docs: Document comprehensive diagnostic history and next steps for crank no-start issue
- Record all previous diagnostic work (3x CKP sensors, 3x ignition coils, 3x ICMs) - Document key finding: grounding ICM signal pin restores spark - Add next diagnostic steps (CKP 5V reference, CMP sensor check, etc.) - Add section for John's notepad voltage measurements - Prepare for systematic continuation of diagnosis
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John Loper II
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**IMPORTANT: All entries MUST include full timestamps (YYYY-MM-DD HH:MM) to track issue progress across multiple days.**
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- **[YYYY-MM-DD HH:MM]:** [Event description - e.g., "Project initiated, diagnosis started"]
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- **[YYYY-MM-DD HH:MM]:** [Event description - e.g., "Diagnostic tests completed, root cause identified"]
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- **[YYYY-MM-DD HH:MM]:** [Event description - e.g., "Parts ordered, awaiting delivery"]
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- **[YYYY-MM-DD HH:MM]:** [Event description - e.g., "Repair completed, van running fine"]
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### Original Issue Occurrence (Several Years Ago)
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- **[Date: Unknown, Several Years Ago]:** Van driven home from Walmart during torrential downpour
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- **Event:** Hit large water puddle, violent splash under vehicle
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- **Symptom:** Complete electrical failure - no electrical activity anywhere on vehicle
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- **Initial Repair:** Something was fixed that restored electrical power, but vehicle would not start after that
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### Extensive Diagnostic Work (Last Few Months)
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- **[Date: Unknown, Few Months Ago]:** Initial diagnosis began - confirmed NO SPARK at all 8 spark plugs
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- **[Date: Unknown]:** Tested spark at ignition coil pigtail to distributor - NO SPARK
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- **[Date: Unknown]:** Removed connector from ignition control module (ICM)
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- **[Date: Unknown]:** **CRITICAL FINDING** - Grounded the signal pin from ECU to ICM → SPARK RETURNS
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- **[Date: Unknown]:** Confirmed ground is present at ICM connector
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- **[Date: Unknown]:** Confirmed battery positive is present at ICM connector
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- **[Conclusion]:** ECU is not sending signal to ICM to fire ignition coil
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### Parts Replaced/Tested (All Failed to Fix)
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- **[Date: Unknown]:** Changed ECU (old ECU had visible corrosion on pins and inside)
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- **New ECU:** Security light present, successfully completed security relearn procedure
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- **Security light:** Now gone after relearn
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- **[Date: Unknown]:** Changed crankshaft position sensor (CKP) - **3 different sensors total**
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- **[Date: Unknown]:** Changed ignition coils - **3 different coils total**
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- **[Date: Unknown]:** Changed ignition control modules - **3 different ICMs total** (including coil on module)
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- **Wiring Check:** Traced CKP sensor wires from connector back to ECU
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- Found both wires: ground wire and signal wire
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- No breaks detected along wire path
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- One wire appears to be ground from ECU, other is signal to ECU
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### Current Status
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- **ECU:** Changed, security relearn completed, no security light
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- **CKP Sensors:** 3 tested, none fixed the issue
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- **Ignition Coils:** 3 tested, none fixed the issue
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- **Ignition Control Modules:** 3 tested, none fixed the issue
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- **Key Finding:** When manually grounding ICM signal pin → engine fires
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- **Proves:** Ignition coil works, ICM works, spark plug wires work
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- **Problem:** ECU is not sending trigger signal to ICM
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### Critical Diagnostic Insight
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**Why grounding the signal pin makes spark return:**
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- The ICM needs a ground signal from the ECU to fire the coil
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- When you ground it manually, you're completing the circuit
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- This proves the entire ignition system WORKS when the ECU sends the signal
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- **The problem is NOT:** CKP sensor, ignition coil, or ICM (all tested 3x)
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- **The problem IS:** ECU is not sending the trigger signal to the ICM
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### Possible Causes (in order of likelihood)
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1. **CKP sensor still not working**
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- Even though 3 sensors were tested, maybe the issue is with the sensor's power supply
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- CKP sensor needs 5V reference from ECU - if this is missing, sensor can't work
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- CKP sensor needs good ground - if ground is bad, sensor can't work
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- CKP sensor air gap - if gap is too wide, signal too weak
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2. **New ECU is also bad**
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- Corrosion damage may have affected multiple ECUs
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- New ECU may have internal damage preventing it from sending trigger signal
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3. **Camshaft Position Sensor (CMP)**
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- Some GM engines need BOTH CKP and CMP to fire
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- If CMP signal is missing, ECU won't fire coils even if CKP is good
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4. **ECU power/ground issues**
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- New ECU may not have proper power or ground to function correctly
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- Check all ECU fuses and relay
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5. **CKP wiring issue not detected**
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- Continuity test may have passed but there could be high resistance
