Hooking tachometer up to Pertronix ignition.

[Cornpatch MO]

Using stock 1968 318 Mopar non electronic distributor with the standard Pertronix points conversion. And the Pertronix coil that a ballast resister is not needed for. ( internal resister. ) As per the instructions , the red wire from the Pertronix is hooked to the positive side (+) of the coil which is also the post that 12 volts from the ignition switch goes to. The black wire from the Pertronix is hooked to the negative side of the coil... It has worked very well for over a year.
Now, I am installing a new Bosh tachometer retro Sun Super . Switch on the back is set to 8 cylinder. One post goes to dash instrument lights....One post goes to 12 volt switched source, so power on when ignition key is turned on. ....one post goes to ground. .... one post goes to the negative (-) side of the coil...
Problem : Turning over the starter motor with the ignition key switch, the engine will start and the tach will show some engine rpm , but as soon as I let off the key for the engine to continue running, it shuts off the engine and of course the tach drops to 0 RPM.. Any input would be appreciated... It has me stymied........................MO

 

[1967coronet]

Mo , that sounds right. Pertronix makes no diff on tach.
Green to coil neg
White to inst lights at the fuse panel.
Mopar has that pesky volt limiter lol for dash lights.
Red to switched 12 volt
Black to a good ground.
Unless you have a half *** ground or hit the wrong spade on the fuse block I would try another tach.

 

[Cornpatch MO]

Mo , that sounds right. Pertronix makes no diff on tach.
Green to coil neg
White to inst lights at the fuse panel.
Mopar has that pesky volt limiter lol for dash lights.
Red to switched 12 volt
Black to a good ground.
Unless you have a half *** ground or hit the wrong spade on the fuse block I would try another tach.

Thanks for your input... The new Bosh " super Sun" retro tach does not have color coded wires , only studs on the back that has a label telling what to hook them up to. Up to installer how he color codes the wires. I triple checked my connections. I think the volt limiter you mention is only for the gauges. My tach light is correct because the variable dimmer works in the tach light. ..... That leaves the power to the tach when ignition switch is turned on.... No places to plug in a wire into the fuse box, so I had to slice in a wire to the black wire coming into the fuse box from the ignition switch. I ran that wire to an auxillary aftermarket small fuse box , so It would be easy to hook up other switched 12 v things if I need to. Used a 5 amp fuse and ran that to the tach.......................
OK... JUST NOW FIGURED OUT THE PROBLEM ! with wifey to run the ignition switch and me on the test light, I found no power to the + side of the coil with ignition switch on ---until engaging the starter motor and then there was power there as long as it was turning over. So I ran a test wire directly from the + battery post to the + coil post.. YES ! that made everything work including the tach. ( BTW , always disconnect you jumper wire from the battery first because if the other end gets grounded, it will smoke that wire in a hurry...Ask me how I know that..)...............
Now I have to find a switched power source, on all the time whether starting or not. to the coil. keeping in mind power is shut off to most everything when the starter motor is turning over..., keeping in mind too, I no longer have a ballast resister.................................MO

 

[Cornpatch MO]

I found it.. %#@*&%^(#$&% ballast resister causing problems, even when it's not there....checked everything I could think of, unhooked everything I had done, to get back to where it did run ...still no power to the + side of coil from the ignition switch..????? Tracing back , I found that when I eliminated the ballast R. I wanted to be able to reinstall it along with the points in case the Pertronix went bad and left me on the side of the road. I had made a jumper wire to bypass the removed B.R. and it had broken the connection... Simple fix, but a bugger to find the problem..............................MO...

 

[33 IMP]

Thanks for the final explanation, not leaving us hangin'. Glad you found it. (How do you like the retro tach?)

 

[Cornpatch MO]

Thanks for the final explanation, not leaving us hangin'. Glad you found it. (How do you like the retro tach?)

There has been a lot of discussion about the Pertronix conversions. I have had three of them on 3 different cars.. No trouble once they are wired right , but I did think the first one quit on me...Turned out, the coil had moved far enough for one of the posts to short out against the intake manifold..My intent is to show folks that the trouble they think they are having , may not be the fault of the Pertronix..
The Bosh retro tach----I got that because I had put a Sun Super Tach in my new Roadrunner back in 1968, and my Sport Satellite I have now, is a tribute car to it... Finding a good looking original Sun tach now , that works , and affordable , is hard to do.. This Bosh looks very much like it, but does not have the Sun Super logo at the bottom of the face.. Seems to work just fine, needle is at rest below 0 RPM but as soon as you turn the key on , it jumps up to O RPM. Needle is smooth motion up to 2000 RPM , but i will give it more RPM tomorrow when I go out for a drive... Chrome band on the front is good , but the tach cup is typical China showing polishing marks... Comes with steering column curved mounting base and a flat one.. also a strip of rubber for under the mount. IIRC it had a large hose clamp for steering column mount . Like I mentioned before, it does not come with any wiring... I was impressed by how it is packaged. In a sturdy plastic box , and the tach securely screwed down to the base... And that plastic box encased in shock resistant formed foam rubber, in a good cardboard box..................................MO