The On Board Computer (OBC)
option (M659) adds a different tachometer to your car with a small display
at the bottom of the tach which displays 6 different functions. It came
standard on all Turbos and cars with Tiptronic transmissions. The functions
displayed in orange letters, figures and symbols include distance to next
fill-up, average MPG, average speed, trip odometer, digital speedometer, and
outside temperature. After a couple of months of thinking about trying to
add this option and finding all used components, I installed the On Board
Computer (OBC) option in my 1996 993 Porsche. Here is a simple write up of
how I did the installation.
Porsche parts required:
OBC Tachometer connector: Not available separately from dealer, need to find a used part off a 993 tiptronic, Turbo, or 6-speed w/OBC option or use the connector off a 1990-1994 964 w/OBC option.
Other parts needed:
Torx T-30 bit (at least 2" long)
M 24 hex socket
M 6 & M 4 Allen socket
Metric hex socket set (various sizes)
M 6x12 or M 6x16 pan head screw (4X)
20 AWG wire (various colors)
Splices or soldering iron & solder
Male and female butt connectors (red)
Wire ties (various sizes)
PVC wire loom cover (optional)
Drill bit set
Dremel or equivalent tool (optional)
993 factory wiring diagram (optional)
Notes on parts:
The OBC tachometer, steering column switch, and the steering column switch cover can all be purchased used to save cost.
The steering column stalk for the OBC is a totally separate piece from the steering column switch and is held on with just two screws and a wire tie so you can buy part 993 613 352 00 01C which is cheaper and just transfer the new OBC stalk to your existing steering column switch regardless of which steering column switch you have.
The OBC tachometer connector is a 26 pin connector that plugs into the back of the OBC tachometer. I do not believe this part is available from the dealer unless you order the entire wiring harness. The connector is made up of a connector surrounded by a plastic shell held together with a wire tie. The connector part numbers are 944 612 551 00 (connector) and 999 652 542 40 (shell). This same connector is used in a couple of different locations in the car. The only differences are the number and the colors of the wires used on the connector. I used a OBC connector off a 1990-1994 964 w/OBC option. This worked out well because this connector has some unused wires which you will need to use later on. The connector to the central indicator (informer) will also work but you will need to modify the position of some of the wires in the connector and the wire colors will not be correct.
Hooking up the OBC tachometer requires the connection of 19 wires. The standard tachometer only has 7 wire connections. 14 of the wires come from the area directly behind the tachometer and from the steering column switch. The remaining 5 wires have to be routed from other parts of the car (DME area, temp. sensor, driver's side kick panel, and the central indicator (informer) ) to the back of the OBC tachometer. Please use a voltmeter to check the continuity of your connections as you go.
*I only had the factory wiring diagrams for the 1994, 1995 and 1996 model years. I am not sure if there were any change for 1997 and 1998. I do not believe there were any but you may want to check with the wiring diagrams for those years to be sure.
** Wire colors given as
color1/color2 means a color1 wire with a color2 stripe. For example a
Red/Black wire color means a red wire with a black stripe. The wire colors
shown on the list are for reference only you may have different colors in
your car depending on the year and what colors you use for your
Remove seat and DME cover
following DIY - How to change
the EPROM chip in the OBC
Remove rear connector as
shown, pull back tape, and splice into pin #6 (white/green wire) coming from
DME (top half of connector). Retape connection with electrical tape when
done. Run wire under carpet and up to dash. Use enough wire to reach under
dash up to the tachometer. Replace cover and install four M 6x12 or M 6x16
pan head screws for stock appearance (I found the M 6x30 screws called out
in the DIY to be too long).
Remove the driver's side
kick panel using a M 4 allen socket to remove the cover from the hood
release handle and then pull the panel off. Pull back tape from connector on
the left (the larger connector), and splice into the pin #4 (green/white
wire) coming from the driver's door (top half of connector). Retape
connection with electrical tape when done. Use enough wire to reach under
dash up to the tachometer.
Remove the central indicator (informer) under the dash using a 10mm hex socket. It is held in place by two plastic nuts. Disconnect the beige connector from the unit by pulling back the black locking tab. At this point you need to add a missing pin to the beige connector for the fuel signal at pin #7 (if your unit has this pin then you can skip this step). To take the connector apart remove the electrical tape around the wires, cut away the wire tie, and separate the beige connector shell from the white center connector assembly. Once you have the white center connector assembly free, pull out the green tab of the connector so that a new pin can be added.
To add a new pin, either
use one of the unused pin from a 1990-1994 964 OBC connector (Pins #16, #17,
or #20) or from a donor central indicator (informer) or other unit
connector. Remove the pin by following the directions above and then use a
safety pin or other pointed objected to press down on the connector pin tab
while pulling the pin wire. Install the new pin in the your central
indicator (informer) and make sure the pin locks in place. Reinstall the
green tab in the connector and assembly the connector reversing the steps
Run two wires from under
the dash for connection of the temperature sensor (red and yellow wire show
here). I ran the wires through a grommet used for the front hood release
handle directly above and to the left of the clutch pedal. I spliced the
wires into the wire harness on the left side of the hood going to the left
lights. Remove the tape from the harness and splice into the blue/green and
brown/green wires. Cover wires with black PVC wire loom tubing if you like.
