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 items on the 911 almost guaranteed to fail is the odometer. Perhaps the most predictable failure on the entire car, the odometer is also one of the most visible. Two distinct design flaws with the odometer coupled with the fact that many 911s have passed the 100,000 mile mark add up to a lot of broken odometers.
The good news is that they can be fixed. The bad news is that the parts to fix them are not readily available. There are some things that you can do to fix them however, with a few tools and couple of hours to spare.
The VDO odometer used in all 911s and many other German cars, has two distinct flaws in its design that can cause its premature failure. One of the problems with the odometer is that some of the important internal gears are made of plastic. With age, the odometer can stick, and if it does, then these plastic gears will grind and become destroyed. The other design flaw involves a pot-metal gear that is pressed onto the main odometer shaft. After many years, this gear often comes loose on the shaft, refusing to turn the odometer any more.
If your odometer has stopped working, yet your speedometer still works, the cause is probably one of the two faults described above. The first step in repairing the unit is to remove it from the car. Follow the procedure described in Project 88 (Replacing the Speedometer) for instructions on removing it from your dash.
Take the unit and place it on a workbench where you will have plenty of room. The first step is to remove the outer retaining ring that secures the face of the gauge. This removal procedure must be performed very carefully, otherwise the edge of your gauge will look like your dog has chewed on it. To remove the outer ring, place a small screwdriver in-between the side of the gauge and the retaining ring. Gently push out with the screwdriver and pry a small section of the ring away from the wall. Repeat this process until you have pushed the retaining ring out from a little more than half of the gauge. At this point, you should be able to remove this retaining ring by pulling on the ring, or by prying it off with a screwdriver. Remember to do this carefully or you will damage the retaining ring. When the ring is loose, it will slide off of the gauge face. Even if you deform the rim slightly, it doesn't really matter because the rim is hidden from view when the gauge is installed in the car.
Now remove the two small screws on the back of the speedometer that hold the entire assembly to the housing. With a little bit of coaching, the internals should slide right out. Be careful from this point on not to scratch the delicate faceplate, or tap, bend, or break the speedometer needle.
One of the main failure points within the odometer is the pot-metal gear that is pressed onto the odometer drive shaft. The shaft holds all of the numbered wheels together and is connected by a worm gear. If the gear is loose, it needs to either be glued or deformed enough to be pressed back onto the odometer shaft. To remove the shaft, simply pull on the small gear that is on the opposite end of the pot metal gear. To prevent the number wheels from coming loose and falling out everywhere, tape a small piece of tape across the assembly. This way, the entire taped assembly can be removed and easily replaced later on.
It can be difficult to remove the gears without taking the faceplate off. Be aware that if you remove the needle, you will have to have the gauge recalibrated. The best thing to do to gain more clearance is to remove the small screws that hold the face on, and then rotate the face so that you can get the wheels out. Again, be careful not to break off the speedometer needle
After you remove the odometer drive shaft, you have a few options to make the repair. You can try gluing the gear onto the shaft, but this is difficult as there isn't too much clearance and working room when the unit is finally assembled. You can place some glue on the inside of the gear, and hope that it will be enough to hold the gear. You can also roughen up the shaft a bit with some sand paper or a grinding wheel. My recommendation is to actually deform the wheel by compressing it with a pair of vise grips or some other applicable tool. Carefully squeeze the wheel at its mounting point until the inside becomes slightly oval. This should be enough to make the gear adhere to the shaft.
If the wheels are still together, and they haven't become separated from the tape, reassembly is a bit easier. If the wheels have become jumbled, then reposition them in their proper order. Make sure that the trip meter wheel with the extra white attachment is placed all the way on the right, and that the wheel with the copper metal insert is placed next to it. Refer to the photo accompanying this project if there is any confusion. To reassemble the odometer shaft, place all of the numbered wheels into the housing and insert the shaft through the wheels, making sure that the numbers stay aligned.
At this point in time, you can set your odometer to any mileage amount that you would like, although it is difficult to put the unit back together without messing up the specific number that you want. Push the odometer driveshaft back into the housing and through the pot metal gear. Depending upon how you decided to deform the wheel, this may take some force. Make sure that all your numbers are lined up before pushing the shaft back into the pot metal gear. Check it by looking carefully at the face on the mechanism. It is possible to have numbers that are half-turned and not even with the other numbers. Getting this right may take more than one try.
The other major problem with the odometer stems from the plastic worm gears getting worn out. Unfortunately, these gears are not available as separate parts that can be ordered. In most cases however, you can head to your own local junkyard and pick up an old VW or BMW gauge made by VDO, and chances are that it will have the same exact gears inside. It's recommended to gather 3-4 of these (they usually cost about $3-$5) at a time to make sure that you get some decent used parts inside. Replacement of the gear is straightforward: simply loosen up the assembly and swap out the gear. It is important to keep in mind the reasons why the plastic worm gears may have been chewed up. If the odometer doesn't spin freely, then your speedometer cable will turn the gear and chew it up again. Make sure that the odometer gears turn freely before installing your new worm gear.
Place the odometer/speedometer mechanism back into the housing and mount it with the two screws you removed earlier. It is possible to test the operation of a mechanical speedometer using a hand drill hooked up to the rear of the housing, but in most cases, it's just easier to reinstall it back into the car. Replace the gauge face and reattach the outer ring by pressing down the ring around the outside of the gauge. Make sure that you clean the inside of the gauge and the glass before you assemble everything back together. With your repair complete, your gauge should work well for years to come!
Removal of the outer sealing ring can be a very tricky process. Use a small screwdriver and carefully push out the edge of the outer ring. Try to minimize the amount of damage that you do to the ring: these are not available as separate parts, and are easily destroyed. One good thing to remember regarding the process of removing the ring is that the edge that is dented and marred is not visible at all when the gauge is installed.
The often fickle odometer mechanism can fail at two different points. The plastic worm gear indicated by the arrow on the left can become chewed up and worn if the odometer sticks. The only way to fix this is to replace the gear. Unfortunately, these gears are not available as separate parts. The other point of failure involves the small pot-metal gear indicated by the arrow on the right. This gear is pressed onto the main shaft, and often becomes loose after many years of operation. The repair is performed by removing and repressing this gear back onto the shaft.
There is a large assortment of rings and seals that hold the top glass to the gauge. Make sure that you don't damage these when you are removing and handling them. A good fix for spots and fading on these rings is to use a black permanent marker to touch them up. Such a easy technique can improve the looks of your dash by a surprising amount.