2001 Blazer, 2wd, 4.3l. Trying to find the ground wire for the PCM. I have a P0335 code and there a new crankshaft position sensor. Think maybe it has something to do with the wiring between the sensor and the PCM. I also want to check the ground for the PCM to make sure it isn't corroded, but I can't find where it is grounded to in the engine bay. The truck starts, the check engine light is on, the RPM gauge jumps all over, and it misfires. Any help would be great. Thanks!

 

My bad, I guess it's called the VCM, not the PCM.

 

PCM == VCM Same difference really.

The most of the PCM grounds run to the back of the passenger side cylinder head. They can be real "fun" to get to.

P0335 =Crankshaft Position [CKP] Sensor A Circuit

You can use the connector pin references in the diagnostic link below to test the wiring for the CKP sensor:
P0339 - CKP (crankshaft position) Sensor Circuit Intermittent
The test methods for an intermittent problem should also pick up a more permanent problem such as the P0335 code typically records.

 

We checked the three wires coming off of the CKP sensor and they checked out fine. The sensor is new, the wires are fine, and the fuse wasn't blown. We can't find the ground for the PCM (only found one ground on the back of the cylinder head and it was going from the cylinder head to the body. We think it might be the PCM itself, so we are looking at getting a new one. Any other ideas would be helpful. Thanks.

 

Did you check power to the CKP sensor while moving the harnesses around? It is possible that something moves and opens the circuit for power to the CKP...

The PCM grounds thru G117 (right rear of engine). After looking this up, I looked into the location in a bit more detail and it is actually on the firewall. G117 above and to the right of the brake booster just below the lip on the top of the firewall.

 

We did that for the other two wires, but not the power wire. We will try and see if that does anything. At the same time, the vehicle is smooth at idle and up to about 2000 rpm, then it starts misfiring and the check engine light comes on.

 

Did you happen to look through the hole in the timing cover at the reluctor ring (the toothed ring that the CKP sensor senses? It is possible that junk has built up in the reluctor ring causing the sensor to read poorly as RPMs increase.

 

Is it common for the reluctor ring and the CKP to be coated in oil. That is about the only thing all over it is a lot of oil. Not sure if that is because of the timing chain or because of the front main seal having a leak.

 

It is perfectly normal for it to be coated in oil. The timing chain is bathed in oil.

 

Make sure the CKP sensor is fully seated in the timing cover. If it starts and runs, the CKP sensor is getting power and ground and the yellow wire is supplying a signal to the PCM. Did you perform the crankshaft position sensor relearn procedure? If the CKP sensor or the timing cover is disturbed, moved, removed or replaced, the relearn must be done. If not, the PCM uses the stored data from the old CKP sensor causing ignition timing to be incorrect. It's also entirely possible that's why the P0335 is being set. The relearn must be done with a "high end" scan tool, the $300 el cheapos that the parts stores use can not access the relearn menu.