How to MegaSquirt Your Dodge Neon or Chrysler 420A Engine

Overview

This guide covers MegaSquirt engine management for Chrysler’s 420A platform (Dodge/Plymouth Neon, DSM 420A, and related “cloud cars”), using robust BIP373/QuadSpark ignition control. It supersedes legacy HEI/VB921 approaches.

Applications

  • 1st‑gen Neon (SOHC/DOHC), DSM 420A (Eclipse/Talon), “cloud cars” (Cirrus/Breeze/Stratus) in 2.0/2.4L variants.
  • 2001–2002 PT Cruiser (later PT and 2nd‑gen Neon—including SRT‑4—use slightly different settings noted below).

Why BIP373 / QuadSpark?

The original VB921 drivers used in early documentation run hot at the dwell these coils prefer. BIP373 (MS1/MS2) and QuadSpark (with MS3) provide reliable dwell capability and thermal margin.

MS3 + MS3X with QuadSpark

Hardware setup

  • VR input: V3.0—jumper TachSelect→VRIN and TSEL→VROUT. V3.57—JP1=1–2, J1=3–4.
  • Cam pull‑up: install MS3X JP7.

External wiring

  • Crank sensor → mainboard DB37 pin 24 (requires pull‑up):
    • External: 1 kΩ between pins 24 and 26 in the harness.
    • Internal V3.0: 1 kΩ from TachSelect to S5 (5 V).
    • Internal V3.57: fit 1 kΩ at R57.
  • Cam sensor → MS3X DB37 pin 32.
  • QuadSpark: wire per MS3/QuadSpark directions. Spark A fires cyl 1&4; Spark B fires cyl 2&3.
  • Injectors (sequential wiring order): A→1, B→3, C→4, D→2.

TunerStudio (MS3/MS3X)

  • Spark Mode: 420A/Neon (2nd‑gen Neon: use 36‑2+2).
  • Trigger Angle/Offset: 0
  • Ignition Input Capture: Rising Edge
  • Spark Output: Going High (Inverted) (required for QuadSpark; wrong polarity can overheat modules/coils)
  • Number of Coils: Wasted Spark
  • Spark Hardware in Use: MS3X Spark
  • Cam Input: MS3X Cam In
  • Dwell Type: Standard Dwell; start at 4.0 ms and tune per coil.

MS2 (V3.0/V3.57) — Direct Coil Control (MS2/Extra 2.0.0+)

Input mods

V3.0:

  • Remove C30; replace R12 with 470 Ω.
  • Wire C30 top hole → S5 (5 V) or 5 V in proto area.
  • Jumper TachSelect→XG1 (remove XG1–XG2 and any other TachSelect jumpers).
  • Jumper TSEL→OptoOut.

V3.57:

  • Remove C30 and the pull‑off jumpers (XG1–XG2 and JP1).
  • Set mainboard J1=1–2.
  • 5 V near boot jumper → JP1 hole #3; XG1 → JP1 hole #2.

Outputs — BIP373 construction

  • Install a BIP373 at Q16 with mica insulator (V3.0: jumper IGN→IGBTOUT).
  • Build resistor‑on‑a‑wire pigtails (330 Ω to the left leg) for each coil output.
  • Input pick‑off points: Spark A from top of R26 (V3.57: U1 pin 7); Spark B from top of R29 (V3.57: U1 pin 8).
  • Outputs: center leg of Spark B → SPR4 (reinforce trace) → DB37 pin 6; Spark A uses DB37 pin 36.
  • Right legs to solid grounds (separate grounds preferred for each BIP373).

2nd‑gen Neon PWM idle (V3.0 only)

  • Remove Q20 and D8; remove R39 and install a jumper.
  • Fit TIP120/121/122 at Q4 (MK‑PWMIAC reference).
  • V3.57 boards and 1st‑gen Neons do not need this mod.

External wiring (MS2)

  • Crank sensor → DB37 pin 24.
  • Spark A (cyl 1&4) → DB37 pin 36; Spark B (cyl 2&3) → DB37 pin 6 (via SPR4).

TunerStudio (MS2/Extra)

  • Spark Mode: 420A/Neon (2nd‑gen: 36‑2+2)
  • Trigger Angle/Offset: 0
  • Ignition Input Capture: Falling Edge
  • Spark Output: Going High (Inverted) (required for BIP373)
  • Number of Coils: Wasted Spark
  • Spark A output pin: D14
  • Dwell Type: Standard Dwell; start at 4.0 ms.

Other Systems the ECU May Control

  • Air conditioning: MS3 has dedicated A/C control; others can use a relay strategy.
  • Alternator: Add an external regulator (older Mopar style). MS3 offers high‑speed field control.
  • Automatic transmission: 4‑speed electronic AT requires OE ECU or MegaShift; 3‑speed SOHC is mechanical (use on/off TCC output).
  • Cooling fans: run from an MS3X output or a relay‑control mod.
  • Fuel pump: MS pin 37 → fuel‑pump relay.
  • Gauges: Cloud cars use networked gauges (retain OE ECU). Neon/DSM: only tach is ECU‑driven—MS3X tach out can drive it; MS1/MS2 can be modified for tach out.
  • Idle: 1st‑gen Neon = stepper IAC (MS2/MS3 support). 2nd‑gen Neon = PWM IAC.

What to Order

MS3 + MS3X (recommended for most 420A builds)

MS2 (assembled or kit)

If buying an assembled MS2, it needs the ignition mods above; we can install them for a fee.

Legal/Emissions: For off‑road/closed‑course use where required. Verify local compliance and any warranty impacts before modifying engine controls.