Pelican Technical Article:
Difficulty Level: 4
Difficulty scale: Adding air to your tires is level one
Rebuilding a BMW Motor is level ten
||This article is
the one in a series that will be released in conjunction with Wayne's
upcoming book, 101 Projects for Your BMW 3-Series. The book will be
256 pages of full color projects detailing everything from performance mods
to timing the camshafts. With more than 350+ full-color glossy photos
accompanying extensive step-by-step procedures, this book should be a staple
in any 3-Series owner's collection. See
The Official Book Website
for more details. The book is due out in October 2005.
[Click on Photo]
Figure 1: Standard 3 Gauge Set on the 914
Many Porsche owners have had problems with the gauges that are located in the
dashboard of their cars. This Pelican tech article will provide some hints and tips
on the following topics related to the VDO gauges found in almost all Porsches:
- Gauge Face Replacement & Refurbishment
- Odometer Repair
- Speedometer Calibration and Check
- Speedometer Recalibration for Different Tires
- Temp/Oil/Gas Module Replacement & Substitution
The VDO gauges used in all the early Porsches have very
similar construction and design. Therefore, the 356, 911, 914, and other Porsche
(and VW, and BMW) gauges are often very similar.
Figure 1 shows some typical gauges from a 914.
Unfortunately, the problems that occur with these gauges are also often common with
many Porsche cars. Although there are many different types of these gauges, this
article will attempt to be as broad as possible, in the hope that much of the information
and pictures provided can be used on many gauges from many different cars.
Figure 2: Scratched Plastic Face
||Gauge Face Refurbishment
Some of the later cars (later 914s, for example) have plastic gauge faces used on all the
gauges. Over the life of the car, these can easily be scratched and dulled, as seen
Figure 2. The solution for this
problem is to replace the faces with older glass ones. These can be cleaned with
minimal scratching and they look much better in general. Pelican Parts can provide
you with all the glass faces that you need for your gauge refurbishment. The
difference is absolutely remarkable, and it's also an improvement that you will notice
every day you drive your car.
All of the VDO Porsche gauges have faces that are
attached with a ring that is bent and wrapped around the outer edge of the gauge.
The factory must have a pretty good tool for performing this, because the seal is both
real good and also very smooth. Any modifications or repair to the gauges requires
the removal of this ring.
Figure 3: Removing Outer Retaining Ring
Figure 4: Removing Outer Ring and Face
Figure 5: Tachometer with Front Face Removed
Figure 6: Using Magic Marker on Rim of Gauge
|| The 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 screw driver and
pry a small section of the ring away from the wall. This is shown in
Figure 3. 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 screw driver. Remember to do this carefully or you will
damage the retaining ring. When the ring is loose, it will come off of the gauge
face as shown in
After you remove the retaining ring, you should be able to remove the various rings and
transparent face that cover the gauge.
5 shows all of the rings that came out of this early 914 tachometer. Depending
upon your gauge type and car, the mounting of the clear face may differ. At this
point, you should perform any repair or cleaning on the gauge that you need to do. Remember
to use a lint-free cloth when cleaning your gauge. Lint dust on a black
background shows up really well if you don't. Another useful tool is compressed air
in a can (usually used to clean computer equipment). Blow out any dust or debris
prior to reassembly.
The substitution of a glass face instead of the plastic
one requires no modifications at all. When reassembling the gauge, be sure to clean
everything carefully and reassemble the gauge in its original configuration. The
retaining ring is reattached by fitting it over the gauge and bending back the edge that
you previously bent away. Make sure that the ring is on tight; you would hate to
have the face of your gauge fall off while driving! When you are finished, you can
touch up the ring of the gauge with a black permanent marker (Figure 6). It's a quick solution that
actually works really well. Your glass gauge faces will be a vast improvement that
you will notice the next time, and every time you drive your car.
Figure 7: Rear of Odometer with Mounting Screws
Figure 8: Speedometer Mechanism
Figure 9: Speedometer Mechanism
Figure 10: Rear of Speedometer Drive
Figure 11: Odometer Drive Shaft
Figure 12: Top View of Odometer Drive
Figure 13: Taped Wheel Set
Figure 14: Odometer Drive Without Shaft
Figure 15: Pot Metal Advance Gear
Figure 16: Right Two Number Wheels
Figure 17: Placing Taped Wheels in Mechanism
Figure 18: Front Face with Numbers Aligned
Figure 19: Final Assembly of Odometer Drive
Figure 20: Side View of Final Assembly of Odometer Drive
The first repair job I ever attempted on my 914 was the repair of my odometer. I had
just gotten to LA, and had some time to kill before I moved into my apartment. I
decided to go look at some 914s in the newspaper in my spare time. I found a gem of
a car (the one I own now) and basically bought it on the spot. I later had to park
the 914 in the parking lot of the hotel I was staying at! Needless to say, when I
took my 914 to get insured, my insurance company wouldn't let me put it on my policy
because the odometer was broken (silly of me for mentioning that to them). That
night in the hotel room with nothing more than a screw driver and a pocket knife, I took
apart my odometer.
