DIYAutoTune.com Engine Management Systems & Tools

How to use Megasquirt with vintage Mopar ignitions

If you haven't already, please read the 'disclaimer' at the top of the parent page here.

Updated 2/15/2006

Engines covered:

Pre-TBI LA series (small block) V8, all B and RB (big block) V8, and all slant six engines when equipped with electronic distributor

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.

Using the MegaSquirt-I (MSnS-E) with Direct Coil Control

This is a straightforward option with a V3.0 board.

MegaSquirt-I PCBv3.0 Mods Required:

Build the VR conditioner circuit, as described in Step 51 of the MegaManual. All our preassembled Megasquirts with the V3.0 board come with this circuit installed.
Jumper TachSelect to VRIN.
Jumper TSEL to VROUT.
You may need to adjust the VR trim pots, R52 and R56, when this is installed on a running engine. A usual base setting is to turn them all the way counterclockwise.
Jumper IGBTOUT to IGN to send to IGBT ignition coil driver signal out of pin 36 on the DB37.
Cut out R57 if fitted (This won't be there on my units, though.).
Get a 330 ohm 1/4w resistor and cut the leads down to about 1/2" at each end. Maybe a bit less.
Tin each end of the resistor with a bit of solder.
Cut a 5" piece of hookup wire (22ga is fine) and strip just a 1/8" or so. Tin the stripped wire with solder.
Melt the tinned wire tip to one end of the tinned 330 ohm resistor tip and let it cool.

Heatshrink wrap this wire/resistor assembly.

Use this wire/resistor combo to jumper the 'top' (top as in when you facing the silkscreen side of the PCB, with the text so that you can read it normally) lead of R26 to IGBTIN on the opposite side of the PCB.

That's it!

External Ignition Wiring

Connect the positive wire from the VR sensor on the distributor to pin 24 (Tach on the relay board). See notes at the bottom of this article for the wiring colors on various distributors.
Connect the negative wire from the VR sensor on the distributor to a ground pin, such as pin 2 (or simply the engine ground on the relay board).
Connect the negative terminal of the coil to pin 36 on the Megasquirt (S5 on the relay board).
You can bypass the ballast resistor (if your car has one) by either splicing the wires at each end together or replacing it with a loop of wire.

MSnS-E Ignition Configuration

Configured for MSnS Spark Output using LED 17.
In MegaTune, you MUST set Spark Output Inverted to Yes.
You will need to adjust the dwell settings based on your particular coil.
The Trigger Angle setting will depend on the physical location of the trigger pickup. Often, an angle of 10 degrees will work. To set the timing with MSnS-E, go to the Spark Settings menu and set Fixed Angle to 10 degrees. You can then set the ignition timing to 10 degrees with the engine running, using a timing light. Once you have set the base timing, set Fixed Angle to -10. This will tell MSnS-E to use the timing map.

Using the MegaSquirt-I (MSnS-E) with a 7 Pin HEI module

If you happen to have a V2.2 board, using a HEI module can be easier than building a VR conditioner input circuit on a board that never had one. This is also a viable option on a V3.0 board, although it does use a bit more hardware (and one more output wire) than the direct coil control option. The MSnS-E Manual has detailed information about how to set up Megasquirt to work with an HEI ignition. This write-up uses slightly different pins to better work with the relay board.

MegaSquirt-I PCBv2.2 Mods Required:

The input mods are to invert the input signal from the HEI module's tach signal. The HEI module will ground pin 24 to trigger a spark input.
Remove D5, D8, and R10.
Replace D8 with a 1.3 k resistor. You may substitute a resistor between 1.3 k and 2.0 k.
Remove the jumper from XG1-XG2. Jumper XG1 to the right (non-banded) side of D5.
Solder a jumper from the lower hole where R10 used to be (the hole nearest U4) to the upper lead of R11. This will provide a 5 volt pull-up.
You will use LED 17 and LED 19 as outputs.
Solder a 1 k resistor from the negative lead of D17 (it's marked with a small - sign) to the right lead of R23 to provide a 5 volt pull-up on the output.
Run a wire from the negative terminal of D17 to jumper X12.
Now you will add the equivalent mod to D19.
Solder a 1 k resistor from the negative lead of D19 to the right lead of R27.
Run a wire from the negative terminal of D19 to jumper X11.
That's it!

