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.
In 1978, Porsche switched from using a numbered temperature gauge to one that only has white and red ranges. While fine for most ordinary cars, this change seems like a downgrade for the Porsche 911. Earlier cars were equipped with an accurate, enumerated gauge that could afford the driver the ability to easily differentiate between when the car was running cool or hot.
Fortunately, it's easy to replace your plain range gauge with one that has accurate numbers. The only small catch is that you must match the sender to the gauge. Each sender is calibrated for a certain gauge within a specified range. If you use the wrong gauge with the wrong sender, you won't get accurate readings.
The first step in performing this upgrade is to obtain the small gauge module that has the numbered oil temperature on it. You can usually find older gauges with these modules in them at local swapmeets, or the module itself is still available new for about $70. Once you have the module, make sure that you order the correct sending unit for it. The most common sending unit is the one that originally came with the 1969-77 911s.
The module can be easily installed into the rear of the gauge. To start, disconnect the battery and simply pull the gauge out of the dashboard (see Pelican Technical Article: Speedometer Removal). The gauges are held in with only a thick rubber gasket, so they should just pull out. Once you have the gauge out, carefully mark the wires that connect to the oil temperature module on the right. You don't need to disconnect the wires that are connected to the oil pressure module. There should be three wires, plus two light bulbs that you need to remove. The light bulbs simply pull out of their sockets: don't be afraid to pull on the wires, or use a screwdriver to pry the light bulbs out of their fixtures.
Now, remove the four small screws that hold the module into the gauge. These screws are sometimes reinforced with a little glob of glue on the outside, so make sure that you use an appropriately sized screwdriver, and be careful not to strip them. Once you remove the four screws, the module can be easily removed. Replace it with the new one and tighten down the four screws. Make sure that you blow off the face of the module with compressed air to clean it before you place it back inside the gauge. Reconnect the wires, and press the gauge back into the dashboard.
Now proceed to your engine compartment, where you will find the oil temperature sending unit to the lower right of the fan, facing the rear of the car. Simply unbolt the unit and remove it. A little bit of oil may spill out so have some paper towels handy. Install the new sender making sure that you use a new metal gasket between the sender and its mount. Tighten the sender down tight, but don't apply too much torque (maximum 25 Nm/18.4 ft-lbs).
When you're finished, start up the car and check to make sure that everything is working ok. While the car is warming up, check the temperature sender in the engine compartment to assure that it is not leaking. Tighten it up a bit if it's wet with oil.
The numbered oil temperature sender module is what you need to start with. This particular module is from a 1977 911 and is numbered up to 340 degrees Fahrenheit. Earlier cars had different variations in the numbering scheme. These gauges are often available at swapmeets, or modules themselves can be purchased new individually.
Installed in the dash, the numbered gauge gives you a much more accurate reading of actual temperatures in your engine. This upgrade is very useful for monitoring the results of upgrades that you perform on your oil cooling system.
Make sure that you replace your oil temperature sender (shown by arrow) when you upgrade your gauge. There are several different types of senders, and each one must be matched specifically to the gauge, or you will get false readings. The switch is located in the rear of the engine compartment, just to the lower right of the fan, facing the rear of the car.