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- Wiring may appear intact but have internal damage
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---
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**Session Log Guidelines:**
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- Use full timestamp format: YYYY-MM-DD HH:MM (24-hour format)
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@@ -172,6 +236,7 @@
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### Before Starting
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- [ ] Engine cranks normally
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- [ ] No spark present at any plug
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- [ ] Security light off (confirmed after ECU swap)
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### After Repair
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- [ ] Spark present at all 8 spark plugs
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@@ -179,6 +244,7 @@
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- [ ] No misfires or rough idle
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- [ ] Check engine light off (codes cleared)
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- [ ] No security light issues
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- [ ] Van is drivable and reliable
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### Verification Steps
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1. Test spark at each plug with spark tester
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@@ -186,6 +252,180 @@
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3. Test drive and check for misfires under load
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4. Scan for codes after test drive
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5. Verify no security light issues
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6. Confirm no codes return after test drive
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---
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## 🔬 Next Diagnostic Steps (Priority Order)
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### Step 1: Check CKP Sensor Power Supply (5V Reference)
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**Why:** CKP sensor needs 5V reference from ECU to operate. If missing, sensor can't work even if new.
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**Test Procedure:**
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1. Disconnect CKP sensor connector
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2. Turn key ON (don't crank)
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3. Use multimeter to measure voltage at sensor harness connector:
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- **Pin 1 (if 2-wire sensor):** Should have 5V reference from ECU
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- **Pin 2:** Should have good ground
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4. If NO 5V → ECU not providing power to sensor (bad ECU or wiring)
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5. If NO ground → wiring issue back to ECU
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**Expected:** 5V on signal wire, good ground on ground wire
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### Step 2: Check Camshaft Position Sensor (CMP)
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**Why:** Many GM 5.0L/5.7L Vortec engines require BOTH CKP and CMP signals to fire coils. If CMP is missing, ECU won't fire even if CKP is good.
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**Test Procedure:**
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1. Locate CMP sensor (on intake manifold, near throttle body)
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2. Scan for codes - check for P0340 (Camshaft Position Sensor Circuit)
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3. Test CMP sensor:
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- Disconnect connector
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- Check for 5V reference from ECU
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- Check for good ground
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- Test sensor signal while cranking (AC voltage ~0.5-2V)
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4. If CMP failing → replace sensor (~$30-50)
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**Part Info:**
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- **GM Part #** (to verify with VIN): Commonly 12559305 or similar
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- **Aftermarket:** AutoZone, NAPA, RockAuto
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### Step 3: Verify CKP Sensor Signal While Cranking
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**Why:** Even if power/ground are good, sensor may not be producing signal
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**Test Procedure:**
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1. Back-probe CKP sensor connector (or use breakout box)
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2. Connect multimeter to AC voltage setting
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3. Have someone crank engine while you measure
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4. **Expected:** 0.5-2V AC signal while cranking
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5. **If 0V:** Sensor not producing signal (bad sensor, bad gap, or no power)
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### Step 4: Check ECU Power and Ground
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**Why:** New ECU may not have proper power or ground to function
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**Test Procedure:**
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1. Locate ECU connector
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2. Check all ECU power pins with key ON:
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- Should have battery voltage (12V) on power pins
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- Check ECU fuses in under-hood fuse box
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- Check ECU relay
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3. Check ECU ground pins:
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- Should have good continuity to chassis ground
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- Check ground points for corrosion
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**ECU Fuse Locations:**
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- Under-hood fuse box: Look for "ECM" or "PCM" fuse (20A)
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- Also check "IGN" fuse (15A)
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### Step 5: Test CKP Sensor Air Gap
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**Why:** If air gap between sensor and reluctor wheel is too wide, signal too weak
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**Test Procedure:**
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1. Remove CKP sensor
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2. Inspect sensor tip for damage or debris
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3. Inspect reluctor wheel (on crankshaft harmonic balancer) for damage
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4. Reinstall sensor and measure gap if possible
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5. Some sensors have a washer that sets the gap
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### Step 6: Consider ECU Replacement Again
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**Why:** If all above tests pass but still no signal from ECU, new ECU may also be bad
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**Options:**
