Chrysler minivan with a U0100 lost communication with ECM and half the modules on the network dropping out intermittently.

U0100 - Lost Communication with ECM on a Chrysler minivan (likely 2008+) points to the high-speed CAN-C bus (500 kbps) between the PCM and other modules. Here's how to check it with a multimeter before disconnecting anything:

1. Locate the CAN Bus Wiring

• Find the PCM (usually near the firewall or under the hood).
• Identify the CAN-C bus wires:
• CAN High (CANH) – typically White/Dark Green (Chrysler standard)
• CAN Low (CANL) – typically White/Dark Blue
• Check the OBD-II pinout at the DLC:
• Pin 6 = CANH
• Pin 14 = CANL

2. Measure Voltage (Key ON, Engine OFF)

• Set your meter to DC volts.
• Measure voltage on CANH and CANL relative to ground:
• CANH: ~2.5V to 3.5V
• CANL: ~1.5V to 2.5V
• If both are 0V or ~5V, the bus is likely dead or shorted.

3. Check Termination Resistance

• Turn key OFF.
• Measure resistance between CANH and CANL at the DLC or PCM connector:
• Should read ~55–60 ohms (two 120Ω terminations in parallel).
• If it reads ~120 ohms, one terminator is missing.
• If it reads 0 ohms, there’s a short.
• If it reads infinite, the bus is open.

4. Check for Shorts to Power/Ground

• With key OFF, measure resistance from CANH and CANL to battery positive and ground:
• Should be infinite (no continuity).
• Any low resistance indicates a short.

5. Visual Inspection

• Check for damaged wiring, especially near the PCM, steering column, or under the battery tray where chafing is common.

If all checks pass, the issue may be internal to a module or require a scope to check for signal activity. But these steps will catch most physical-layer faults safely.

Do not use a standard multimeter (DMM) to diagnose CAN bus integrity. A DMM measures average DC voltage; it cannot detect the differential square-wave signaling, bit errors, or termination issues causing U0100. You will get false "good" readings on a dead or noisy bus.

Use a scope or a dedicated CAN bus analyzer. If you must verify wiring integrity with a meter before scoping, do this only with battery disconnected and ignition off:

1. Check Termination Resistance (Pin 6 & 14 at DLC): Set meter to Ohms (Ω). Measure between DLC Pin 6 (CAN High) and Pin 14 (CAN Low). Spec: ~60 Ω (two 120 Ω resistors in parallel). > 100 Ω: Open circuit in bus wiring or a missing termination resistor (usually at ECM or ABS module). < 50 Ω:* Short between CAN H/L, or a module pulling the bus down (unplug modules one-by-one to isolate).

1. Check for Shorts to Power/Ground (Key Off, Battery Disconnected): Measure Pin 6 → Ground, Pin 14 → Ground, Pin 6 → Pin 16 (B+), Pin 14 → Pin 16. Spec: OL (Open Loop) / > 1 MΩ. Any continuity indicates a harness short.

1. Check Continuity (End-to-End): Disconnect ECM connector and a known-good module (e.g., ABS). Meter on Ohms: Probe CAN H wire at ECM pin → CAN H at ABS pin. Spec: < 1 Ω. Repeat for CAN L.

The Real Diagnostic Path:

1. Scan Tool: Check which modules are missing. U0100 + multiple dropouts usually = Bus Short or ECM Power/Ground loss.
2. Scope the Bus (Key On/Engine Cranking): Look for clean 2.5V recessive / 3.5V dominant (CAN H) and 2.5V / 1.5V (CAN L) square waves. Missing termination = rounded edges/ringing. Short = flatlined ~2.5V.
3. Load Test ECM Power/Grounds: Voltage drop test on ECM B+ and Grounds while cranking. A bad ground kills the transceiver.

Chrysler Specific: Check the TIPM (Totally Integrated Power Module) connectors for corrosion/water intrusion—it's the central gateway. Check the ECM ground G100/G101 (left front fender/engine).

If you lack a scope, stop. Guessing with a meter wastes hours. Rent/borrow a PicoScope or Snap-on/Vantage.