Types of Mopar ignition systems
Chrysler offered these engines with several different distributors. The earliest ones used breaker points. Chrysler made an electronic ignition standard in 1973, which used both a mechanical and vacuum advance. These distributors used a variable reluctor type sensor, much like the later GM HEI and Ford Duraspark. Chrysler began introducing electronic spark timing control in 1976, and by 1977 brought out their infamous Lean Burn system, which used a distributor with no advance mechanisms at all. The Lean Burn system kept the same VR sensor as the earlier systems. This combination of a computer controlled timing on a carbureted engine is something of a rarity in the automotive world.
One thing about Chrysler ignitions that can create a bit of confusion is that Chrysler chose to name their ignition module an Electronic Control Unit. While this shortens to ECU, this is not at all the same sort of ECU as an engine control unit, at least not in its earliest incarnations. Such units as the popular Orange Box are simply ignition modules with no computing capability (the Lean Burn ECUs are another story).
In 1988, Chrysler introduced a new distributor design along with throttle body injection for some versions of the 318. This one uses a Hall effect sensors similar to the one Chrysler had used in their K-car engine control systems. The 360 soon acquired the same system. When the Magnum engines came out, Chrysler designed yet another ignition triggering system, this time using an eight notch crank trigger and a Hall effect sensor in the distributor with a single tooth trigger. These two distributor systems, along with the points distributors, are outside the scope of this article. Here we will cover the VR sensor based systems, as they are the most commonly retrofitted Chrysler electronic ignition.
Chrysler distributors will physically bolt on to any other engine in the same family: All LA distributors are interchangeable, all slant six distributors are interchangeable, all B distributors are interchangeable, and all RB distributors are interchangeable. It's even possible to put earlier distributors onto Magnum engines. So when planning a Megasquirt buildup, you can mix and match distributors to get the easiest to configure distributor on the engine of your choice.
Before rolling out the Hall effect distributors, Chrysler built several different variations on their "classic" VR type distributor, not including the number of different advance curves:
One VR sensor, with mechanical and vacuum advance
Two VR sensors, with mechanical and vacuum advance
Two VR sensors, with no advance mechanism
One VR sensor, with no advance mechanism
Most of the distributors you will see are either of the first or fourth kind, with the remaining two being seldom seen designs used with the earliest versions of the Lean Burn system. When setting up Megasquirt on your Mopar, you can use any of these four types. The most straightforward is the fourth type, with just one VR sensor and no advance mechanism. If your distributor has an advance mechanism, you must lock out the mechanical advance and disconnect the vacuum advance. If there are two VR sensors, simply ignore the second one. While the Lean Burn system has a bad reputation, the distributors are just as reliable as the earlier ones, if not more so. One of the biggest sources of problems with the Lean Burn system appears to have been Chrysler's decision to mount the computer on the air cleaner.
Many popular aftermarket distributors for classic Mopars also use VR sensors, including most of the choices from MSD and Accel. These directions will also apply to those distributors if you lock out the advance mechanism (if any).
Chrysler was not big on separate ignition modules and timing computers - when they brought out computer controlled timing, they went straight to integrating the ignition module into the timing computer. The Orange Box and electronic control units are not meant for computer controlled timing. Consequently, the easiest ways to control timing with Megasquirt are to either use a seven pin GM HEI module or, like Chrysler did, have the computer control the coil directly.
