1988 S10 Blazer, 2.8l, auto, 4wd.

I live in Utah, where emissions testing is mandatory. I guess I have a major electrical harness issue, because my ECM shows 1.5V on the pin where the O2 sensor connects when I turn on the key, thus I get a code 13, bad sensor which of course means auto failure of the emissions test, plus because of that, its running really rich and would fail no matter what.

I just blew $50 at the junkyard for a second ECM, from an identical truck, same problem, 1.5V at the sensor input pin, with or without the sensor connected, once I turn on the key.

Removed the connector from the ECM, and the sensor disconnected, and zero volts, just as expected. Plug the connector into the ECM, leave the O2 disconnected, and 1.5V with key on. Hook up the sensor, 1.5V. Drive the truck to get into closed loop, and engine light comes on.

Scope shows that the O2 sensor is actually working based on the appearance of a waveform in the millivolts level, (not a great scope) riding atop the DC 1.5 volt level that the ECM is generating, which it should not be doing.

Can't have two ECMs with identical problems, something in the truck harness is screwed up. There is no way for me to track that down, and I doubt anyone else can either.

Therefore, prior to driving the truck to the junkyard and getting $100 for it, even tho the entire driveline and front end are all new, or at least under 50K miles, I thought I would ask if anyone can suggest something I could try.

I am out $85 for the O2 sensor install, and $50 for the ECM, in addition to a new ignition module and coil that I put in a few weeks ago, added to the new engine, and all the other parts. Its too much, even tho I absolutely love the truck.

Is there any way to find out details on the wiring, and the ECM? The manual I have shows the block diagrams, but its not complete, and lacks details I need.

Can anyone help me keep my S10 Blazer????? :-(

John

 

Im sure Gimpy and Swartkk will jump in here with some suggestions to help you out brother!

 

This AM, I went to the junkyard and bought another ECM. They 'claim' that they test them all in working vehicles, no returns, $50. What can I do, gotta start somewhere, so I brought it home and plugged it in. Exact same symptoms, so its not the ECM.

Anyone need an ECM for an 88 2.8l auto, 4wd? :-)

As it sits, the O2 sensor is not even considered as a problem. All the testing I have done today, I have done with both the sensor connected, and disconnected.

Wired the ECM right to the battery. 1.5V at the pin for the sensor......

With the key on, the voltage on that pin present, I disconnect the smaller of the two harness connectors to the ECM (I now have a spare, so I break the rules now and then <shrug>) and the voltage at the sensor pin fades to zero, which is what I need, after a few seconds, as in a capacitor discharging.

So, realizing that the problem started when we reinstalled the engine last time, we disconnected everything that goes to the wire harness, that we would have pulled when the engine came out. At that point, all things disconnected that we know of, turn on the key, and 1.5v at the o2 sensor input pin, with or without the O2 sensor connected.

Sometimes we can get fixated, but I don't believe for a minute that it has anything to do with the sensor, or any ground issues. I feel confident I have beaten that horse to death. I could be wrong, but that is my mindset after doing all that I can think of.

Given that removing the smaller of the two harness connectors (c2?) eliminates the 1.5V at the sensor input, the next step is to cut every wire on that connector, until the 1.5V goes away. Lots of cuts, and lots of crimp connections to put it back together, but at the moment, its the only next step I can think of, since as far as I know, I disconnected everything on the harness, but must be missing something, so this is how I have to go about finding it. Hoping that a cut wire, will point me in the right direction.

Besides, if I cut them all, minus power and ground, and it fails to reduce the voltage at the sensor pin, it doesn't matter, its going to the crusher in two weeks anyway <shrug>

I REALLY hate to lose to a car/truck. Really, really, really hate to lose. Never have yet. This will be the first time in nearly 40 years I have had to give up, and toss a vehicle. Its got lots of great new parts, wheels, tires, brand new engine, etc, but not worth giving it to GM, or some clip joint to see if they can figure it out. The shop hourly rates would be over the top for sure on that method.

Still up for any ideas, but shoot, there is only so much I can do without schematics. The block diagram in the manual doesn't give me what I need, and the library doesn't have anything useful. I wish I could spend a day with the GM manuals that the dealers have :-)

John

 

Can i make a suggestion? Unplug the o2 sensor, and the harness to the computer, with the key off, then check for continuity from the power wire at the sensor plug, on the truck side, to chassis ground. Make sure you don't have a partial short to ground. Also you can probe into the back of plugs with a thin mutlimeter probe, with the plug plugged in, thus checking the voltage on that wire without having to cut it, you shouldn't ever have to cut it. Also can you read schematics? Get a copy for your truck and they should tell you what wires go to what pins in what plug, also where they end up going to.

 

Thanks. I appreciate the ideas. I have gone over the grounds and power lines and found no problems. Not sure how to phrase it, because I really like input and am grateful for things to try, but I am unable to see how a partial short, or even an open ground, can cause the problem I have. A partial short, shouldn't create voltage at an input pin, from within the ECM......