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- Try another known-good ECU (borrow from friend, buy from junkyard)
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- Have ECU professionally tested/repaired
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- Consider reprogramming existing ECU if corrosion can be cleaned
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---
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## 📋 Existing Diagnostic Data (From John's Notepad at Home)
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**Date:** [To be entered when John shares notepad contents]
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### CKP Connector Voltage Measurements
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John measured voltage on **every pin** of the CKP connector at various points. This data is critical for determining:
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- Whether CKP sensor is receiving proper 5V reference
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- Whether ground circuits are intact
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- Whether any voltage signals are present
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**Action:** Wait for John to share notepad contents → enter voltage readings into this section
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### ICM Connector Voltage Measurements
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John measured voltage on **every pin** of the ignition control module connector. This data is critical for determining:
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- Whether ICM is receiving proper power (12V battery)
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- Whether ICM ground is good
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- Whether ECU trigger signal is present
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- Whether any other pins have unexpected voltage
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**Action:** Wait for John to share notepad contents → enter voltage readings into this section
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### Continuity Testing Documentation
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John performed continuity tests on:
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- **All wires between CKP sensor and ECU**
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- **All wires between ECU and ignition control module**
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**Results:**
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- John believes all wires showed continuity
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- No obvious breaks detected
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- **Note:** Continuity test may pass even with high resistance - voltage drop testing would be more definitive
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**Action:** Wait for John to share notepad contents → enter specific wire numbers, pin numbers, and continuity results
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### Why This Data Matters
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Having the actual voltage measurements will:
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1. **Eliminate guesswork** - We'll know exactly what voltages John measured
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2. **Identify patterns** - May reveal which wires are dead vs. which have voltage
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3. **Pinpoint the issue** - May show exactly where the signal is being lost
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4. **Save time** - Can focus on the specific wires/circuits that are failing
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---
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## 🛠️ Tools Needed for Next Steps
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- Digital multimeter (must measure AC voltage)
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- Back-probe pins or thin wires for testing connectors
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- OBDII scanner (to check for CMP codes)
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- Basic hand tools to remove CKP sensor and CMP sensor
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- Service manual for connector pinout diagrams
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- **John's notepad** - Contains critical voltage measurements
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---
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## 🔗 Technical References
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### GM 5.0L Vortec Ignition System
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- **CKP Sensor:** 2-wire magnetic sensor
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- **Signal:** AC voltage (0.5-2V while cranking)
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- **Power:** 5V reference from ECU
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- **Ground:** Good chassis ground
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- **CMP Sensor:** Needed for timing reference
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- **ICM:** Receives trigger signal from ECU based on CKP+CMP inputs
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### ECU Trigger Signal to ICM
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- **Signal Type:** Ground pulse from ECU
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- **When:** ECU fires when CKP and CMP signals indicate correct timing
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- **ICM Function:** Completes coil ground circuit when triggered
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- **Result:** Coil fires, spark at plugs
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---
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## 📝 Lessons Learned
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1. **Water damage can be insidious** - Corrosion may not be immediately visible
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2. **Systematic testing is critical** - John's methodical approach (3x each part) proved valuable
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3. **Grounding test was the key insight** - Proved ignition system works when triggered
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4. **ECU corrosion is likely culprit** - Old ECU had visible corrosion inside
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5. **Multiple ECUs may be affected** - If water damage was severe, could affect multiple ECUs
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6. **Detailed documentation matters** - John's voltage measurements from notepad will be invaluable
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7. **Continuity testing has limits** - May pass even with high resistance - voltage drop testing more definitive
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---
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## 📊 Revision History
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| Date | Change |
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|------|--------|
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| 2026-03-22 15:53 | Added section for CKP and ICM voltage measurements from John's notepad |
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| 2026-03-22 15:48 | Comprehensive diagnostic history documented from John's recollection |
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| 2026-03-21 | Initial creation - Crank no-start, no spark diagnosis started |
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## 🧪 Revision History
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