Installed a 86 3.2 into my car with a tach from the donor. Everything works but the tach. It reads about double what it should. Any ideas why it's off?

If the tach is from a 4, you will need a tach adapter or you can send it out to get it converted. There is a member here who converts them.

this is what you would be looking for ...........

Link to Tach Adapter

He posted the tach is out of the donor, so I'm assuming its a 911 tach.
You shouldnt need a tach adapter, maybe hooked up wrong

Are you using a distributor or different type of signal, like a distributor-less ignition? The signal patterns and voltages can be different.

Doesn't that need the transducer from the 911 also. . ???

That engine in it's native state uses a speed sensor on the 915 transmission.

hi there, my dad is teaching me how to convert tachometers. I'm working on a 6 conversion tach this past week. I'm almost most done with the tachometer i still have to do the paint and calibration. I will post pictures soon!

You have many different options. You can buy a tach adapt, or follow my tach conversion thread (if you want to do it yourself) or buy a 911 tachometer or wait for my son to finish his tachometer conversion.

My son asked me for starting a weekly allowance. Instead of handing out free money, I asked him if he would be interested in converting tachometers. This past week, we worked together on converting a 914 tachometer over to a modern tachometer. I asked him what we should make this tach work for, and he responded 6 cylinder! I agreed, since there are some folks on here that swapped out their type 4 to a 6 cylinder.

If you can wait a few months, I'm working on a totally new design which is universal for the 914 tachs and 911 tachs which will span from the late 60's to the early 80's. I'm using a new air core motor that is micro sized, and a better circuit that will make the tach more responsive and accurate thru out the whole rpm spectrum.
Click to view attachment

Nice work.

My new 10k tach seems to stall sometimes if the idle gets below 500. Then it sticks on zero until I blip the throttle a few times and bring it back to life. It's currently set up pulse/mag (?) for the twin plug dizzy I have.

Would this new system be a solution (and what might be the problem)?

Your not the only one with this issue, this is why I'm designing a drop in circuit board that is universal for all Porsche tachs up to the early 80's. Your issue could be with the dwell setting, CDI box, or even the tach. VDO did a wonderful job designing their tach's with what they had to work with back in the day. They had mechanical spring/mass systems with little dampening. The tach signal has a lots of ringing to deal with. This is where modern technology comes in play. There is a time where trying to fix an old problem just doesn't make much sense, where upgrading with newer technology while keeping the original look makes better sense.


My new design is a better circuit which will accept all signals strong or weak. Using a solid state design to reshape the input wave form, and ultimately drive an air core frictionless movement. I'm even entertaining the idea of installing memory on this circuit board for high rpm play back. Just trying to figure out how to do this without installing a button somewhere.

This new design will have jumper settings for 7k tachs up to 12k along with different cylinder engines for each range. The circuit board will be the same diameter as the inside of the gauge can. The outer circumference will have a halo of LED lights, and my custom RGB controller already built onto the same board. A truly universal drop in replacement that kills 2 birds with one stone!


Sorry for the thread highjack

If it is of any help:

I just spent the last few weeks figuring out how the tach signal (from the coil) works and how to convert it to being read by a microcircuit.

I basically used the megasquirt design to step down the 400 Volt pulse that the tach sees to something manageable to trigger a 555 timer "one shot" then the output of the 555 gets read by a arduino and the tachometer is displayed in digital numbers (Hz).
Once you get the Hertz value (coil pulses per second), you then need to know if the engine is a 4 or 6 cyl, and if it has multiple spark firings per cyl firing. Then you do the math in the CPU.
The same circuit could easily drive a DC motor of the type used in standard tachometers using a digital 5DC output in PWM mode.

I even built an ignition mockup complete with points a coil, and a spark plug to bench test it.

total in parts is about $15 and fits on a circuit board the size of a silver dollar.

I know that what you are doing is different but I could lend help if needed.

Just get a 911 Tach. I used the same 911 tach with my 2.7 with webers that I am now using with my 3.2 with the stock DME fuel injection. Works great, no problems. If you have a 911 tach then something is wired wrong.
I also used a 914 trans with both motors. It should have nothing to do with the trans. 915 trans has a sensor for an electronic speedometer. Has nothing to do with the tach.

The tach is from an 86 911. I'll have to check the wiring again. The signal comes from the DME with the 3.2 setup.

Gully, I have the same setup, 86 3.2 with the 86 tach. Mine works perfectly fine with no modifications, etc. I did wire it wrong the first time, as the numbers on the plugs aren't 'logical' to me at least. Do you have the info on how you wired it?

I'll have to pull it out to look at the back of the tach. The wiring from Patrick Motorsport has the tach wire label from the DME. I wonder if I have it going to the wrong terminal on the tach. Can't remember which one, need to find my diagrams again.

The fact that it seems to read double suggests something is wrong at a basic level making it count a double signal rather than a single. I am in the same boat, but a ways behind you. Would you mind posting pictures of the sensor set up? Or did you go with the 915 transmission?

Any chance you are using a aftermarket CDI?

Some of those fire double pulses. That would explain your 2X number.

Stock/rebuilt DME unit. Everything else is stock OEM parts except the air intake. The sensors at the flywheel are new & stock configuration. I running a Dr. Evil 914 tranny.