Hi, and thanks for jumping in.

The O2 sensor is hooked to the purple wire, that goes to the larger, of the two connectors. At the moment, I am in my jammies, and can't see the manual, so I cannot say the designator, but its the larger connector. How do I know I have it connected right? Because I can trace the wires, I use a meter to verify it, and its the purple wire, so there is no way, no how, no chance, that I am not monitoring the pin for the O2 sensor at the place where it enters the ECM .

The wiring is just as it should be. If there are typos, its because I have written it so many times, I have just gotten it wrong once or twice. :-) Sorry. This absolutely is NOT an O2 sensor issue. The purple pin, on the larger connector, without the O2 Sensor connected, shows 1.5V with the key on. This is bad. Real bad. Can't be, but it is. That's why the ECM throws the code 13 since its over the limit for voltage, by a lot.

Connecting the sensor, makes no difference, but as I noted, if I connect the sensor, it works fine, and delivers the 300mv-600mv sawtooth wave, (best guess with my not so great scope) only it rides on top of the 1.5VDC that is present at the ECM so the ECM sees a solid 1.5VDC and turns on the engine light.

Since I never cut any wires, there is no way its wired wrong :-) All I did was add one of those splice connectors to the purple, sensor wire, right at the ECM, so I could hang an idiot clip off it, and monitor it with a meter at all times. Nothing else has changed.

All is hooked up. Turn on the key. Read 1.5V at the purple O2 sensor line. Doesn't matter if the sensor is connected, I leave it off, its irrelevant to this. Pull off the smaller connector, and the 1.5V dissipates to zero, slowly, over about four seconds. its a cap discharging. Plug the smaller connector back in, the pin jumps up to 1.5V. Its redundant to say, but I am aware that its not cool to plug and unplug with the power on, but the problem was there long before I began pulling the plugs. I bought a second ECM, same problem, so I am not doing any damage at this point with unplugging it powered on.

What I don't know, is what voltages I should be seeing on each pin of the smaller connector. (or the bigger one, for that matter) The power ones are obvious, the block diagram shows me where most of the others go, but they don't tell me what I should see with regards to voltage levels.

As I mentioned above, I decided not to cut, but instead am gonna hit the junkyard and get parts to build a "breakout box" so I can isolate lines at random without damaging the harness, but no guarantees that would get me where I want to be.

All I can say is that if I did cut, which I won't <g> and I cut every wire on the smaller connector, the 1.5V at the sensor input to the ECM WILL go away, because unplugging it, makes it go away. What that leads me to, is unknown.

most of those pins, are zero, which may or may not be ground, and the power lines are at 12V. A few are at 5V, and one is 9V. I can see whatmos of them are on the block diagram, but that doesn't tell me if 9V is what I should be seeing, so my only hope is to pull one wire at a time, and observe. If I get lucky and find the one that drops the 1.5V to zero like it should be, then I will have to figure out where it goes, and why its affecting an output on the ECM. Seems very odd, even more odd I am sure from the outside looking in, but I am trying my best to explain it. I am not an electronics neophyte by any means, and reading schematics and diagrams are as easy as eating ice cream to me. Its what I have done most of my life. It may be automotive, but the basics are the same <g>

You said "It just takes checking the stuff against what ought to be there." and that is the key. I have no way to know, what is supposed to be at each line. If I had another truck right next door, I would compare, but I don't have that luxury, so I am up in the night when I probe a pin, and find voltage. Should it be there, or not? Is it too much, or too little? The chilton block diagram doesn't give out that kind of information.

That is why I am stumped. I will create a method to disconnect every damn wire without cutting, if I have to, but that will likely leave me holding a wire, and wondering what it should be reading. Therein, the problem.

It has nothing to do with the O2 sensor. That is for certain, and I don't at all mind be reminded that I may have something connected wrong, so thanks, but I haven't cut anything, and so I cannot have it wired wrong. All I could be doing wrong is probing the wrong wire from the sensor, but I am positive its the purple one, on the bigger connector. The ohmmeter confirms that, and I also keep my 24 year old son around, to make sure his old man isn't losing his mind, and he double checks everything, and never takes my word for any of it, which is fine with me. :-)

I am open to anything you suggest, and I am sure you are right that few, if anyone, knows the inside of the ECM, but the problem exists with either of the two I have, so I don't think its inside the ECM. If I was a rich man, I would take it to the dealer and make them figure it out, but I don't have hundreds upon hundreds of dollars to give away while they spend five or six hours debugging it :-)

What blows me away, and keeps my head spinning, is that I have disconnected every sensor, relay, and connector in the engine comparment, as well as pulling every fuse but the ECM power, and still.......

