Re: Oer tach msd ignition update
Sounds like you've really been through the wringer with the MSD ignition and your tach.
I have a few ideas for you to try... I'll need to give you some background for them to make sense.
Factory "point" ignitions don't work the way a lot of people think. It isn't the case that the points open and one spark results.
Here's what really happens.
First, the points close, and electricity flows into the ignition coil. That sets up a powerful magnetic field within the coil. Then, the points open as the distributor bumps them open. Suddenly the magnetic field is all this stored energy, in a magnetic field, that wants to collapse. As each magnetic field "line" cuts through a wire, it generates electricity. There are a whole bunch of wires there, so a whole bunch of voltage happens.
The coil dumps out electricity, which creates high voltage, which sends a spark to the distributor, to the rotor, to where the rotor points, to the wire, to the spark plug.
But the magnetic field doesn't just neatly collapse to zero magnetic field in one spark!
There's a thing called a "condenser" in the circuit. That's the thing bolted next to the points. Everyone else calls it a "capacitor". It's a storage device. As the coil is dumping electrical energy to the spark plug, it's also sending electrical energy to the capacitor, charging it up and up and up. Finally the coil runs out of energy ...
... and the capacitor is now full, so the energy "sloshes back", and now the capacitor sends electricity right back into the coil ! As the coil magnetic field spins up again, this time high-speed, another spark happens; as it sloshes into the capacitor again, another spark, and so forth. The cap and coil complement each other; the electricity sloshes back and forth, just like water sloshing in a bathtub.
Electrical engineers call this "a capacitor-coil oscillator" (or an "LC oscillator", where L is the coil and C is the capacitor). You can use the values for the coil and cap and determine very accurately what frequency they'll "resonate" or "ring" at. (That's how we tune radios, by the way.)
So what *really* happens is when those points open, the coil and capacitor *ring*, *exactly* like they were a gong that was just struck. "Gonnnnnnnnng!" Eventually they wind down, but a forest of sparks happen while they're ringing. They put out a high "plus" spike, then a high "negative" spike, then more +, then more -, and so forth. It all settles down well in time for the coil to start charging up for the next cylinder.
If you remove the capacitor, your car won't start. That's how important those extra sparks are.
MSD and other aftermarket ignitions talk about how They Generate Lots Of Hot Sparks each time the points open and kinda imply the factory ignition doesn't, but that is not the case.
Engine Gnd<--Batter --> IgnKey --> Coil+ COIL Coil- --> Points --> EngGnd
The tachometer is generally connected at the wire between the coil and the points, which means it's at 0 volts (ground) when the points are closed, and whatever the coil is at (through the resistance wire to +12) when the points are open. This wire is also full of "gonnnnnnnng" stuff ... oscillations at the LC frequency, until the points re-close and it's clamped to ground again.
How the heck do ya figure out RPM from that messy a signal??? Bear in mind the engineers back then didn't have onboard computers to help them. (See also: those wonderfully accurate (?) car clocks).
While I'll need to drag an oscilloscope down to my Camaro to be sure, I'm going to bet that what happens is the tach watches for any non-zero volt signal to either come off of ground, or go to ground. (We call this "edge triggering".)
Now, I'm not a formal Electrical Engineer. I'm on the Computer Science side of things. But here's some thoughts.
Your tach may very well *want* the messy signal of a capacitor-coil "ring", that rings from high positive to high negative voltage back to high positive. Digital electronics can get into trouble with negative voltages in a hurry, and who knows how your HEI ignition was engineered?
I'm wondering if MSD just generates a simple square wave from +12V to 0 that corresponds to "points closed" and "points open", and none of the ringing that a tach just might expect. Again, I'll have to 'scope this to check. (I have an MSD ignition, so I can look).
Another thought: You may have a ground issue, where the MSD square wave is being generated relative to one ground, and your car's dash panel is at another ground. If the dash panel works like I think it does, then even a few ohms of resistance would be quite critical. Actually, the more I think about it, the more this makes sense. (Heck, I got tripped up by corrosion inside a battery clamp; this was trickier than it sounds.)
On the ground side:
1) Measuring the voltage, if any, between the battery ground pole (not the clamp, the actual battery negative post!) and the MSD ground connection;
2) Measure the voltage, if any, between the battery ground pole and the dashboard tachometer negative (ground) trace.
You should not see even 0.01 volt.
On the +12 side:
3) Measuring the voltage, if any, between the battery +12 pole, again, the battery post itself, and the MSD +12 connect (the MSD does not use the built-in resistance wire that the old coil used, I believe; did that accidentally get used, or did you run a new wire?)
4) Measuring the voltage, if any, between the battery +12 pole and the dashboard tachometer positive trace.
You should not see even 0.01 volt.
If you see *any voltage at all* on these tests, then there's resistance that should not be there, causing a "drop" in voltage that your meter is seeing. You may have to run a seperate +12 and ground for your tach, but the best solution would be to fix the ground problem.
You might also:
5) Take the tach to the engine compartment, and connect its +12, ground, and signal to the MSD unit right there. If it starts working, you have yourself a ground problem, or just possibly, -- a flaky wire --. You just might have a wire bad enough to trip up the MSD but good enough if used with a messy stock signal, which have really big spikes in them.
These are old cars. I was 12 when my 1970 SS was built. The ground connections to frame can get pretty tatty. (Heck, some of my connections are a little tatty... *grin*). I've run two very thick ground straps to engine and to frame from the battery just because of this sort of thing, which eliminated these problems for good.
6) If you have older-style fuses that are about 1 inch long with glass windows, then maybe you've gotten burned the same way I did.
I picked up some fuses at Radio Shack and installed 'em as needed, and things worked erratically or not at all. A long and painful debug later, I found out those actual fuses had a resistance of 1.0 ohms, each. It boiled down to, these were "110 volt" fuses not intended for cars. At 110 volts, 1 ohm matters less than at 12 volts (although it's still a damned crummy trick!)
Let's say your ignition coil pulls 10 amps (which is reasonable) and you have one of these #%#@@#$ fuses in the circuit. E / IR ==> 10 volts will try to drop over the fuse. In reality, the coil and the fuse will then fight it out as to who drops more voltage between them, but it's a great way to make sure things go flaky after the fuse box. You'll see widely varying "+12" voltages after the fusebox depending on load. You may start thinking you're losing your mind. I was sure wondering what the heck was going on!
It's easy to check if someone put these type of fuses in, once you know it's possible. Look for measurable resistance across the fuse. Good fuses tend to be marked For Automotive Use.
By the way, Watts = IR^2, or current in amps X resistance squared. Let's assume you're pulling 10 amps on a 110 volt circuit. 1 ohm squared is 1. 1 x 10 is 10. That's 10 watts of heat coming off that blasted fuse. Generally fuses are in sealed areas and have no way to dump the heat ... so this is a doubly crummy trick.
Hope this helps some,