Chevrolet Uplander Base 3.9L, V6, MFI, OHV, VIN W, Eng Desg LGD will not start

Chevrolet Uplander Base 3.9L, V6, MFI, OHV, VIN W, Eng Desg LGD
MILEAGE:   200,000


Customer Concern:  The engine will not start and it bucks the starter while cranking. There is no spark from the 1-4 coil but it has good spark from the other coils. Tried a new coil but that did not help.


Tests:    1. Ground the secondary coil terminals and disable the fuel injectors. This will stop the engine from bucking the starter and prevent the cylinders from flooding with fuel.

2. Scope check the pattern on the IC 1 signal at the coils Purple wire, pin B while operating the starter.

3. Compare to the IC 2 signal, Orange/White wire pin A. It should be the same.

4. Inspect for a poor ground at the Electronic Control Module (ECM), ground G113, at the engine to the transmission stud above G115.

5. Check ECM grounds to be less than 50 mv.

6. Check the Crankshaft Position (CKP) sensor signal for a glitch in the pattern.

7. Check under the CKP sensor for rust at the engine block and clean to bare metal so the sensor mounts flush.


Potential Causes:  Failed Crankshaft Position (CKP) Sensor Signal
Failed Electronic Control Module (ECM) Ground
Failed Electronic Control Module (ECM)
 
Confirmed Fix:  Cleaned Crankshaft Position (CKP) Sensor Mounting Surface