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Discussion Starter · #1 ·
Hi all, I just joined recently - just a long time shadetee guy mostly just to work on my own cars.

I know there are already like 100 P0340 posts (especially if you count other forums), so sorry. I've read tons already. (And I started with this not reading any b/c I know what a CMPS is and how they work). I've just got what I think are weird things - not present in other posts I see - and IDK how to interpret.

Anyway...'02 Envoy, 4.2L with 187,4XX miles. I'm 3rd owner but very well kept over it's life. (1st owner transferred a huge stack of dealer receipts to 2nd who is a local mechanic where I am).

Maybe irrelevant - something like 2 yrs ago it threw a P0340. I did the normal thing - clear the code and see what happens. Nothing happened so I forgot about it.

About a year later it came back and now it hangs around. The first thing I did was just throw a new sensor at it - but unfortunately a big box parts place aftermarket. (Cheap enough and I didn't have time for mucking about at the time).

The second thing I did was ignore it ... for like the last year. Partly b/c - and the (first weird) thing - is that the truck has no problems. It starts fine. Idles fine. Runs great. Gas mileage is normal - just as crappy as it always was. (I rarely verify, but the computer bounces between 18.4-18.5 avg). Cruise control used to work even with this code, but now doesn't so maybe that's derivative - or not. None of that, except maybe the cruise control, is "normal" for lack of cam signal, as far as I know.

Anyway, I've got 12V on the red wire where it belongs (KOEO). I've got ground on the pink wire where it belongs (even w/out key on - is that normal, as I thought it grounds thru the PCM?). I checked signal (brown) by pulling the sensor, plugging in, putting KOEO and just flopping a big socket in front of it. In doing this I discovered that my crappy aftermarket sensor was dead. (Probably DOA, but I never checked). But I had kept the orig and it cycled - the only anomaly is that my power (red wire) voltage was like 12.4 or so and the sensor only pumped out 10V. Is that enough of a drop to call it dead?

I also ohmed out the signal wire to the PCM and that had continuity. That doesn't 100% clear the wire but it doesn't have an obvious problem. Anyway, I also count all of that as weird - except for maybe the 10V part. (I keep hoping to find something just plain old broken!)

Another weird thing - it only sets at initial start up. If I turn to KOEO, scan and clear codes and then start the truck I can literally drive around for hours and hours without the code setting. But it does register as pending. So the very next time I start the truck is when it sets. This makes no sense. This is continually monitored and the darned thing either has a signal or it doesn't. So I don't know what to do with that.

Anyway, I kind of feel like I just need to order up an AC Delco sensor and put that in. But a) I do actually hate throwing parts at things and b) I already threw one at this one (though it was probably DOA - my bad as I didn't check).

Suggestions welcome.
 

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'05 Chevy TB EXT
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1. Be Ye veri careful on "ohming out" any circuits on a computer driven vehicle --- or toaster or electric toothbrush for that matter. Computers don't like it when/if the meter uses a 9V battery to run that part of the device. Shoving 9V down a 5V pipeline is rife with unforeseen consequences.
2. It may take a few million thousand hundred dozen revolutions to go from "soft code" which sets a Boolean Flag, but won't trip the SES .... to "hard code" which takes the Boolean Flag and finally makes it over the goal line and THEN it turns the SES light on.

HTH.

Thanks for the excellent written layout.
 

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Discussion Starter · #3 ·
1. Be Ye veri careful on "ohming out" any circuits on a computer driven vehicle --- or toaster or electric toothbrush for that matter. Computers don't like it when/if the meter uses a 9V battery to run that part of the device. Shoving 9V down a 5V pipeline is rife with unforeseen consequences.
2. It may take a few million thousand hundred dozen revolutions to go from "soft code" which sets a Boolean Flag, but won't trip the SES .... to "hard code" which takes the Boolean Flag and finally makes it over the goal line and THEN it turns the SES light on.

HTH.

Thanks for the excellent written layout.
Thanks v much. I generally disconnect harnesses before running any continuity checks. I did back-probe at the sensor, but that'll happily take the 9V. At the PCM I had the plug off, so the PCM itself was out of the picture.

Here's a half-baked theory. If I get the hall effect sensors well enough - a voltage is supplied from power and then returned when the magnet creates the field. This one gets the car's 12V and at max magnetic field (magnet right on a wheel tooth) just returns 12V on the signal. But if the thing is getting weak, maybe it will only return 10V, and maybe that's not enough for the PCM.

But the 12V talk is "loose." I get about battery voltage with the KOEO. But I did just verify that it's getting alternator voltage (about 14.5V) with KOER. So now the signal return will probably be something more like 12V, which should be enough. I don't own an oscilloscope tho, so I don't think I have a way of getting signal with the thing running. But this could explain why I can clear the code and drive around for hours without having the code set ... until the next time I go KO with only battery voltage and then it gets the inadequate signal again...? First KO before ER it gets inadequate signal and sets the flag. Driving around it's always getting adequate signal. Next KO before ER is gets the inadequate signal again - boom second flag sets the code.

In this case, I just have a weak sensor. How half-baked is that thinking though?
 

