery on my 1999 996 cabriolet. Need to get front hood open first. Don't have the red posts in my fuse box. I heard of an emergency hood latch somewhere near the front end . Can any one help?
January 24, 2015
Followup from the Pelican Staff: The emergency release is behind the left side of the rear bumper. Once the rear bumper is release, you can add a jump pack to the jump start points and power up the vehicle to open the front hood. - Nick at Pelican Parts
Comments: Need to know the amperes that the 911 uses thankyou
October 29, 2014
Followup from the Pelican Staff: What year and what part? - Nick at Pelican Parts
Check out some other sample projects
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