Well, enough of the history. The problem of the
Porsche VDO odometer breaking is very common among all early Porsches. I was amazed
to find that my 1960 356 odometer was almost exactly the same as my 914 odometer and had
the same problem (just a few parts that were metal in 1960 were now plastic in
1974). The odometer didn't work, and when advancing the trip counter, the mileage
would advance by one mile. Having fixed this before on my 914, I quickly fixed the
one in my 356.
The basic problem with the odometers lies in the fact
that a pot-metal gear that was originally press-fit onto the odometer drive shaft often
comes loose. When it is no longer adhering to the shaft, there is no way for the
odometer to turn. The solution, in a nutshell, is to remove and disassemble the
mechanism, and to reattach this gear to the shaft. Unfortunately, this is not as
easy as it sounds. For this project you will need the following:
Beware that the first time you put everything back
together, you may make a mistake, and have to do it over again. This has happened to
me a couple of times. One way to prevent this is to check to make sure everything
works correctly after you finish a particular step.
The first step in fixing your odometer is to remove the
outer retaining ring and gauge face. This was described earlier in the section, Gauge Face Refurbishment. Once you
have the face removed, the next step is to remove the speedometer and odometer mechanism.
Remove the two screws located on the back of the gauge, as shown in
Figure 7. With a little coaching, the
internals of the gauge should slide out.
Note: All of the photos that accompany
this section show the gauge with the speedometer face and needle removed. You
should not remove the speedometer needle and gauge face. This will require
you to recalibrate your speedometer - not an easy job. The only thing that you need
to remove to perform the odometer repair is the odometer drive shaft. The rest of
the photos show the internals of the speedometer drive primarily for curiosity seekers.
Disassembling the speedometer assembly may certainly affect its accuracy in the
future. The speedometer mechanism, detached from the odometer mechanism, is shown in
Figure 8 and
Figure 9. This speedometer mechanism
mates into the back of the odometer mechanism, as shown
The main failure point within the odometer is the
pot-metal gear that is pressed onto the odometer drive shaft. The shaft holds all of
the number wheels together and is connected by a worm gear. This shaft is shown in
Figure 11. The pot metal gear, shown on
the right, needs to either be glued or deformed enough to be pressed back onto the
shaft.To remove the shaft, simply pull on the gear that is on the opposite end of the pot
metal gear. This gear is shown on the left side of
Figure 12. To prevent the number wheels
from coming loose and flying off everywhere, tape a small piece of tape across the
assembly. This way, the entire taped assembly can be removed and easily replaced
later on (Figure 13). It has also been
brought to my attention that the wheels are difficult to remove without taking the
faceplate off. Beware that if you remove the needle, it may be difficult to
recalibrate the gauge (see below). The best advice that I have is to remove the
small screws that hold the face on, and then rotate the face so that you can get the
After you remove the odometer drive shaft (Figure 14), you have a few options to make the
repair. You can try gluing the gear onto the shaft, but this is difficult as there
is little clearance 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 vise grips or some
other applicable tool. Carefully squeeze the wheel at the points shown in
Figure 15, until the inside become slightly
oval. On the 356 odometer, I found that the flanges on the wheel were a bit bigger,
making this a much easier process.
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. The two right most wheels are
Figure 16. Make sure that the
wheel with the extra white attachment (shown on right) is placed all the way on the right,
and that the wheel with the copper metal insert is placed next to it. To reassemble
the odometer shaft, place all of the number wheels in the housing as shown in
Figure 17. Then insert the shaft through
the wheels making sure that the numbers stay aligned. At this point in time, you can
set your odometer to anything that you would like, although experience tells me that it is
hard 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, as shown in
Figure 18. 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 - it does for me each time. The final
odometer mechanism is shown reassembled in
Place the odometer/speedometer mechanism back into the
housing and mount it with the two screws you removed earlier. You can test the
operation of the unit using a hand drill in the calibration procedure described below.
