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Is there a way to bench test a tach?

Dealing with a car that came in running a FITech 30002 EFI with a MSD 8920 tach adapter wired in under the dash.  There is no MSD box presently in place, though I wonder if there may have been one prior to the FITech installation. Lots of taped off abandoned wires, some added, some stock.

Tach will not indicate with the adapter connected or when bypassed and re-connected as stock to the negative pole on the Ford e-coil it is using.

Downloaded FITech instructions show no way to tie in a tach or indicate that the 8920 adapter is required. The FITech wiring diagram I show is one of several options provided and is how I think the system was installed, being the only layout that shows using the black wire to the coil with no MSD 6AL box.

Downloaded MSD 8920 instructions all show them connected to MSD boxes with no reference for use with the FITech or other systems.

Without a pretty obvious solution, like a bad tach, I have advised the operator to continue enjoying his ride this season without a tach until the creative wiring and fusing in place can be unraveled and rearranged over the winter. There are literally 30+ fuses installed throughout this car, along with a few circuit breakers too. Its confusing.


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  • FITech wiring
  • 78 firewall wiring
  • 78 custom fuse layout
Last edited by larryw
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If your tach really is inoperative and you take off the face of the tach (difficult to do without breaking something and there are NO individual parts available), inside you'll find a matchbox sized printed circuit board screwed down, with precision 1% value glass resistors soldered on. After 50 years, a resistor may crack, which opens the circuit. You may need a strong magnifying lens to see the crack. Desolder and it will confirm its condition by falling in two pieces no matter how careful you are. Replace the resistor with its exact 1% value. If you're lucky, thats all thats wrong.

Note- Even if you use precision grade 1% resistors with the exact same color code, this will likely still change the calibration of the meter slightly, so electrical recalibration may be necessary.  Some of the other components are electrically  'trimmable'. Then reassemble the tach again without cracking or denting the plating on the chrome face-surround.

So bottom line, unless you are a trained electronics tech,  you may be better off to just replace the tach entirely. They have never been cheap.  All this is why few 'speedo rebuild' shops actually do more than clean the face of a Veglia tach during their refurbishing (not really a rebuild). All known tachs in DeTomaso cars are the same- Pantera, Mangusta, Longchamp, etc. Not sure about other brands but a broken tach has some value so don't throw it in the trash. Good luck.

Without an ignition box, the Pantera's tach connects to the negative coil terminal. No tach adapter is needed.

The blue wire or the black wire (not both) connect to the negative coil terminal. The blue wire is used if the FiTech is NOT controlling the ignition timing.  The black wire is used IF the FiTech is controlling the ignition timing, and the distributor's advance MUST be locked out.

Check with Jon Haas (Pantera Electronics) about repairing the tach.


Last edited by jb1490


@larryw posted:

Is there a way to bench test a tach?

I never got a chance to verify my notes actually worked and hate to cause damage to someone elses . . .  But here is youtube of bench testing tachometers.

In leu of the "transformer" of unknown ratio, I've heard a battery charger provides the wave form to trigger the tach.

However.   I haven't done this myself and don't think I could "anlysis" any more

Agree with John, iIts probably only the issue of the electronic ignition; if you have a distributor with simple points and connect the Veglia tach to the coil, it probably will trigger. The Veglia tach itself is fantastically common. They are neither precision devices, as I can tell the differences between application on 4 or 8 cylinder are rotation of a trim pot. Opening the speedo/tach are actually easy (completely different ballgame from the 2" gauges)...which probably explains why bezels are completely unavailable for the big gauges.

  A "Tach Amp" can splice the difference between a digital trigger (which I suppose the EFI unit outputs) and the Veglia. Suggest this as a cheap start   

Last edited by leea

...a whole replacement DeT tach is not hard--they come up all the time at about $200. If you want this one repaired, Bosswrench is correct that a place like Palo Alto speed can do it for about twice that. If you want to move the movement from one working tach (even a Fiat 124 tach you can buy for as low as $20), you still will want some way to move the needle to the right place (calibrate against another tach).

The assumption is that the tach is really bad--honestly, connecting the +12 to the tach input won't kill it (it is isolated by a resistor and cap on the input, so really no current will flow). So if the other tach you've measured the circuit against is another 60/70's Veglia tach, OK, the other may be bad--but any other $15 modern tach from Hong Kong will switch off a low voltage square wave. (the Veglia will not)--Lee

Last edited by leea

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