Re: 1978 Chrysler 70hp igniition question and CD information
Your engine has the Motorola Magnapower system. If it is indeed a 1978, then someone has swapped out the elecrtric eye distributor for a points distributor. Not much difference but the electric eye is a LITTLE more precise and never needs gapping.
That ground wire is absolutely essential. The distributor body is insulated from the mount and the rest of the engine by grease in the bushing. Without a solid ground, the points which are grounded through the body of the distributor will not go to ground when closed and will not signal the CD box to dump the capacitor voltage into the coil--NO SPARK!
NOW: The CD box has TWO power sources. 1. The red wire is always powered whether the engine is on or not. It charges the capacitors in the CD box. That is why if after sitting a long while with the battery disconnected, when you go to connect the battery, you will get a spark.
2. The blue wire powers the electronics inside the box and is only on when the switch is in run or crank position. When you turn off the key, the box gets no power, the capacitors stay charged, but no signal is sent to dump the capacitors into the coil.
How it works: (Generally, and some possible minor errors but basically correct.) Capacitors are constantly charged by the battery, regulated by a transistor. The CD box sends a voltage to the points. When the points open, the voltage has no where to go except signalling the gate transistor to dump the capacitors. At this point, voltage builds up very quickly in the primary of the coil because the capacitors dump almost instantaneously. Also, because they dump so fast, the voltage drops almost instantaneously. The magnetic field collapses and the secondary now generates about 60,000 volts to fire the plug.
Of course, at full throttle and 5000 RPM this is happening 20,000 times per minute so you can see that there is a lot of work the capacitors are doing. A lot of heat builds up in the CD box. That is one reason the newer Prestolite system is superior: Two boxes with four sets of capacitors and four coils. Each unit is doing only 1/4 the work that the Motorola unit is doing AND it is more precise because there is no distributor.
If it starts well and runs well until the CD box heats up, then that is typical failure mode for a capacitor: It will short out when hot but behave normally when cool. This would indicate that the Cd box is failing. These boxes are "potted" completely and are not repairable. Well, you can have them repaired but it costs just as much as buying a new one. First try to borrow a known good one to be certain it is the box, then try to buy one on the auctions.