Retape harness with electrical tape when done.
Install the temperature
sensor behind the left side of the front bumper grill. If you remove the
front bumper this step will be easier but I chose to do it without removing
the bumper. To remove the bumper follow the DIY -
How to remove the front bumper cover.
If not following these steps: remove front turn signal light, bumperette,
and fog light following directions in owner's manual. The connector for the
temperature sensor is already installed in the car (usually tied around your
light wire harness) and is shown below on the left. The OBC temperature
sensor installs on bracket above existing temperature sensor for the climate
control unit. Simply push the new sensor through the hole and make sure it
is fully seated in place. If your bumper is removed this will be easy. If
not you need to reach through the fog light opening and install the sensor
that way. After the sensor is installed, plug the connector into the sensor.
You may also want to check the continuity between the temperature sensor
wires installed in the car and the temperature sensor connector before
plugging the connector into the sensor.
At this step you will
remove the steering wheel, airbag, trim cover, and the gauges. First
disconnect the battery. Undo the screws holding on the airbag using a Trox
T30 socket drive. Disconnect the airbag connector and remove the airbag
(store the airbag with the padded side facing up). Remove the nut from the
steering wheel using a M 24 hex socket. Turn wheel straight ahead and mark
position of steering wheel to steering shaft. Lift off the steering wheel
and the spring washer. Undo steering wheel trim cover screws on the front
and both sides of the unit. Remove the screws from the steering wheel
contact unit but do not remove the unit. Now you can remove all the gauges
from the dash by gently pulling and twisting each gauge until it pops out
from the dash.
Now you can attach the OBC
stalk. Like I said above, the OBC is a totally separate piece from the
steering column switch and is held in place by two screws and a wire tie so
you do not have to remove your entire steering column switch to install the
stalk. You can just transfer the OBC stalk from your new column switch to
you existing column switch using the screws and a wire tie. You can install
the entirely new column switch if you like but that requires removal of the
contact unit and your existing steering column switch assembly.
Next I took apart the
connector at the end of the OBC stalk and extended all five of the stalk
wires using red female butt connectors that were partially crimped (extend
the wires so they are long enough to route under the dash and up to the
tachometer). You could also splice and solder the wires to extend them or
choose some other method that works. Once the wires have been extended, run
them in the opening directly under the steering column and under the dash.
Now run all the wires from
under the dash up to the tachometer area. There should be a total 10 wires.
I labeled the ends of all the wires using masking tape so I would not get
confused later. To run the wires from under the dash up to the instrument
gauges I added a hole and grommet almost directly under the central
indicator (informer). Make sure not to drill through any wires or vent
ducts!!! You could also try to run the wires through the existing grommet
under dash. I did not think there was enough room to do these but this could
vary from car to car.
Splice into pin #4 (red
wire) of clock connector harness for the +12V power connection and into pin
#2 (white/red wire) of the speedometer harness for the speed connection.
Retape connections with electrical tape when done.
Now we are almost done!!!
You now need to splice into the 7 wires from the existing tachometer
harness. You can splice into the tachometer harness using plastic splices,
solder, or whatever makes you comfortable. Make the wires long enough to
allow for routing behind the tachometer. Once all wires were in place, I
added male and female butt connectors to all of them and to all of the of
the wires on the beige OBC 26 pin connector. 38 connectors in total. You
could also choose to eliminate the butt connectors here and just solder the
appropriate wires together from the OBC connector and the spliced wires.
Make sure all of the
appropriate wires are connected together using the wire list. Use wire ties
and/or electrical tape to bundle and secure the wires together. Route wires
to ensure that none of them will get pinched by any gauges, screws, or
etc.. Install gauges, contact unit, trim cover, steering wheel (torque to
33 ft lb), and air bag (torque to 7 ft lb) by reversing previous steps.
Reattach battery and check operation of OBC Tachometer!!!!!
Pelican Technical Article:
993 On-Board Computer Retrofit DIY
Torx T-30 bit (at least 2" long), M 24 hex socket, M 6 & M 4 Allen socket, Metric hex socket set (various sizes), M 6x12 or M 6x16 pan head screw (4X), 20 AWG wire (various colors), Splices or soldering iron & solder, Male and female butt connectors (red), Voltmeter (optional), Electrical tape, Wire ties (various sizes), PVC wire loom cover (optional), Rubber grommet, Drill bit set, Dremel or equivalent tool (optional), 993 factory wiring diagram (optional).
Porsche 993 Carrera (1995-98)
Outer temp sensor, OBC Tachometer, Steering Column Switch, Steering Column switch cover, OBC Tachometer connector
Digital display of car system statistics.