jiggerjawCommunity MemberJune 19, 2022 at 2:03 pm
So I’m lost. This is quite a long story so maybe get some popcorn. If you don’t want all the history, there’s a TLDR at the bottom.
Initially, my Saturn just stopped starting. It gave me codes for camshaft position sensors which was weird since I had just replaced them. I replaced them again and it still had codes for camshaft position sensor. I figured I would try the next cheap thing that was going to be needed soon anyways and replaced the crank sensor.
Reset the codes, still had no start but this time for no crank sensor. I reinstalled it, it was definitely in place correctly. I decided with all the weird stuff happening, it was probably the ECM. I backprobed the crank pins at the ECM connection and used a regular voltmeter to try to judge if it was getting a signal. The signal wire seemed to bounce around at ~2v which seemed to make sense with a 5v reference. I ordered a new ECM.
New ECM arrives from flagship one, I plug it in and the car starts. Yay! But it says the crank position sensor needed relearned. I have a knock-off Tech2, tried the reset but it didn’t work. The RPMs just read zero. I try for a while to do the reset with the car running kind of rough, presumably because the crank timing needed relearned. It had a mechanical failure light but seemed like it was running okay. At this point I had been without a car for like 3 months and decided to take a victory lap around the neighborhood.
Car seemed like it was okay for a few blocks. I decided to take it on the highway to see if everything was kosher. As soon as I get up to highway speeds, it gets rough. Really rough. I immediately slow down, take the next exit, and limp it home. I thought the crankshaft relearn must matter much more than I had thought.
When I had the initial camshaft position codes, I messed around with them to look for shorts, etc. I had unbolted one and had it sitting in it’s hole. Well… It obviously came out of it’s hole and the entirety of my engine oil managed to get pumped out of the top of my engine. I didn’t know while driving because the mechanical failure light was already illuminated.
Ever the optimist, I decide I’m going to buy a low mile eBay engine and do the swap. I find an exact match from a 2007 Saturn Ion. Do the engine swap, and I’m still getting a no crank code. I’m convinced it must be a bad ECM, and get it replaced under warranty. New ECM comes after another 3 months. Same exact thing. Zero RPMs at the tach, can’t do a crankshaft relearn. The car actually runs well. It has a long crank to get started but then seems to run normally after. I read somewhere that it’s possible for the engine to run off the camshaft position sensors if it needs to.
So now, I’m convinced it was the new crankshaft sensor. That one had a lifetime warranty so I swapped it out. No dice, same issue. Now, I’m out ~$1500 and countless hours of labor. I decide it’s time to stop throwing parts at it and really work to diagnose it.
I get an oscilloscope since that’s what the youtube guide I stumbled upon suggested.
TLDR, start here:
There are three wires on the crank sensor. Wiring diagram says 5v ref, signal, and low ref. With the car “On” but not running, I have 5.2v at the 5v ref and signal wire. Nothing at the low ref. When the car is running, the 5v ref stays 5v and the signal wire jumps around. However, it does not produce a square wave. It was at ~2v but seemed really noisy and continuous. Perhaps my settings on the oscilloscope?
It’s been hard to find something that lays out the crank circuit in an easy to understand way. I understand there’s the 5v ref, and the 5v gets pulled down by the magnetized reluctor wheel on the signal wire. But should the signal wire have 5v coming backwards to the sensor? I saw one diagram that said the two are connected with a resistor back at the ECM, but that didn’t really make sense to me. It would be easier to just cut the 5v in a continuous 1 way circuit. I cut the signal wire to see if I could read a solid square wave hooking straight to the sensor (to rule out downstream wiring issues). I found that the side coming from the ECM on that wire has a constant 5v.
If this is just how it’s supposed to be, does anyone have any input as to why two different ECMs are telling me that I don’t have a crank signal and my old ECM with a different engine also said no crank signal…? To me that screams wiring issue, but I can’t seem to find anywhere that it’s shorted. Any ideas on where to go next would be appreciated. I don’t imagine it’s the reluctor wheel as this engine only had 60k miles
Engine: 2.4L Ecotec
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