Replace the gauge face and reattach the outer ring as described in the first
section. Your gauge should work well for years to come!
For the non-adventurous, Pelican Parts repairs these
odometers, replacing the faces, and guarantees the results.
Send us some email if you would like this service
performed on your gauge, or if you have any questions.
Figure 21: Calibration Setup
Speedometer Calibration and Check
Do you think that you're speedometer might be internally uncalibrated? You can check the accuracy of your speedometer by using the following
Remove the speedometer from the car along with the trip
meter reset cable. For cars without the cable (356 gauges), you can get one from a
914. Place one end of the reset cable into the back of the gauge, and clamp the
other end in the chuck of a hand drill.
21 shows this setup. Run the drill in reverse until the speed
reaches about 60 mph. Then wait until the right dial on the trip meter reaches zero.
Start the stop watch and carefully watch the trip meter. After exactly one
minute, the trip meter should read exactly one mile more than when you started. If
you reach the one mile point before the time is up, your speedometer is reading low.
If you reach the one mile point after the time has elapsed, then your speedometer
is reading higher than it should. You can use this method to check the speedometer
at different speeds too. If you're adventurous, you can use this method to
recalibrate your gauge by removing the needle and replacing it on it's shaft. This
may or may not work depending upon how your speedometer became uncalibrated.
Speedometer Recalibration for Different
Charles Davis (email@example.com) adds the following
information on recalibrating your speedometer for different sized tires:
A spedometer is an eddy current device...ie, a permanent magnet rotating
close to a conductive disk or cup causes electrical current to flow in small rotating
eddies. These currents, being "shorted out" by the continuous conductive disk,
cause a drag torque. The torque produced is a function of several things, but primarily
the velocity of the magnet and the flux produced by the magnet. The torque winds up a
spring on the needle until the spring torque is the same as the eddy current torque.
To "calibrate" one, there are several ways:
1. change the spacing between the magnet and the disk or cup. The cup design was used
(I believe) to eliminate the spacing as an assembly variable, so changing it with a cup
type is difficult.
2. change the spring constant. I dont know how this would be done.
3. change the amount of "magnetism" in the magnet.
In practice, Ive only seen the last method used. The process is to
magnetize the magnet fully, then demagnetize it a little until the speedometer produces
the correct result. Normally, a speedometer is calibrated by rotating the input shaft at a
stable, constant rate (such as 1000 RPM) and adjusting the magnet until it gives a known
reading. Some speedometers have the input-output value printed on the case somewhere...not
sure about VDO gages though...I see some "funny" numbers on mine, in addition to
the part number, but none seem to be this type number. This type of calibration would make
the speedo correct for the factory original tire size. It would be needed because magnets
loose a little of their strength with time.
To compensate for a different size tire, you would have to do the math to determine the
% difference to a standard tire (use the rolling radius spec) and then tell the guy doing
the calibration to make it X% more or X% less.
Another method is to leave the speedo alone and add a small "transmission" in
the drive line. I think these were common in old rally cars (the ones bafore computers
took over). These "transmissions" could be adjusted to give many gear ratios
(near 1:1), and thus correct for speedo errors. I think they did this by having hundreds
of gear sets available? (Wayne, ask Tom...he might know about these? I dont think
they are used much now...perhaps someone has an old one they would sell?) The
"transmission" was small...it could be hidden under the dash.
Hope some of this helps.
Figure 21: Gauge Module Removed from Rear
Figure 22: Interchangeable Gauge Module
||Temp/Oil/Gas Module Replacement and
gauges used on many Porsches have interchangeable modules that can be switched quite
easily. The process is simple; simply unscrew the module from the rear and lift it
out of the housing.
Figure 21 shows the
rear of a gauge while
Figure 22 shows a 911
druck press (oil pressure) module. The swapping of modules is useful for a wide
variety of tasks:
- Swap 6V with 12V modules when converting a 356 to from 6V to 12V.
- Replace your expensive and hard to find 912/356 temp sender with an inexpensive 914
one. Replace both the sender and the temperature module in the gauge.
- Add a temperature gauge to 914s that originally did not have one.
- Add an oil pressure module to a 914 combination gauge.
- Easily repair your gauge by swapping only the module that is broken.
Well, there you have it. If you can think of
anything we left out, please let us know.
Please remember that Pelican Parts strives to offer great customer service coupled with
reasonable prices on everything for your 914. Help keep the tech articles coming by
letting us earn your business.
