The history: was sent out to redo the face of my 6000rpm factory tach (on my S code tribute car) to an 8000rpm look-alike. I put in a Petronics II module in the distributor. Seems to only read a constant 500rpm. Kind of at a loss as to what the check.
Plus, can't send it to the best tach man we used to have any more if I need too.
I have a similar problem with my tach and I have a Pertronix setup currently that I have not yet replaced for points. However, Bob Gaines provided a very elaborate explanation on how to fix the Pertronix problem. My tach probelm began when I took my car in to fix the 12 volt connection issue and the place hooked up a third wire from the coil to the parking light switch I believe it was. That is when my tach seemed to hover at 1500 to 2000 rpm and has not gone away. Here is Bob Gaines explanation on how to get the Pertronix right. See below.
If you are getting power from the coil then your problem may be because the Pertronix is not getting a full 12 volts like the instructions says that is necessary for the module to work properly. The coil source is a cheater way of getting power for the module . It will work but since it is not a full 12 volts (9 1/2 volts I think when key is in on position through the resistor wire) can sometimes cause problems. To get a full 12 volts typically a wire is connected in the harness at the ignition switch shortly BEFORE it turns into resistor wire for the coil and run this extra wire out to the module for power. The problem you describe is a common problem with Pertronix and the change to full 12 volts like the the factory says the module requires is the common way to typically solve it. Another way to get 12 volts out to the module without running a extra wire is to eliminate or bypass the resistor wire in the harness to the coil but then you MUST run a after market coil that has a built in resistor . Most if not all aftermarket coils are made to run on a full 12 volts or the lower voltage of the resistor wire because they have a INTERNAL resistor so only the lower voltage amount gets to the coil windings. If you run a full 12 Volts to a Ford yellow top coil it will heat up to a point that you can't even touch it before failing or exploding! :o I don't know how many times I have heard the Pertronix problem stories and getting power at the coil is a root cause on the vast majority.
More from Bob Gaines:
It is pretty simple to figure what wire to use at the ignition switch for full 12 volt power. There is a short wire with a bullet connector that comes off the ignition switch that your tach wire plugs into . You tap into the wire someplace between the ignition switch and the bullet connector and run that wire out to the pertronix. You are all about day 2 modifications so the extra wire should not offend your sensibilities. I would run a black wire so that it blends in more with the existing harness but that is just me. If you want to get creative you can tape wrap the extra wire into the harness at the firewall and let it exit out closer to the coil so that it is harder to distinguish as extra. Best of luck on how ever you decide to solve your problem.
Yes the coil was factory installed on the passenger side. Yes I can see that you are getting power from the coil terminal. I am familiar with your style coil and it has a internal resister so it can be used with the stock resistor wire that steps down the voltage to the coil or with a modified wiring with full 12 volts from the ignition switch bypassing the resister wire wrapped inside the stock wiring harness. The resister inside the aftermarket coil steps down the voltage inside before it gets to the windings. The problem is that if you have a stock wiring harness with the stock resistor wire wrapped inside then your aftermarket coil terminal is getting less then the full 12 volts and consequently since that is where you are getting power for the module is suppling less voltage then what the module needs to work properly. As has been said before sometimes the module will work properly at least for a while with the lower voltage and sometimes it will act up and stop running similar to the symptoms you describe.
By not doing what Pertronix explains in the instruction to do (full 12 volts to the module) it is a roulette wheel of chance of how long it will work without problems. It may work for a good long time or it may fail in a month. The possible failure is because you have it connected to the coil power which is not a full 12 volts. Murphy's law dictates that it will fail at a inopportune moment. I would run the dedicated 12 volt wire to the module as the instructions say is required for problem free operation.That way you take the roulette wheel out of the equation.
I did not want to become the Pertronix answer guy but when I see suspect incorrect information it tends to bother me. I went to the Pertronix website to check about the coil. The ignitor 1 requires a 1.5 ohm or more coil. The ignitor 2 requires a 0.45 ohm coil or more. I must have missed it but I did not find any information on the ignitor 3 . The Ford yellow top coil falls within the Pertronix parameters. That is if it is working properly. Most aftermarket coils fall into compatible category too. I think there is a Pertronix flamethrower coil that will match up with their ignitor modules. Something that has not been mentioned but I was reminded of when going over the installation info is that the spark plug wires must be resistance type wire . Interference from the solid core wire will damage the module. That reminded me also of the urban legend that 289 hipo in a GT350 was supposed to have solid core SX wire. That is false . The regular production GT350 used spark plug resistance wire that was marked "resistance wire" form the factory. This has been documented by vintage factory pictures.
Ignitor1 1.5-3.0 ohm
Ignitor2 0.45-1.5 ohm
Ignitor3 0.32-0.6 ohm
Lower resistance will burn down your coil and higher resistance will make less spark on the spark plugs
Well, I spliced in a new wire before the pink wire under the dash. And ran that out to the coil. And, I bought the Petronic's coil that they advise.
But, I will check the voltage out at the coil wire but it ought to read 12 volts.