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HomeTech Articles > Converting A Mechanical Tachometer to Electric

Pelican Technical Article:

Converting A Mechanical
Tachometer to Electric

Difficulty Level 2

Difficulty scale:
Adding air to your tires is level one
Rebuilding a 911 Motor is level ten

Figure 4
Figure 1:
Late Model Electric Tachometer

Figure 4
Figure 2:
Threading Cable From Center Tunnel

Figure 4
Figure 3: Threading Cable into Engine Compartment

Figure 4
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.

   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.

Comments and Suggestions:
Wayne's Friend Comments: This article is a troll.

I'll be converting a tachometer from mechanical to electric in the near future, and I'm currently doing research to avoid unnecessary mistakes.

This article immediately caught my eye!

I thought, "Hmmm... This looks useful. Wait a minute, difficulty 2!? Only one level harder than putting air in your tires? Wayne must be a total boss. Here I am putting in days worth of research, and this guy does it with almost no effort at all. How pitiful I am."

Then I read a brief history of Porsche and their tachometer setup, followed by the truth.

"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."

What this article should really be called is, "Pelican Technical Article: Buying a Tachometer From Us and Running a Few Wires Through your Car"

However, even that may be a lie, after reading the first comment.

As mentioned before, the article states, "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."

Then BG asks what the tachometer costs and if comes with a warranty. I'm not a mind reader, but if I had to guess, I would think he/she was referring to the tach that "Pelican Parts can provide".

But not Wayne. The same Wayne that laughs in the face of tachometer conversions, writes deceptively titled articles, and apparently turns away potential customers.

No, not Wayne. Rather than giving BG any information about the tachometer that "Pelican Parts can provide", Wayne just says to search for a used one online as I did.

Look Wayne, I goof up like this too sometimes. I hope you had as much fun laughing at yourself as I did. But seriously, please change the title of this article.
February 21, 2017
  Followup from the Pelican Staff: Thank you for your feedback. As you probably know the parts environment for the 356 has been in a constant state of change since this article was written in the 1990's. We apologize if you felt that the article was mis-titled. The reality of the below comment is accurate NOW. When the article was written however we were able to obtain SO MANY more new parts for the 356 model. We wish we could have bought them ALL, so that we would have a warehouse full of new old stock Porsche stuff to sell! I hope this didn't cause too much emotional frustration, and we truly do appreciate the feedback and your continued support of Pelican and the Porsche brand. You rock! - Casey at Pelican Parts  
BG Comments: What's the cost of an electric tach? Is there a warranty with it. Thanks.
February 16, 2011
  Followup from the Pelican Staff: You'd have to find a used one on the Internet. I think I paid $200 for mine about 10 years ago. I imagine that they are more difficult to find these days. - Wayne at Pelican Parts  

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