After publishing this article, we received the following tips from Steve Anderson:
Name: Steve Anderson
Location: Richmond, VA
E-Mail firstname.lastname@example.org <mailto:email@example.com>
Vehicle: 1976 911 Targa with 1977 3.0 engine
Recently, I encountered problems with the speedometer in my
1976 911. I read your tech tip article entitled, "Porsche Dashboard Gauge Repair and
Refurbishment, Odometer Repair", and found it to be very informative, just as each of
your articles have been. However, I was experiencing additional problems with my
speedometer and I was also dealing with an electronic speedometer rather than a cable
driven speedometer. But fortunately your article gave me enough information the confidence
to tackle the troublesome speedometer and make a successful repair. I am including the
symptoms that I experienced and the steps taken to resolve the problems, in case they
could be useful to others. My terminology may not be correct, and due to this, if this is
used please feel free to correct it as needed. Thank you for providing such an informative
After resetting the tripometer in my car, I realized that
neither the tripometer nor the odometer would function. A constant clicking sound could be
heard coming from the speedometer and the reset button was jerking with each click sound.
Also the speedometer needle would not go beyond 90 kmh. Nor would the needle drop back to
0 kmh. The needle would remain at 90 kmh. I found that by pushing on the tripometer reset
button would allow the speedometer to continue advancing but would then not go beyond 110
kmh. I would have to push the tripometer reset again to either allow the speedometer
needle to reset to 0 kmh or to continue advancing. And neither the tripometer nor the
odometer would advance in mileage.
Using your tech tip article, "Porsche Dashboard Gauge
Repair and Refurbishment, Odometer Repair", the speedometer was dismantled. I then
loosened the retaining bolt holding the electric gear drive motor in place so that the
remaining gears could be rotated freely. This is where I found the clicking sound being
heard. Still pushing the tripometer reset button had no effect.. The speedometer needle
could manually advanced but would not advance beyond 90 kmh unless the tripometer reset
button was pushed, then the speedometer needle could be advanced to 110 kmh and the same
problem would occur. I found one of the (for lack of proper terminology) locking tabs that
is used to lock or unlock the numbered wheels on the tripometer to reset to 0, was not
going back to its proper position. With this tab in the up or unlocked position the gears
were attempting to turn the numbered wheels but were slipping or not engaging properly.
Also with this tab in the up or unlocked position, the tab would catch and stop what
appears to be the speedometer needle counterweight. By lightly pushing this tab back down
to its proper position, the speedometer needle could be advanced through the full range of
the speedometer without catching and stopping as it did previously. I then tested the
tripometer and odometer by turning the gear closest to the drive motor. The tripometer and
odometer advanced correctly without the clicking sound. The tripometer was reset in order
to check its functionality. This test was also successful. Then I repeated each of these
steps several times to test the complete functionality of the speedometer. Once I felt
that the problem had been corrected, the main drive motor was tightened back down and the
speedometer was put back together then reinstalled into the car. A final test drive was
performed to verify the problems had been corrected and the tripometer reset would operate
correctly. Now all is back to normal.
Fritz Lohss, firstname.lastname@example.org
I would like to thank you for your article on
VDO speedometer repair. It was very helpful and gave me the encouragement to do it myself.
I have experience as a locksmith and that helped me to perform this task.
I have some suggestions that should improve the
article. After exposing the speedometer from the housing with a small screwdriver lightly
try to rotate the pot metal gear. If it rotates then proceed. Pull the shaft out so that
the pot metal gear can be removed. Loosen the gage panel screws to permit swinging the
gage panel that will permit dropping the pot metal gear in your hand. Be careful not to
disturb the gage needle. With the pot metal gear out, centerpunch the hub (outside), I did
it two times l80 degrees apart.
Reinstall the pot metal gear and use a small
phillips screw driver shaft to facilitate the alignment from outside the housing. Lightly
tap the shaft to its position. Check to ascertain that the pot metal gear does not rotate
and is part of the shaft per above directions. Make sure that the gage needle is on top of
the stop peg and tighten the two gage panel screws. Make sure all the numbers line up to
the gage face. Reassemble the gage. Before installing the glass, use a battery variable
speed drill with the switch on counter-clockwise setting. From a finishing nail pound and
grind a square on the head that will fit the square speedometer drive. Set the trip meter
to zero and run the drill for several miles to check the instrument. If
everything is okay recrimp the ring around the glass cover. This should fix your
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