The problem appeared, when we reinstalled the engine after having it out to do the main seal. I pinched a wire or severed it? We have gone over it a dozen times, and can't find any sign of such a thing, but anything is possible. There really is not all that much going on in there. Its just OBD1, not exactly a super computer :-)

thanks for the info. As I said, I don't mind being wrong, or suggestions that I am, but so far, I am as sure as I can be, that all is connected right, and I just mistyped connector numbers because the book is in the garage, and I don't bother to get up and get it. Sorry for that confusion.

Much obliged for your time and expertise.

John

 

I'm on the same page as you now, i feel like i'm sitting in the truck with you haha I liked the part about keeping you son close by to make sure you're not loosing it haha. I like the break out box idea. Did you check the powerless wires for continuity to ground? I spent some time trying to find a schematic on the net and nothing. I love and hate wiring issues like this one, so frustrating but so rewarding when you finally get it. keep trucking buddy. Yeah if you can find the wire that is the problem, that would be a huge step forward. It sounds like Gimpy has some info that could be a big help at that point. Oh yeah i forgot to ask... any other codes? or just that one? I'm a heavy duty mechanic and i've seen some really weird codes that ended up being related. also some really weird causes haha. Good luck and let us know what you find.

 

So I guess you're looking at the same W/D I am. I just can't see where you're getting migrant 1.5v from though. You say it's coming from the O2 sensor terminal, right?

 

Yes. It is comes -out- of the ECM terminal pin that is the purple wire, which connects to the O2 sensor. It doesn't come from the sensor, or the wire itself. It comes into the ECM, through something on the smaller of the two main connectors.

I am glad I am not the only one staring at that diagram, and getting no clue where to look :-)

I am pretty sure I am gonna be looking stupid when I finally, if I finally, figure this out, because its not real logical, what I am seeing. The ECM is hardly a computer. Its just a state machine that reads voltage levels and does certain things when it sees certain levels. It hardly seems reasonable that voltage can get re-routed around the inputs and outputs, but its what I see, and I can't get around why that is.....

Of course, its cold raining this AM, and I have to go to the pick and pull to get the wire harness and connectors, and I am starting up with a bitchin cold since yesterday..... :-( The clock is ticking, and I am close to calling it.

Thanks.

John

 

Did you say you pinched or caught the wires on something on engine install? The wire harness that the O2 sensor is hooked up to appears to be the "small harness" that when you unplug the voltage goes away. I would try to unloom this harness and see if a couple of wires are nicked and transferring stray voltage over to another wire that is feeding the ECM this 1.5V. Ir's the only possibility that I see if before the pinch job happened everything was OK. Does this make sense? Or at least as much sense as what's happening? I'll think about it some more. Don't give up. Sounds like you have a nice little truck there.

 

I put forth the notino that I might have, or must have damaged something in the harness when I put the engine back in, but I have no proof of that, other than the things I am seeing.

I agree with your methodology, but here is the reason I think I am over and done with it. Unless I know what voltages I should be seeing on what wires under what condition, there is little point in measuring, and searching for what you are suggesting. If I check a wire, and it shows as grounded, there is no way to know if it supposed to be grounded, or not. If I see voltage that is not 12V, is it right or wrong? No docs, no way to do that.

The O2 sensor is on C1, the big connector, while the offending voltage occurs when I plug in C2.

I spent the morning in the rain and mud at the pick and pull yard... Tons o' fun. Got some connectors and built a "breakout box" for the C2 connector. Its smaller, so that was easier to start with :-)

The results are in, after plugging and unplugging, and a variety of other checks, and here it is: C2 - A6, pink and black wire is power according to the Chilton W/D. When I disconnect everything from C2, and just plug in that wire, the voltage at the O2 pin input on C1 goes up to the 1.5V. If everything else is connected, it makes no difference, but then, that is because there is no power on the C2 connector. So, one more time, if everything is connected into C2, except the power, all is well, but by the same token, the car is dead, as the fuel relay, and a host of the ignition stuff needs power there. Once I put on that power, there is my 1.5V buddy on the C1 side, for the Os sensor. Clear as mud? :-)

At this moment in time, here is what has happened. Did something to the wiring harness or a sensor when we put the engine back in. Short, crimp, cut, broke, whatever. This broke the ECM and caused the 1.5V where it doesn't belong.

Went to the junkyard, and bought another ECM. It was pulled from a truck that was running, anything can happen, but there is little chance it had the same problem, and I think it was a good ECM. However, I plugged it into the harness that blew up the first one, and that just blew up the second one because the harness has a problem that "eats" ECMs.

