Well my orig equipment ribbon tach was reading low before my engine died and after overhauling my engine there was no fire and I found out the coil didn't have full voltage. Now that I've jumpered the pos side of the coil I've got it running, but no tach. So I'm just learning about the coil and tach. I'm thinking the neg side is not making a strong enough current pulse to trigger the tach but the coil IS making enough current pulse to power the ignition. I'm wondering if the negative coil run straight to the neg post of your battery would be sufficient. It would be an easy test. I've got a Sunpro aftermarket tach to help me figure mine out. Good luck and let us know what you find out.
Also, I've had Pertronix as well for about 10 yrs which replaced the dual points that were there.

 

I'm wondering if the negative coil run straight to the neg post of your battery would be sufficient.

Unfortunately, that wouldn't do anything but give you no spark and burn up your coil. It's the cycling pos-gnd-pos-gnd on the (-) coil terminal that gives you spark, so a constant ground just causes runaway coil charging and no "un-ground" for it to spark.

Look at it this way. When the points (or PerTronix) are closed the circuit is grounded and power flows through everything including the coil. The (-) terminal would obviously test as grounded under this condition. When the points open (or PerTronix 'off') the grounding stops and the primary winding's magnetic field collapses, giving you a spark from the secondary windings. At the same time, since there is no grounding, the (-) terminal will show POSITIVE voltage as it's the open end of the line from the (+) power source. Make sense?

Okay, then let's throw one more thing in for tach use. When the circuit is broken (distributor points open) for the coil to fire, not only does the (-) terminal go positive, but the effect of coil firing is a 'ringing' through the windings, causing small higher voltage pulses on the order of about 100 to 400 volts. Be aware that most tachometers that signal off the (-) coil terminal actually use the higher voltage pulses to trigger. Simply switching 12v on and off will not work well with most of them.

What can cause the pulses to be weak? Poor block grounding to the chassis (usually a ground strap from the rear of a cylinder head to the firewall), poor tach grounding, or a weak condenser can do it. If you no longer have a condenser with electronic conversion, you can increase the pulse intensity with a small coil capacitor from an EFI car, commonly called a radio suppression capacitor. They are usually located very close to the (+) terminal of the coil and can be any shape (cylinder, cube, etc.) but share the +12v at the coil terminal and then ground nearby. First check your grounds, and then try adding one (from any make or model is fine) to see if that doesn't help.

David

Random ignition capacitor (RF condenser/suppressor) from a BMW or Volvo or some other euro import I can't remember, but they're all functionally the same. Ford EDIS distributorless coil pack capacitors usually look similar but can be cylindrical or blocky U-shaped. Whatever... lol. Grab one off the first wreck you see. They are free to $2 at my local 'yards. Wire goes to coil (+) and tab to ground:

 

Your Coil is NOT supposed to get 12 Volts at ANY time except for the Cranking position.

That coil is designed to run on about 6 or 8 volts through a resistor wire. The Tach works off the pulse of the discharge in the coil not off the voltage.

 

Ok if I'm understanding what your saying then the tach I just bought and installed yet still needs wired won't work because I have a pertronix set up in my distributor? I know in my 70 Stang I added a tach but it had the Mallory unilite set up which has that window kind of rotor which I'm guessing works more like the stock points set up.

 

Yes it will still work if it's hooked to the Neg side of coil. The pertronics works just like points but with no metal to metal contact.

In a points system the coil is sending the pulse to the points pretty much all the time.

When the points close, the electricity flows out into the dist cap and into the spark plug then jumps the gap to ground at the ground strap setting off your fuel charge.

When the Distributor lobe reaches the set parameter on the pertronics (same place the points would be closing in the case of points) and set's off the pertronics unit electromagnetically, THAT pulse which is then released from the coil is the signal for the tachometer to begin it's calculations of the rpm's by counting the signal pulse electronically and dividing it by 8 or 6 or 4 (as per the application) to come up with one RPM, then it displays it on the tachometer face for you


The Tachometer feels and records the RELEASE of the coil's energy the instant the spark crosses the gap on the plug to the ground strap, not when the points close or when the pertronics sends the energy to the plug. But when it releases across the Gap.

Rick, if you set it up just like a normal points setup it will work fine.

 

I have had no issues running a pertronix and a tach..Are you sure you are wiring the tach properly?..What colours are the wires?..You normally have a red for + black for - and a white for the light and a green to go to the - side of the coil..

 

Excellent! Guess I'll start hooking up the wired tonight. Thanks guys!

 

Thank you for the input, nothing that indicates a issue outside the norms on my specific application (fe390) so I can credit it to dumb luck that "I" get 2 non functional units. I guess i will have to go new off the shelf with a retro look rather than retro piece.
Sure wish I could find someone who could repair that old Dixco Illuminator, not the best tach out there by any stretch but it was rather unique even in 1981.

 

BTW - although the electronic parts in old tach's are often bad (especially dried-out capacitors), many of them only suffer from oxidized solder connections. A guru on another site recommended simply re-melting the solder joints, and sure enough, about half the tach's I try this with makes them work perfectly. Maybe worth a try as it's a cheap fix.

David

 

Very true!

I've repaired Hundreds of old Tube and Solid State Radios and Televisions simply by doing that very thing! A soldering gun is your best friend in the world of electronics! Even better than a multi-meter if you re-solder everything

 

Back when, I had a 3-4 inch dia tachometer strapped on to the column of my 1967 Fairlane, and it was getting to be only hours away from the trade-in deal. The dealer had a make it look stock and normal, no frills. So the tach, and other things had to go, and right now too. I had my friend over to help get it stripped and ready.

He essentially ripped the tach off the column and the coil. The wires were colored yellow, green and red. I know the red was for the illumination bulb, but which ones went where after that I don't know. Only thing it was accurate and worked good. Last day for that.

Next weekend, we simply began connecting the other taken off items (and the tach) into the new car. Tach did not work, it sort of got pinned by simply running up the engine, and seconds after closing the throttle, it remained high, and then wandered down. Broken then. Tried it again years later on and same thing.

Seems the older tachometers did not have circuit protection, and if you hooked them up wrong for even one second they would blow and act screwey thereafter. I think not having the original wiring diagram did me in then. For which wire went where, as I only knew to hook up the red wire a certain way.



Wm.

 

Don't toss that Dixco! I can't put my finger on them at the moment, but there is (were) more than one outfit that specialized in repair and restoration of Dixco tach's. Mostly the GM versions (like T/A hood tach's), but they're all the same inside. I also saw a website some time back that showed how to swap modern guts into the old housings. Maybe worth some searching.

David[/QUOTE]


If you happen to recall it please feel free to pm me with it. I tried the sun piece on another engine and verified that it is indeed a dead piece, no desire to try and repair that one but would put an effort towards the dixco. If nothing else, maybe when it is cold out this winter I will have time to try reheating the solder connections.

 

I know this is a Captain Obvious statement but assuming one or both tachs in question are adjustable for 4 6 8 applications, make sure they're set to 8. Hard to get a V8 to run with a tach set on 6.

Experience does not grade on a curve