Pelican Technical Article: Converting A Mechanical Tachometer to Electric

Figure 1:
Late Model Electric Tachometer

Figure 2:
Threading Cable From Center Tunnel

Figure 3: Threading Cable into Engine Compartment

Figure 4:
Attachment to Ignition Coil in Engine Compartment

The early 356 cars originally came with mechanical tachometer that worked similar to the speedometer. In these cars, a mechanical cable extends down the center tunnel of the car and is connected to an adapter on the engine. The cable spins as the engine crankshaft turns, and the tachometer in the dashboard indicates the proper RPM. In the later years of production, the 356Cs were equipped with an electric tachometer, shown in Figure 1, similar to the type used on the more modern 914, 911 & 912s. The older mechanical tachometer is a source of much frustration for many 356 owners, as the 40 year old mechanical components often fail. In addition, 356 owners who have installed a more powerful 912 engine in their cars (like myself) need to convert to electric because the 912 motor lacks the mechanical drive needed to connect to the tachometer cable.

The actual conversion of the tachometer is not an easy task and requires significant modification to your gauge, as well as many replacement parts from a later-year tachometer. Therefore, this conversion of the actual dash-mounted instrument will be covered in a later article. Pelican Parts can provide you with an original 356C electric tachometer, or one that has been already converted to electric, or we can convert your original tachometer to electric. Either way, the installation is relatively simple.

The first step is to remove your old tachometer. Disconnect all of the lights by pulling them out of their sockets in the rear of the tachometer. Then disconnect the drive cable at the back of the tachometer gauge. The gauge is held into the dash by two hand-tightened screws which constrain a small bracket than extends over the back of the gauge. Loosen the screws on the back of the tachometer, and remove the bracket. Depending upon the year of your car, you may have to remove the steering wheel and drop the steering column down. To remove the steering wheel, see the technical article, Removing and Replacing a Porsche Steering Wheel. After the wheel is removed, then loosen the two steering column retaining bolts on the bottom of the steering column. At this time, you should be able to drop the column down far enough to remove the tachometer from the dash. Depending on how long its been since the gauge was last removed, it may require some force to remove the gauge. Push gently, and make sure that you don't damage the chrome strip around the outside of the gauge. Once the gauge is out, you can clean up the inside of the compartment and the edges of the gauge opening.

At this time, it would be wise to figure out if you would like to keep the original tachometer cable in the car. I recommend using the outside sheath of the tachometer cable as a protective sleeve for the electrical wire that needs to run down the center tunnel of the car. Jack up the car and disconnect the tachometer cable from where it attaches to the motor. Take a large pair of wire clippers and cut off the end of the inner cable that plugs into the tachometer, inside the dashboard. Now get some tape and tape the end of a spool of wire to the inner cable. I recommend using a thick gauge like 16 or 18 for longevity and strength while pulling through the tunnel. Now, slowly pull the inner cable out of the car from where it used to connect to the engine. The wire should follow suit through the tunnel of the car. At this point, the wire can be pulled through, and enough slack taken up to route the wire through to the engine compartment. The electric wire is shown leaving the center tunnel in Figure 2. For the path of the wire, I chose to route the wire next to the gas line. The wire is shown entering the engine compartment in Figure 3. I secured the line at many points with several nylon cable ties. Once the wire is routed into the engine compartment, attach it to the negative terminal on the coil, as shown in Figure 4.

Behind the dash, use a wire tap to route some power from the temperature gauge on the left. You should also route a ground line from the temperature gauge as well. Make sure that both gauges are running on the same voltage. Clip a wire connector onto the back of the signal wire that is connected to the coil in the engine compartment, and plug it into the signal input terminal on the tachometer. Start up the car and the tachometer should work. Be careful of the lights hanging out in the back of the dash. Make sure that the positive leads from the lights don't come in contact with any metal structure that is grounded. Make sure that the signal wire is not touching ground, or the car will not start. This actually creates a handy ignition cutoff switch if implemented properly. See our technical article, installing An Easy and Inexpensive Ignition Cut-Off Switch for details. If the gauge doesn't work, then check your electrical connections, and the fuse box as well. Check also to see if the temperature gauge is working.

After the gauge is determined to work, reconnect the gauge lights, and replace the gauge in the dash. You will probably want to use a new rubber seal. Reattach the steering column, and replace the steering wheel onto the column. That should do the trick. You shouldn't need to worry about replacing the mechanical elements of your tachometer again.

If you have any questions about the procedure in this article, feel free to drop us a line or give us a call.

Pelican Technical Article:

Converting A Mechanical Tachometer to Electric

Difficulty Level 2

Converting A Mechanical
Tachometer to Electric