Therefore, my only option, is a new, or built by me, wiring harness, which is not going to happen due to time constraints, and lack of desire anymore. Its a safe bet, in my mind right now, that any good ECM that I plug in, is gonna fail, caused by the wiring/sensors in the the truck. I wish I knew someone with the same truck, because I would like to plug one of my two bad ECMs into it, and will bet the farm, that the problem goes with the ECM. I was suprised to learn while knee deep in the muck at the scrap yard, that every year connector, is different. Pins that are used on the 88, are not used on an 89, and on and on. I can't see that the ECM is interchangeable with anything other than the original truck harness. Thought they would work in most late 80's, early 90's models, but that is not what I saw today. Interesting.

Now the only part of that that I can't quite get down with, is that there is not a whole lot of wiring going on in the engine compartment. Short of the grounds that go to the back of the left side head, there just isn't anything I can conjure up, that would get pinched, or shorted. Always careful around the exhaust and stuff. Its a little ugly around the coil and distributor, but not close quarters and not so tight that something would pinch. Its not the first engine I ever installed, but there are no guarantees each time.

Far as I can tell, my only choice is to replace the entire harness, lacking docs that tell me what to expect if I measure voltage and shorts to ground or power. Anything less, would just blow up another ECM. Too much work, too little reward, too many $50 ECMs.

That's my story for tonight. The truck is shared between me, my son, and my daughter, and they will decide its fate. I wanted to do rock, paper, scissors, but they said they would rather think about it for a few days :-)

New 3.4l engine for 95 camaro, $1700. Trans rebuilt less than 100K ago, brand new tires and wheels over $1000 worth, basically less than two year old front end parts all the way round including CVs, both rear axle seals replaced, new brakes, just replaced coil and the ignition control module. Its a bitch to think about tossing it away, but I think there comes a point of diminishing return. I told them to consider parting it out, or even just selling it whole and telling whomever, what's going on. I also told them they could park it alongside the garage, and take their time in warmer weather, pulling each wire back through the harness, and making it all new again, but to me, that counts as diminishing return.

Its up to them now. I feel bad because I have lost to a damn truck, and that has never happened before, but building a new harness, without specs on what a good one would read like, doesn't make a whole lot of sense to me.

That's my day. I am gonna go lay down and see what happens tomorrow.

Thanks for all the help. I really appreciate the support.

John

 

You know the more I think about it I think you are just one wire away from finding your problem. If when you connect the "small connector" then I think you could tell from the W/D what is supposed to have power and not. If you check the connector and have power to one terminal where it's probably not supposed to be that might be it. I realize you don't want to eat up another ECM, but you could be reasonably sure of the incorrect wire and address it before you plug in another ECM.

 

Dredging up an old issue :-)

The way to go was to back off, hide the truck in the garage, and think about what to do. That brings me back here to ask a question of the experts.

Toying with the idea of rewiring the harness. However, as I was talking it through with my son, we came up with a question that maybe someone here can validate?

If the theory is that there is a short in the harness, or a sensor that is doing something wrong and blowing up the ECM, then how can I explain the fact that I don't get any other codes besides the original O2 sensor? If one of the wires was shorted, or whatever, wouldn't that cause a failure that should throw the appropriate code?

To try and prove the point, we disconnected several of the sensors, one at a time, and each one cause the ECM to put out the failure code, so the ECM is still working, but for the O2 sensor port, and all the sensors are being monitored properly.......

Almost convinced to rewire, then this line of thinking came up, so now its not looking like rewiring would be of much value, since it doesn't appear that the harness is bad in the first place.

Anyway, been a while since I brought this all up, and real close to the junkyard, but not wanting to do that. Just that we cannot think of a good plan of attack.

If anyone is still listening :-) be interested in what this says to you.

John

 

hey i just wanted to ask, are you getting the code, 13 or is it 14 that always comes first? it just means that the ecm is working properly. if it doesn't come first then the ecm is toast.

 

The 13 is all I get. That is the O2 sensor code. When I replaced the ECM, it got toasted, and does the same thing so its practically impossible to fix the electrical problem that bangs the ECM, since the only way to prove it is fixed, is to pay $50 and plug another in. Not an elegant solution.

What I have trouble with is, if the wiring harness or one of the sensors is bombing the ECM, how come I never get any other code, like temp sensor, or map sensor? I was going to rewire the harness, but when I got to thinking about it, it doesn't make sense. If I uplug the map sensor or the TPS, or any other, I get the appropriate code. If the harness had a short or open, or bad voltage level, it would throw a code, but it only gives me 13.

Given the presence of 1.5V at the O2 sensor input, with the sensor disconnected, its hard to blame anything but the ECM itself. Ain't supposed to be that voltage there.

Been through 3 ECMs so far. Turning into a waste of moeny. What I need is a truck like mine, sitting next to it, so I can compare the levels on the ECM inputs and find out which is messed up. Anyone willing to sit at their ECM and write down all the voltages and grounds with a cold engine, key on, not running? :-) This one blows the ECM without ever starting the truck......


Other than something like that, I have no way to proceed.

thanks a lot for the ideas.

John