One of the easiest tasks to perform on your manual transmission Carrera is to change the transmission oil. The Carrera transmission is what is known as a transaxle. It includes all the standard components of a normal transmission, plus an integrated differential. This design is possible because of the rear-engine design of the Carrera. The transaxle design is more compact and theoretically lighter in weight since you don't need a dedicated differential.
The differential and the transmission both share the same lubricating fluid. It's very important to make sure that the fluid in your transmission is at the proper level, otherwise your transmission will experience significant wear. The synchro rings and sliders all depend on a slick surface in order to match speeds when shifting. If your transmission is low on oil, the wear on these components will accelerate significantly. In addition, shifting the car will be more difficult. One of the first things that you should check on a 911 Carrera that is having problems shifting is the level of the transmission oil. Keeping the differential and all the associated gears well lubricated should also help increase your fuel mileage.
The transmission oil also helps to keep temperatures down inside your transmission. The engine is one of the primary sources of heat for the transmission as it conducts and radiates through and around the points where the engine and transmission are mounted. The transmission also creates heat itself as the gears and synchros turn within its case. Keeping the transmission fluid at its proper level helps to mitigate heat problems. Having a large reservoir of oil to spread the heat throughout the transmission helps to keep temperatures down. On some of the higher-performance Porsche transmissions, there is even an external transmission cooler that operates similar to the engine cooler.
I recommend that your transmission fluid be changed every 30,000 miles or about once every two years. This number is a rough estimate and may vary depending upon your use of your 911 (track versus street). There are many moving parts in the transmission, and they have a tendency to drop small microscopic metal particles into the oil. Specifically, the synchro rings wear down slowly over time, actually with every shift. While the transmission bearings are not as sensitive to oil contaminants as the engine bearings, they can still exhibit wear from these particles in the oil.
The Carrera transmissions have two plugs for filling and emptying the transmission oil, located on the side and the bottom of the case. A 10mm hex socket is all that is required to remove both the drain plug and the filler plug.
The first step in checking or filling your transmission is to gain access to the plugs. Jack up all four corners of the car (Pelican Technical Article: Jacking Up Your Porsche 911 Carrera), making sure that the car is perfectly level with respect to the ground. Then remove the rear plastic under body trays.(Pelican Technical Article: Replacing Your Automatic Transmission Fluid on your Porsche 911 Carrera).
If you are simply checking the level of oil in your transmission, start by removing the filler plug on the side of the transmission. This is the plug that you add fluid to. Simply stick your finger in the hole and see if you can feel fluid at the bottom level of the hole. If you can feel the fluid level with your finger, then your fluid level is about right or perhaps will need only a little topping off.
If you cannot feel the fluid level, then you will need to add transmission oil to the case. If you are planning on changing the oil, then remove the plug on the bottom of the transmission case. It's a wise idea to try to empty the transmission oil when the car is warm, as this will make the oil more viscous and it will flow out easier. Make sure that you have a drain pan capable of handling at least four quarts of transmission oil. Check the fluid in the pan to see if you see any unusual metal pieces, or grit in the oil. The transmission holds about 2.8 liters (3.0 quarts).
While the fluid is emptying out, you can use this time to clean out the drain and filler plugs. The bottom drain plug should have an integrated magnet in it that traps metal debris. Using a cotton swab or a paper towel, carefully clean out all of the black debris and particles that may have found their way in there.
Replace the bottom plug on the transmission, but don't tighten it too tightly (18 ft-lbs, or 25 Nm, maximum). These plugs do not have a tendency to leak (transmission oil is thicker than engine oil). If it does leak later on, you can always tighten it a little more. Now, add transmission oil to the case. The best method for this is with a hand-operated oil pump. These are available from most auto parts stores and attach to the top of the plastic transmission oil bottle. They work very similar to the liquid soap dispensers you find in most bathrooms. Pump the transmission case full of fluid until it just starts to run out the filler hole. Replace the filler plug and clean up the few drips that might have run out of the hole. Tighten down the filler plug in a similar manner to the drain plug.
The automatic transmission cars also have a differential built-in to the transaxle. This differential uses the same type of fluid as the manual transmission and must be checked and filled in addition to the automatic transmission fluid. On the automatic cars, there is no drain plug, however, so the gear oil must be drained by loosening the outer differential cover (see Project 39 and Figure 5). Or, you can possibly get around this by inserting a fluid vacuum pump into the fluid fill hole and sucking out all of the old fluid. You top off the fluid and fill the differential in a similar manner to the six-speed transmission. Simply unscrew the plug and fill until the fluid starts flowing out of the hole. The automatic transmission differential uses the same fluid as the manual transmissions and takes about 0.8 liter (0.85 quart). Tighten up the drain plug to 22 ft-lb (30 Nm).
In many cases, generic transmission gear oil that meets or exceeds SAE 75W90 will suffice perfectly fine. Also very effective are the Porsche factory lubricants (typically manufactured by Shell Oil) or Mobil Delvac Synthetic Gear Oil 75W-90. In addition, if you have a limited slip differential (LSD), be sure that you get transmission fluid that is appropriate--using a fluid that is too slippery can reduce the torque bias effects of the differential and make it less effective at distributing torque.
This photo shows a typical six-speed Carrera transmission with both the drain plug (insert upper right, yellow arrow) and fill plug visible (blue arrow). You only need a 10mm hex socket in order to remove either plug (insert upper left).
Shown here is the side of an automatic transmission. The blue arrows point to the differential cover bolts that need to be removed in order to loosen the cover and drain the differential gear oil. If you can't empty the differential by simply loosening the cover bolts, then you need to remove the center bolt and pull out the stub axle. The red arrow points to the fill plug. Top off the fluid to the edge of the bottom of the fill plug, just like on the six-speed transmission (insert lower left). The upper leftPictures shows the differential cover removed.
The six speed Carrera transmission should be filled approximately 11mm below the bottom edge of the fill plug in the side of the transmission. Use a paper clip to measure or you can just stick your finger in and see if you can touch the fluid.
Shown here is the side of an automatic transmission from a Carrera. The green arrows point to the differential cover bolts that need to be loosened to drain the differential gear oil. If you can't loosen the cover bolts then you need to remove the center bolt and pull out the stub axle. The red arrow points to the bottom of the fill plug.