Join Date: Sep 2016
Location: Not in Boise but nearby.
I don't think your ECM is bad. It could be but you are a long way from assuming that yet.
What you are describing sounds like an intermittent problem with the wiring to the cam sensor that is getting worse and is now constant. That's a good thing because if it is happening now it is a lot easier to find than if it is not happening now. Computers don't often have intermittents. It is possible but rare.
Try not to disturb anything you don't have to so as not to inadvertently "fix" the problem before you have a chance to actually find it.
Disconnect the ECM connector and the connector at the cam sensor.
There are three wires from the ECM to the cam sensor. They are solid Red for 12 volt reference, Pink/Black for sensor ground and Brown/White for the cam sensor signal.
The ECM has so many wires going to it that it is often hard to tell what the colors are, especially after 19 years of aging and dirt. By looking at the wires on the sensor connector it is easier to tell what you are looking for at the ECM. I find that a spritz of brake clean or carb clean makes wire colors much easier to see.
When probing connectors on ECMs and sensors it is very important to be careful not to stick something into the connector that will spread it open. I've had to re-form a lot of terminals after someone jammed a nice fat wire in there to test with. The best thing to use is a terminal like what it is supposed to connect to, if you can find one.
The ECM has two connectors C1 and C2. Both have 80 pins in two rows of 40 each. There is a solid block in the middle that separates the two rows into 20 on each side. If you look carefully (you might need a magnifying glass) you will see that some of the pins are numbered so you can tell which they are. C1 is blue. Socket 61 of C1 is the Pink/Black wire for ground. It will be on the top row just to the right of the center divider. Socket 73 of C1 is for the Brown/White cam sensor signal. It will be 8 from the right end on the top row.
The Red power wire is in socket 39 of connector C2 which is red but is otherwise very much like C1. It will be the first one in from the right end of the bottom row. You could test for 12 volts by probing the red wire at the cam sensor connector before removing the connector at the ECM. If there isn't any power on the red wire it may be broken (see below), the ECM may be bad (unlikely) or connector C107 may be bad (likely).
C107 is on top of the engine on the driver's side (unless you are Harnold) and connects the engine wiring to the chassis wiring. Sockets B3, B4 and B5 in connector C107 are for the cam sensor. Check the colors. I would have big suspicions about C107. I wouldn't be surprised if disconnecting and connecting C107 would fix the problem for a while. Is not unheard of for this connector to do exactly what you are seeing.
Once you are sure that C107 is good and that you have good power to the cam sensor then check that the ground is good. If that is okay then check that the signal wire is good. The sensor signal can be tested by removing the wire from either the ECM connector (best), removing the wire from connector C107 (not as good because it doesn't test the wire from there to the ECM) or probing the wire directly somewhere near the ECM. Connect a multimeter to the wire. You should see a solid switching between 12 volts and ground or if your multimeter will do duty cycle it should be between 45% and 55% as the engine cranks. If you see anything else you may have a short to power, short to ground, broken wire or a brand new defective sensor.
Personal experience with hall effect sensors, like the cam sensor, suggests that when they work they are excellent but when they don't work they can be a real pain. I have had lots of brand new sensors be bad out of the box. I might suggest that you get an AC Delco part just to be sure.
If you don't find anything then try clearing the code again and see if it will start. If it does then that sucks for you because it is very hard to find a problem that isn't there. The next thing to try is to follow the wiring harness from the ECM to the cam sensor. Try flexing it a lot, pulling on it and twisting it to see if the P0340 code will reset. It takes 2 seconds of bad signal on two ignition cycles to set the code, unless it is already set and active.
There is a common wear point near the a/c compressor where the harness rubs through and shorts out. That is a real big possibility. A forum search should get you some pictures of the situation.
You need a scanner with data capabilities so that you can monitor the cam signal while cranking. Hopefully you won't have a signal now but when it is fixed you will have one.
The engine will continue to run once started, even if the cam signal is lost. It will not start without it though. That means it won't strand on the road you but it might leave you sitting at a fuel pump. If the code is set but the test the ECM runs passes the engine will start, even with the CEL on. If it does this three times in a row with full warm up and cool down between starts it will turn the CEL off but it takes 40 successful run cycles with no failures to clear the code from memory, or you can use a scanner to clear it.
Last edited by mizterwizard; 10-28-2019 at 02:32 AM.