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I think --- and I'm sure I'll be sternly punished if I'm wrong -- that the 12V you're seeing isn't the digital signal that the sensor creates ... and is what the ECM/BCM/PCM/?BM is looking for and to which it reacts.

The 12VDC is just the carrier and the zero-ones are the data.

FTR: I have a 2-channel oscilloscope that I have used in some of the most wonderful ways to test things --- and I have been known to waste hours finding these things, new to me.
 

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Here is a procedure to test the camshaft position sensor using just a multimeter along with some explaining as to what is going on with the observed results.


I find this series of "tutorials" to be quite good.

Just my 2 cents worth.

Good Luck!
That's the biggest weakness of a digital meter --- whereas an old timey analog meter (the one with the needle that moves) is a better device.

Actually, some fine wire, a magnetized sewing needle and some buttons and a piece of tin foil to create an amateur galvanometer would be about as good.
 

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Truer words have never been spoken. Sometimes an analog meter is superior to a DMM.
 

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Discussion Starter · #8 ·
I did run through all of those checks (which is why I know I have voltage, ground, and signal). And those tutorials are great, I agree! The only part I didn't follow to a T was cranking the engine by hand. It must be different for other years or maybe body styles, but in my '02 Envoy (SLT) the cooling fan is so crammed up against the front of the engine that I don't see how anyone gets a socket on the crank. That's why I just pulled it and subbed a socket. Same thing, really. (Though I have failed to mention that I've pulled the oil cap quite a few times to eyeball the reluctor wheel and all appears clean and shiny).

So the only part I'm still unsure about was in getting the 10V. Sure, the guy says 12V - but almost no one gets exactly 12V. 12.3 ... or ... 12.4 ... or even 11.9? I just find saying "the meter should read 12V" to be the ambiguous part because it almost never will.

I do have an analog meter. But if I wanted to put that to use, would it tell me about the digital signal? I figured I'd just be looking at a vibrating needle...

Thanks for all of the comments. I appreciate the time and expertise.
 

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2004 chevy trailblazer_lt
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I know what you mean about the ambiguity of someone saying about 12 V. I figure if it's between 10.5 and 13.5 V it should be good. Of course we all know that all these measurements were made with NIST certified and calibrated multimeters that cost a gazillion bucks!

As far as using an analog meter (and I am just assuming this because I've never really done it) you would likely see the
needle deflect to the left hitting the stop, then to the right then to the left, then to the right... when trying to read a low level AC signal, so yup, a vibrating needle in my opinion. Now if you had one of the really fancy analog meters that has the 0 in the middle of the display, that would be neat to watch!
 
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Discussion Starter · #11 · (Edited)
Ok, a new and very embarrassing part of the story that I don't want to tell... But that would be rude with people being so helpful and it might help someone else later on in time. It doesn't solve MY problem so it's irrelevant to my situation. But maybe not to others!

I've just been playing with the Envoy lately as I've also got A/C system issues. I went to the garage to play with that aftermarket sensor I bought. I have a 9V set up for playing with things (like A/C actuators which I HATE - wth is wrong with physical cable connections and stuff?) So I was going to power up the sensor with 9V and see what it put out for signal. (Sort of like, "is there a typical voltage drop between voltage supply and signal return on a hall effect sensor?") And what I discovered first is that the pins on the aftermarket sensor were all bent. They're really thin and flimsy (very much unlike the oem which are pretty hearty). So when I threw a sensor at it, it was never really hooked up right.

So then I (embarrassingly) thought - aha! I'm just a dumba**. Straightened the pins. Made sure the connector went in right...but still only produced about 10V with metal to the magnet. No difference.

So yeah, I totally screwed up the first sensor install. But unfortunately figuring that out didn't change my story. I still get a correct voltage supply to the sensor. The ground checks out. The sensors both produce about the same voltage and the signal wire has continuity to the PCM.

I was born in '67 and most of the cars I grew up riding in had no computers. I might just go to the boneyards, not to find parts, but just to go find an old ... something that doesn't need computerized engine controls?
 

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I think we've all been there at one time or another. Admitting to it is what separates us men from the boys!

I fully understand the desire to return to the good ole' days of no computers!!! Lord I miss my '76 Buick Electra, '73 Impala and the '68 Olds Cutlass Supreme! Talk about easy to work on !
 

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Discussion Starter · #13 ·
I think we've all been there at one time or another. Admitting to it is what separates us men from the boys!

I fully understand the desire to return to the good ole' days of no computers!!! Lord I miss my '76 Buick Electra, '73 Impala and the '68 Olds Cutlass Supreme! Talk about easy to work on !
LOL. I once had a '64 Ford F100 step sider (3 years older than I am). Straight 6, 3 on the tree, and more rust than paint. Only a 2WD, but it heated my house (hauling firewood) for a good part of the '90s. For the room it had under the hood I could almost just jump right in and stand next to the engine while working. And, of course, no computers or sensors. Just 12V wiring, switches, relays, fuses and such. I had a factory service manual for it. LOL. It looked more like a Haynes/Chilton than today's FSMs that seem to need a forklift to carry them around.
 

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We don't carry books around any more --- I had Mitchel On Demand and it was available to any of the guys who worked with me on their own PCs that they kept n their bays.

Nowadays, we have more than all we could ever need on a cellphone.
 
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