Introduction to the MJ Auto Electric Architecture

The Jeep Comanche (MJ), produced from 1986 to 1992, features one of the most unique and heavily debated electrical architectures in domestic truck history. When navigating the MJ auto electric schematics, technicians and restorers must first identify the specific era of the vehicle, as the transition from French-engineered electronics to American corporate standards created a split personality under the hood. Understanding these wiring diagrams is not just about tracing lines; it is about recognizing the physical failure points inherent to the harness routing, bulkhead connectors, and grounding strategies of the era.

This complete wiring diagram walkthrough will dissect the core circuits of the MJ, focusing on the Electronic Fuel Injection (EFI) systems, the infamous bulkhead connector, and the critical grounding networks. Whether you are troubleshooting a no-start condition on a 1988 Renix 4.0L or upgrading the charging system on a 1991 Chrysler High Output model, mastering the MJ auto electric system requires a methodical approach to wire gauge specifications, pinout configurations, and voltage drop testing.

The Great Divide: Renix (1986-1990) vs. Chrysler (1991-1992)

Before opening a wiring manual, you must identify which Engine Control Unit (ECU) your MJ utilizes. The diagrams for these two systems are fundamentally incompatible.

The Renix Era (1986-1990)

The early 4.0L inline-six engines utilized the Bendix/Renix fuel injection system. The Renix ECU is a 35-pin (or 24-pin on early 2.5L models) rectangular unit mounted in the passenger kick panel. The wiring diagrams for the Renix system are characterized by a lack of a centralized Power Distribution Center (PDC) and a reliance on inline fusible links. The Renix Crankshaft Position Sensor (CPS) is a 3-wire Hall-effect/magnetic pickup that is notoriously sensitive to wire shielding degradation. According to Haynes Jeep Cherokee and Comanche Repair Manuals, the Renix ECU requires a dedicated, unshared ground path to the engine block to maintain accurate injector pulse width calculations.

The Chrysler Era (1991-1992)

Following Chrysler's buyout of AMC, the 1991-1992 MJ models received the Chrysler 60-pin Powertrain Control Module (PCM) and the 4.0L High Output (HO) engine. The wiring diagrams shifted to use a centralized PDC under the hood, standard ISO mini-relays, and a completely different sensor pinout architecture. The Chrysler CPS, for instance, operates as a 2-wire variable reluctance sensor, meaning the wiring diagrams will show a twisted pair of signal wires rather than the 3-wire shielded harness of the Renix.

Walkthrough: Reading the MJ EFI and Ignition Diagrams

When reading the factory MJ auto electric diagrams, power flow typically originates at the battery, passes through a fusible link (usually 14 AWG or 12 AWG, color-coded Red or Black), and enters the main harness. Let us break down two of the most critical circuits you will need to trace.

Crankshaft Position Sensor (CPS) Circuit Deep Dive

The CPS is the primary synchronization signal for both ignition spark and fuel injector sequencing. A failure here results in an immediate no-start condition with no check engine light (on Renix models, as the ECU does not have enough cranking time to register a code).

  • Renix 3-Wire CPS Pinout: Pin A (Shield/Braid - bare copper wrapped around the inner wires), Pin B (Ground - Black with White tracer), Pin C (Signal - Gray with Black tracer). The signal wire routes directly to Pin 11 on the 35-pin ECU connector.
  • Chrysler 2-Wire CPS Pinout: Pin 1 (Signal - Gray/Black), Pin 2 (Sensor Ground - Black/Light Blue). These route to Pins 24 and 4 on the 60-pin PCM, respectively.

Expert Troubleshooting Tip: When testing the Renix CPS, do not back-probe the connector with a standard multimeter probe, as this can spread the female terminals and cause intermittent signal loss. Use a T-pin and measure AC voltage while cranking; a healthy Renix CPS should generate a minimum of 0.5V AC.

The Fuel Pump and ASD Relay Circuits

The fuel pump relay circuit is a common failure point due to voltage drop across aging contacts. In the Chrysler-era diagrams, the Automatic Shutdown (ASD) relay and Fuel Pump relay share a common 12V ignition feed (Dark Blue/White wire, 16 AWG) on Pin 86. Pin 30 receives constant battery voltage via a 10 AWG Red wire protected by a 20A fuse in the PDC. Pin 87 outputs power to the fuel pump via a 14 AWG Dark Green/Black wire. If the fuel pump is receiving only 10.5V under load, the factory 14 AWG wire is suffering from voltage drop. Upgrading the Pin 30 to Pin 87 bypass circuit with 12 AWG SAE J1128 compliant primary cable is a standard performance modification for the MJ.

