The Porsche 964/993 OBC Liquid Crystal Display (LCD) is placed in the revolution counter and is backlighted by a normal instrument lighting bulb. This bulb will of course fail over time, but the transistor inside the rev counter controlling the light bulb also have a tendency to fail. Most garages will use expensive spare parts or specialized repair shops to mend this, but it is a fairly easy job to do yourself. A failing transistor will either extinguish the light entirely, or it can dim or flicker, maybe temperature dependant.
It is possible to remove the tachometer without removing the steering wheel and steering stem trim. You then do not have to worry about the air bag and steering wheel at all, greatly simplifying the process.
Many people regard this part of the job as the most difficult, it is a tight fit between the steering stem trim and upper part of the dashboard. However, it is certainly possible with the help of some soap water and patience.
The instrument is press fitted into its opening, and a rubber gasket holds it in place. Start by using a flat screwdriver or similar to lift the instrument ring a little out from the dash. Then start pulling gently with your fingers. Beware, using tools and force can make the instrument ring with glass to come loose. Pull the instrument housing, not the ring! If you first pull out the adjacent instrument, speedometer or oil meter, it is very easy to push the tachometer out of its opening without putting any strain on the instrument ring.
You can remove the adjacent speedometer first to get a better grip on the back of the tachometer.
Remove the rubber gasket as soon as it is out of the opening, just pull it out over the instrument ring. Leaving the gasket there will make it very difficult to pull the instrument out over the tight lump on the steering stem trim and steeering wheel.
Disconnect the harness at the back by lifting the lever as soon as the rev counter is out of the dash opening. Remove the instrument out to the right of the steering stem trim. Pull it by hand, and get a mate to slightly wriggle the steeering wheel to help the movement. Use soap water on both the steering stem trim and upper dashboard lip for lubrication!
Open the instrument
Remove the two screws at the back and pull the backlid gently off. There are several connectors inside between the sandwiched circuit boards. The transistor in question is mounted on the circuit board in the backlid, remove the three screws marked with red rings to be able to remove the board.
Check all bulbs
Be sure to check that all the bulbs inside are OK. There are two (yellow) 1,5W bulbs for the rev counter scale backlight, and one (blue) 0.9W bulb for the LCD backlight as well as thre others in indicator lamps.
Turn the bulbs several times back and forth when reinserting them to ensure a good electric connection with the circuit board tracks. The Porsche bulbs are quite expensive, about EUR5 and EUR10 per piece.
The transistor in question is marked with a red ring on the picture. There are several versions of this board, but all of them have the transistor at the same spot. There are also SMD versions of this PCB, those do not have the same component failure problems. The transistor is a BS170 N-channel FET, available on eBay or electronic component suppliers as RS Components. An alternative is a 2N7000, but be aware, that type has a different pin configuration and must be mounted the other way around. See the pictures for details.
This type of solder job require some skill with the soldering iron. The circuit board is a through-hole plated type, that make it a bit difficult to remove the old transistor. One alternative could be to cut all three component legs and pull them out one by one while heating the solder pad. Use a vacuum solder pump or similar to clean the pads and holes. An alternative is Chipquik. Be sure to clean the solder spot with alcohol after soldering.
Refitting the instrument
Test the instrument by connecting the harness in the car before you press it into the opening. The crevice between the steering wheel and steering stem trim can be a bit difficult to get past on the way in. Use soap water for lubrication and a flat screwdriver to give the instrument ring help over the crevice. A helping hand can be good here too.
Needed spare parts
0,9W blue bulb: Porsche part # 964 641 920 00
1,5W yellow bulb: Porsche part # 999 631 027 90
N-Channel FET transistor: BS170 or 2N7000