MegaSquirt-I PCBv3.0 Mods Required:

The input mods are to invert the input signal from the HEI module's tach signal. The HEI module will ground pin 24 to trigger a spark input.
Remove D1 and D2, if they are installed. Replace them with jumpers.
Remove the jumper between XG1 and XG2 and any jumper leading to TachSelect, if fitted.
Run a jumper wire from TachSelect to XG1.
Run a jumper wire from OptoIn to one of the two +5V holes in the Proto Area.
Remove C30.
Replace R12 with a 1.3k resistor. You may substitute a resistor from between 1 to 2 k.
Jumper OPTOOUT to TSEL.
You will use two LEDs as ignition outputs.
Connect a 1 k resistor from the +5V source in the proto area to the negative lead of D14. The negative LED leads are marked with small - signs on the board.
Run a jumper wire from the negative lead of D14 to the IGN jumper hole.
Connect a 1 k resistor from the +5V source in the proto area to the negative lead of D16.
Run a jumper wire from the negative lead of D16 to the IAC1A jumper hole.
That's it!

External Ignition Wiring

Bypass the ballast resistor (if your car has one) by either splicing the wires at each end together or replacing it with a loop of wire.
Connect the positive terminal of the coil to the stock Mopar coil wire and to the + pin on the HEI module.
Connect the negative terminal of the coil to the C pin on the HEI module.
Wire the distributor's VR sensor to the HEI module, positive distributor wire to N and negative to P. See notes at the end of the article about which wire is which color.
The R pin is the HEI module's tach output. Connect it to pin 24 on the Megasquirt, or the Tach pin on the relay board. Note that the DIY Autotune wiring harness has a shielded wire - connect only the inner wire to this pin!
Connect the E pin to pin 36 (relay board terminal S5) if you have a V3.0 board. If you are using a V2.2 board, use pin 27 or relay board terminal S2.
Connect the B pin to pin 25 (relay board terminal S1) with either board.

MSnS-E Ignition Configuration

Under Codebase and Output Functions, set HEI Ignition to "GM 7pin HEI" and the other options to Off.
Set LED 17 function to Spark Output A, and LED 19 function to Spark Output B.
In Spark Settings, set the Trigger Angle to 10 degrees and set Spark Output Inverted to No.
To set the timing with MSnS-E, go to the Spark Settings menu and set Fixed Angle to 10 degrees. You can then set the ignition timing to 10 degrees with the engine running, using a timing light. Once you have set the base timing, set Fixed Angle to -10. This will tell MSnS-E to use the timing map.
Under dwell settings, set dwell control to fixed duty and the spark output duty cycle to "Minimal for HEI4."

Using the MegaSquirt-II PCBv3 with Direct Coil Control

If you're assembling your MegaSquirt-II from a kit (Part# MS230-K):

For the most part, stick with the standard assembly documentation at http://www.megasquirt.info/ms2/V3assemble.htm. I'll just be covering any departures from that doc here.
Step 51: You'll be installing the VR input circuit (currently all steps under 51). You do not need the components in step 50, although it's handy to install them in case you ever use the Megasquirt on a different car (or decide to convert your Mopar to a ignition system that uses this circuit).
Step 52: Set the jumpers for VR input. That would be TachSelect to VRIN, and TSEL to VROUT.
Step 65: You will use IGBT High Current Ignition Driver Circuit. That means you complete step 65. Install everything just as this step directs you to (which does mean no R57) and install the jumpers to enable the circuit. (IGBTIN to JS10 and IGBTOUT to IGN).
That's it!

If you're modifying a DIYAutoTune.com MS230-C MegaSquirt-II Assembled ECU:

Input Mod:

You will be using the VR input circuit. Connect the TachSelect jumper to VRIN, and TSEL to VROUT.

Output Mod:

There will be a jumper wire from JS10 over to IGN. Desolder and remove this (don't just cut it, you'll need the holes clear of solder).
Install new jumper wires to enable the IGBT High Current Ignition Driver Circuit (IGBTIN to JS10 and IGBTOUT to IGN)

That's it-- you've got your Megasquirt-II Engine Management System fully prepared to wire up to the Mopar distributor pickup, and drive the stock coil directly using the VB921.

External Ignition Wiring:

Connect the positive wire from the VR sensor on the distributor to pin 24 (Tach on the relay board). See notes at the bottom of this article for the wiring colors on various distributors.
Connect the negative wire from the VR sensor on the distributor to a ground pin, such as pin 2 (or simply the engine ground on the relay board).
Connect the negative terminal of the coil to pin 36 on the Megasquirt (S5 on the relay board).
You can bypass the ballast resistor (if your car has one) by either splicing the wires at each end together or replacing it with a loop of wire.

Megatune Ignition Configuration:

Trigger offset = 10° (this will vary, depending on the distributor orientation, see notes at the end of the article)
Ignition Input Capture to 'Falling Edge'
Cranking Trigger to 'Trigger Rise'
Coil Charging Scheme to 'Standard Coil Charging'
Spark Output to 'Going High (Inverted)'

You will need to set the dwell to match the particular coil you are using.

More information on setting Trigger Offset:

You must also set the initial position of the trigger (called the 'trigger offset'), then check it using the Trigger Wizard in MegaTune (Tools Menu). The trigger offset setting will vary according to your distributor position (where it is in rotation) but you'll need to set it properly... Basically you use the Trigger Wizard and adjust the 'trigger offset' and/or twist your distributor until the advance number in the Trigger Wizard matches what you're reading with your timing light. The +/- buttons on the trigger wizard will adjust your trigger offset. You'll need to use these buttons and a timing light to make the number on your light, and the big number on the left in the Trigger Wizard, match up.

Here's the information on this direct from the MegaTune Manual:

Before tuning your advance table, be sure to use a timing light to verify that your 'trigger offset' is calibrated. Changing the Trigger Offset in MegaTune will not change the displayed advance, instead, it changes the actual advance as seen with a timing light. Your goal is to make these two match.

To do this, get your engine warmed-up (otherwise the timing moves as the temperature increases) and idling, then use a timing light to verify to be certain your actual advance as shown by a timing light equals your the advance display on the advance gauge in MegaTune. (8° in this case). (Note that positive numbers denote BTDC, and negative numbers denote after TDC.)

Image courtesy of Bowling and Grippo

Using the MegaSquirt-II PCBv3 with a HEI Module

If you're assembling your MegaSquirt-II from a kit (Part# MS230-K):

For the most part, stick with the standard assembly documentation at http://www.megasquirt.info/ms2/V3assemble.htm. I'll just be covering any departures from that doc here.
Step 50: You'll be installing the 'Hall/Optical Input Circuit' (currently all steps under 50a). If you see any steps that say 'only do this if you are triggering from the negative terminal of the coil', you aren't, so don't.
Step 51: You don't need these components, but I'd recommend installing them anyways in case you ever decide to work with direct coil control.
Step 52: Set the jumpers for Hall/Optical/Points. That would be XG1 to XG2, OPTOIN to TACHSELECT, and TSEL to OPTOOUT.
Step 65: You will not be using the IGBT high current driver. Jumper JS10 to IGN to bring the signal out on pin 36.

That's it-- you've got your Megasquirt-II Engine Management System fully prepared to work with a HEI module.

If you're using a DIYAutoTune.com MS230-C MegaSquirt-II Assembled ECU:

There is no need to modify this ECU to work with a HEI ignition - it includes every mod you need for this application, right out of the box.

External Ignition Wiring

Bypass the ballast resistor (if your car has one) by either splicing the wires at each end together or replacing it with a loop of wire.
Connect the positive terminal of the coil to the stock Mopar coil wire and to the + pin on the HEI module.
Connect the negative terminal of the coil to the C pin on the HEI module.
Wire the distributor's VR sensor to the HEI module, positive distributor wire to N and negative to P. See notes at the end of the article about which wire is which color.
The R pin is the HEI module's tach output. Connect it to pin 24 on the Megasquirt, or the Tach pin on the relay board. Note that the DIY Autotune wiring harness has a shielded wire - connect only the inner wire to this pin!
Connect the E pin to pin 36 (relay board terminal S5) if you have a V3.0 board. If you are using a V2.2 board, use pin 27 or relay board terminal S2.
For the B pin, you will be using a relay, as described in the MegaManul's section on HEI modules. You will need to wire a relay so that the coil gets power from a wire that is energized with the key in the Run position but not Start. Run a wire from the VREF terminal on the relay board to one of the switch terminals of the relay, and run a wire from the other pin on the relay to the B pin on the HEI module.

Megatune Ignition Configuration:

Trigger offset = 10° (this will vary, depending on the distributor orientation, see notes at the end of the article)
Ignition Input Capture to 'Rising Edge'
Cranking Trigger to 'Trigger Rise'
Coil Charging Scheme to 'Standard Coil Charging'
Spark Output to 'Going High (Inverted)'

Dwell is very low; try around 1 ms and adjust as needed.

More information on setting Trigger Offset:

Here's the information on this direct from the MegaTune Manual:

Image courtesy of Bowling and Grippo

Mopar ignition wire colors

Chrysler used several different color schemes on their distributor wires, and aftermarket distributors often use their own color schemes. Here is a list of sensors and their polarity.

Distributor	Positive	Negative
Mopar with advance	Orange	Black
Lean Burn	Yellow or white	Blue or black
MSD distributor	Orange and black	Violet and black
MSD Flying Magnet crank trigger	Violet	Green

A note on temperature sensors

Megasquirt can read the Mopar coolant temperature sensor if you calibrate it with EasyTherm (for MS-I) or the temperature sensor calibration tool (for MS-II). However, Megasquirt cannot share the stock temperature sensor with the stock temperature gauges. Chrysler used thermal temperature gauges well into the 1980s with a mechanical voltage regulator. This results in a lot of voltage noise at the sensor if it is connected to the original gauge. If you want to retain the existing temperature gauge, your best option is to install a second sending unit for Megasquirt.

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