The Infamous Bulkhead Connector (C100/C101)

No discussion of the MJ auto electric system is complete without addressing the main engine-to-dash bulkhead connector. Located on the driver's side firewall near the brake booster, this multi-pin rectangular connector is the bridge between the engine bay harness and the interior cabin harness. Due to its low mounting position and lack of a robust weather seal from the factory, it is highly susceptible to capillary action wicking and green copper oxide corrosion.

Bulkhead Connector Pinout & Wire Color Reference

Below is a reference table for the critical pins on the 60-way bulkhead connector that frequently cause electrical gremlins, including dim headlights, erratic gauges, and alternator overcharging.

Pin Number Circuit Function Wire Color (Typical) AWG Size Common Failure Symptom
14 Alternator Voltage Sense Red 18 Overcharging (15V+) or undercharging due to high resistance.
22 Fuel Pump Power Feed Dark Green 14 Fuel pump starvation under heavy load; engine lean stumble.
45 Tachometer Signal Gray 20 Erratic or dead tachometer needle at idle.
58 Starter Solenoid Trigger Yellow 14 Intermittent no-crank; requires multiple key cycles to engage.

Restoration Advice: If you find corrosion on Pins 14 or 22, do not simply clean it with electrical contact cleaner. The copper strands inside the insulation often turn black and brittle. You must cut the harness back at least 6 inches from the bulkhead and solder in new pigtails using heat-shrink butt connectors with adhesive lining to prevent future moisture ingress.

Grounding Strategy: The Hidden MJ Failure Point

The MJ utilizes a distributed grounding system. The factory diagrams show ground points designated with a 'G' prefix. The most critical for engine management are G100 (located on the engine block near the dipstick tube) and G101 (located on the passenger side of the engine block near the motor mount). The dash harness relies on G200, located behind the passenger side kick panel.

From the factory, these grounds were secured using 10 AWG or 8 AWG braided metal straps or black insulated wires with ring terminals. Over decades of heat cycling and engine movement, these straps fray, and the ring terminals oxidize, creating a high-resistance ground. Because the Renix ECU calculates injector pulse width based on a 5V reference and a ground return, a poor G100 ground will cause the ECU to see a false voltage offset, resulting in a massively rich running condition and fouled spark plugs.

Upgrading the Grounding Harness

To properly upgrade the MJ grounding system, bypass the factory braided straps entirely. Use 4 AWG or 2 AWG flexible welding cable. Crimp heavy-duty copper lugs onto the cable, coat the inside of the lugs with dielectric grease before crimping to seal out oxygen, and secure them to the engine block and chassis using Grade 8 hardware. Ensure you sand the mating surfaces on the engine block down to bare, shiny metal before torquing the bolts to 25 ft-lbs. This single modification resolves approximately 40% of all unexplainable sensor-related diagnostic trouble codes (DTCs) on the MJ platform.

Diagnostic Decision Tree: No-Start Electrical Faults

When an MJ presents with a crank-no-start condition, follow this structured diagnostic path before replacing expensive components like the ECU or fuel pump.

  1. Verify Battery Voltage at the PDC/Fusible Links: Set your Digital Multimeter (DMM) to DC Volts. Probe the main feed stud in the engine bay. If voltage is below 11.5V during cranking, the issue is battery health or starter draw, not the EFI system.
  2. Check the ASD/Fuel Pump Relay Control Side: With the key in the 'ON' position (engine off), probe Pin 86 of the Fuel Pump relay socket. You should read battery voltage (12V+). If 0V, trace the Dark Blue/White wire back to the ignition switch for a broken circuit.
  3. Verify CPS Signal: Connect an oscilloscope or an AC Voltmeter to the CPS signal wire. Crank the engine. If no AC voltage or square wave is present, the CPS is dead, or the flexplate/flywheel sensor ring is damaged.
  4. Test Injector Pulse: Use a Noid light on injector #1. If the Noid light flashes during cranking, the ECU, CPS, and ground circuits are functional. The fault lies in fuel delivery (pump, filter, or pressure regulator).
  5. Inspect the Bulkhead: If the Noid light does not flash, and the CPS is verified good, disconnect the C100/C101 bulkhead connector. Inspect the ECU power and ground pins for green corrosion. Clean or repin as necessary.

Authoritative References

For comprehensive, vehicle-specific wiring diagrams and torque specifications, always consult the following authoritative resources: