Backwards reading Tach

[Caparco]

So I hooked up my restored and recalibrated tachometer had trouble with the wiring so I wired it direct to a switch power supply and direct to the MSD ignition coil negative,
  I do not have a full MSD ignition just a conversion kit distributor and the MSD coil.
  Strange sporadic things are happening,
   on the road it seems to operate fine around 2k, which seems normal
  But on the highway, with a burst of speed I've seen it go backwards, and very inaccurate readings, anybody seen this before or could try and guess what I did wrong?
  My guess wouldn't be there shop that rebuilt it, looking at my errors first.
Thanks for the help, tips and advice

[Bullitt-]

Having never heard of this issue I was surprised at the number of search returns I found.
Nothing stands out as most problems never seem to have a resolution  that I could tell except for a decade old topic about Corvette conversions that had an issue...

  You mention yours has a conversion kit on it replacing the points... I wonder if it could be a PerTronix Ignitor III... Those seem to cause issues with tachometers 

[Caparco]

Wow wicked nice of you too research this for me and look into it,
I myself didn't find much at all maybe I just didn't word it right,
  I'll have to look into that it was over 20 years ago when I did the ignition kit, one of the first things I've done in the car LOL
  Nevertheless it's definitely an order now for me to give  'Gauge specialist' a call
First thing Monday morning, I mean I sure don't want to break anything with what they fixed and maybe they can nail down the issue
  I'll report back with any findings
Thanks again! Good work

[Caparco]

I heard back from Gauge specialists
He knew the problem staright away,
  And yes it is the MSD compatibility with the stock tach. As I have just the super coil, and not the whole MSD ignition, he said that 'sometimes' it does work fine but sometimes it doesn't, the pulses are too fast for the stock tach, and it gets erratic,
MSD makes a little box , the tack adapter,
For just this problem and it will make the compatibility run more smoothly.

[70vert]

surprised a search didn't turn this up, maybe didn't specify MSD. Their "tach corrector device" has been around for 30+ years (probably since they first came out with the 6), I still have one from the 90's when I was running an MSD in my R/T. Glad you got it resolved.

[dodj]

Tach adapter is for when you are running the msd ignition box.  You said you just have an msd coil?  Maybe I misunderstand?
Be interested to see if it makes a difference.

[Caparco]

I gave him MSD a call just to be sure and apparently you are right The adapter only works with the ignition box, but a new question arises apparently I had it hooked up wrong
He asked if I had a voltage or current type of tachometer two wire being the current type and three wire being the voltage type obviously we have the current tachometers in these cars right? So that explains it,
It's supposed to connect to the positive side of the coil not the negative like a voltage tank does maybe that's why I was getting the backwards reading,

[Bullitt-]

  Stock tach gets it's signal from Negative coil terminal.....

[Caparco]

Right but with a MSD coil it's considered a current type tachometer, and it goes to there positive.
I guess I'm going to try it hopefully I don't blow anything and see how it works

[Caparco]

Okay LOL scratch everything I said!

In order to go to the positive side of the coil there should be power on the trigger wire with the key on, that didn't happen, now knowing that the tach adapter, only works with the ignition box which I don't have I'm back to square one I don't know what to do,
  Maybe I should just put in an ignition box, don't know if I'll do that right away I just got my wires all straightened out with the ballast resistor and the ammeter bypass,
Should just leave well enough alone without a tach. Obviously the gauge specialist assumed it would work once I mentioned the word MSD.
Three wire or two wire tach on the Challenger, it's obviously a voltage type tachometer.

[Bullitt-]

  Did you ever look to see what conversion you have in the distributor?

[dodj]

Stock tach is current type tach.
To me, the issue may be the tach itself. You said you hooked up your restored tach. And issues became apparent. Assuming that is the only change......
You also stated trouble with the wiring....
So check the stock wiring diagrams. Make sure you have it hooked up like before the rebuild.
If the wiring is correct...I suspect the rebuild may be the issue..
Just my 

[Caparco]

No..
Sorry, what do you mean?
It was over 20 years ago I bought the electronic ignition conversion kit. From the old points distributor.

[dodj]

No..
Sorry, what do you mean?
It was over 20 years ago I bought the electronic ignition conversion kit. From the old points distributor.

Your first post said you hooked up your newly rebuilt/calibrated tach and had wiring issues hooking it up.
After connecting, it did not work correctly?
Do I misunderstand?

[Bullitt-]

No..
Sorry, what do you mean?
It was over 20 years ago I bought the electronic ignition conversion kit. From the old points distributor.

Pertronics has had a popular electronic conversion for many years, from what I understand they make 3-vesions, the model IgniterIII is multispark similar to the MSD ignition and will cause issues with our tachometers.. There may be other similar products on the market.