Back in the early '80s I had a Mallory Tachometer with built in rev limiter and shift light. Two dials, one for each function. It was a four wire hook up, power and ground, lights, and tach signal. I used it with HEI ignition. How did the tachometer kill the signal through the tach terminal? Was it grounding the wire to make it hold the set rpm? I do know it killed all cylinders at once. I was on another forum and they were looking at how GM used a relay and kick down switch to momentarily kill the ignition, ( points ignition ) this let the Overdrives kick out without a load on the drivetrain on an earlier Chevy truck. Not wanting to kill the power source to HEI, it made me think of the Mallory tach. Any idea how this worked?
This might get bumped to the OT section. I had one too. I liked it a lot. Very responsive and accurate. I searched for this and found that MSD has flooded the results to the point of nothing else showing. You should be able to see that result. I was able to find this which I have no experience with, but is a different option. https://www.pegasusautoracing.com/productdetails.asp?RecID=5313 If you want the OG, they are out there but not cheap. https://www.2040-parts.com/nos-mall...r-built-in-rev-limit-shift-light-wow-i628251/ BTW 2040 parts seems to be a site that just s****es the net and posts results. I'd imagine it's fairly simple to program something to 'set' the limit then ground the tach signal if that is exceeded. Both the MSD and the Pretronix are small finned boxes, so a bit of heat is involved. With the rise of turbo stuff and banging against the limiter, I'd think there would be more options, but most or all of those are probably part of the ECU, not standalone. DIY? https://www.ecosia.org/search?method=index&q=combustion+engine+rev+limiter+arduino
The fellow doesn't need any limiter, they just need to momentarily kill the power to let the overdrive drop out. I thought using the tach terminal like the Mallory tach did might be easier on the module than just shutting off the whole power source to distributor. Maybe it doesn't matter, it just made think about how Mallory did it. If it's a simple ground to the tach signal, a relay could do that pretty easy along with the kick down switch. I know it would be easier to wire up since you wouldn't need as big of wire or high amp relay as you would if working on the power feed side.
If it's just a momentary kill, a relay should be easy. The only concern would be the ignition coil field collapsing, with the attending voltage wave. I'm not a certified electrical engineer (far from it!) but if you don't get an answer from one of those, all it's going to cost is some wire and a relay if it does croak something. Since you are doing this with the OD switch, you don't even need the logic of the Arduino I showed.
On an overdrive the relay was used to supply power to the OD solenoid. When you floored the throttle pedal a switch under the pedal closed a circuit to a set of grounding points inside the OD solenoid. The other side of the circuit was connected to the coil primary on the distributor side. The kickdown switch also killed power to the OD solenoid. So the solenoid is not energized and the ignition circuit is interupted. With no torque on the sun gear, the spring inside the OD solenoid is able to retract the pawl that was locking the sun gear to the case and you go back to direct drive. Retracting the solenoid also opens that set of points that allowed the ignition to be interrupted. It all happens in a split second and you never notice the ignition being interrupted.
So will it work with HEI ? Can a simple ground to the tach signal interrupt the ignition just long enough to drop out of overdrive? It would be a much simpler wire and relay job if it will.
2 minutes with a jumper wire will prove it to you! https://wireblueprint.com/wp-content/pic/gm_hei_distributor_and_coil_wiring_diagram_mkw.jpg
On a HEI the tach wire terminal is the same as the Neg post on a conventional coil. On the Mallory Tach the Rev Limiter simply grounds the green signal wire to the black wire. Some drag bikes use a micro switch to temporary kill ignition while clutchless shifting and in road racing you can get this feature for a dog-box [Flat shifting] where a micro switch is connected to a load cell on the shift lever.
Sorry for the tangent: my late Alfisto cousin had a string of Alfasuds. Those had mechanical rev limiters in the form of a calibrated spring-loaded counterweighted slider in the distributor rotor. Encasing that in silicone allowed the little flat-four to spin to 8500rpm — which it did consistently enough.
https://www.weirdfacts.com/en/origin-of-phrases/origin-of-phrases-b/3906-balls-to-the-wall That one ought to give the censors a fun time.
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