We don't need to go over my skill set :-) but I have been in electronics for 30 years, so if I had schematics, it would probably solve the issue, but I have none, and haven't been able to procure a set. All I have is the block diagram in the back of the chilton manual and there are wires at the ECM that do not appear in the manual diagrams. Nothing at the libraries covers it. I have been watching auction sites for GM manuals for a very long time, but there are none right now, and I have less than two weeks before it goes to the crusher, so my options are limited <shrug> I just love the fact that the state will make me throw the truck away because of their damn emissions testing.

Today is cut the wires day, because I see no other way to proceed. Something in the wire harness, through the smaller connector (I forget the designator) causes the O2 sensor input pin, to rise to 1.5V when the key is turned on. The o2 sensor itself has become totally irrelevant because it is not causing the 1.5V, and most assuredly cannot 'talk' to the ECM over the 1.5V DC level that exists. In fact, the sensor actually does put out the expected signal levels, and when connected to the ECM, those levels ride atop the 1.5V, which is why the ECM throws the code 13, as the input level is way out of spec.

Your point is a good one, I can turn on the key, and probe the connector with my scope, no problem, but since I don't know where all the wires go, it will be impossible to tell if the levels I will see are supposed to be there or not. It seems to be from a capacitive source, as the 1.5V dissipates slowly when I pull the smaller of the two connectors off when the key is on.

That is why I have to remove each wire one at a time, to see which one is causing the rise in voltage on the o2 input pin, then hope to figure out where it is coming from. A second ECM behaves exactly the same, and as I lay awake thinking about it, I cannot imagine what on earth is causing voltage, on an input pin. I am somewhat convinced that I have fixated on something, and won't get past it, until I do this, an lacking any other course of action, I have no choice.

I will try everything I can think of, no matter what, but time is not on my side. I wish I had schematics, but I don't therefore, I have no choice but to cut and poke and hope. Plus, before anyone says anything, I know unplugging the ECM with the key on is not a great idea, but I have no choices, and thanks to the junk yard, I have a spare just in case :-)

Gotta go buy the splices so I can put the wires back together and be able to drive it to the junkyard and not be stuck with it in my garage forever <g> I guess AAA could come get it. The scrap yard takes off $50 from the $200 they offered if they have to come and get it...... Oh well.

Always much obliged for ideas and suggestions. Thank you very much.

John

 

Send a PM over to Gimpy Blazer, and dont scrap the truck just yet. Hes got his hands into some good books Im sure he would get you the help that you need.

Also, I took my truck for testing a couple weeks ago, and stopped just down the road from the testing station, unhooked the battery, hooked it back up, and went for the test before it could get into the closer loop. Had no engine light on until I was pulling out of the testing station AFTER I had already passed.

 

That would keep the light off, bu it won't pass the emissions test if its not in closed loop, since its running full rich, thinking the the sensor is bad. :-(

I believe Gimpy Blazer has said he would look into his books, but I haven't heard anything. I will bug him again, hope I don't get on his bad side :-)

The truck becomes scrap officially, on the 28th due to the government, so I haven't got much time to do anything with it. Kinda sucks, but with no docs, even finding an offending line into the ECM may not help much, and without schematics, I may be just chasing ghosts around and around.

Sucky thing is, every fuse (but for the ECM power, and every sensor on the truck has been unhooked, and makes no difference. Still that damnable 1.5V at the O2 input pin......

Thanks a lot,

John

 

hey man sorry to talk as though you were new to wiring, most people are so i find speaking simply is usually best. no offense man. i must have been confused, the 1.5 volts is on the sensor wire going back to the ecm? or on the 5v wire coming out of the ecm going to the sensor? And where are you testing it from? Is your electronics experience in automotive? HaHa my buddy an electrician and couldn't find his butt if you turned the lights out, when it comes to automotive haha. Just different animals. For the record, i'm not trying put you in that category. I feel bad for you situation. Oh and unload the pins from the plug, don't cut.

 

Absolutely not a problem. I know you were helping. No way to tell who you are talking to. I just wanted to find a way to make it known I am not a newbie. No big thing.

Picture this. the ECM is on the floor of the truck, all connected, and the O2 sensor is -not- connected. I have verified that all the 12V wiring is good and solid, no intermintent's or anything and the grounds are rock solid everywhere. I have even connected the - terminal of the battery, to the ground line on the ECM, just in case. Turn on the key, and measure the voltage right at the ECM pin (I forget the number) which is the wire directly from the O2 sensor (which again, is not connected) Meter says, 1.5V. Not right at all. There is nothing connected there. The voltage is coming from inside the ECM.

Connect up the sensor, turn on the key, and the pin right at the ECM shows 1.5V The sensor itself, is just not an issue here. The 1.5V is coming -out- of, being generated by, the ECM from within. If I didn't see it, I would not believe it. Tried a second, known good ECM, same result.

Key is on, there are two big old connectors to the ECM. The O2 sensor pin is a part of the bigger one, so I pull off the smaller one, and watch the 1.5V dissipate over about four seconds or so, like a capacitor discharging, and then, the O2 Sensor pin at the ECM, reads zero volts. What the heck????

At the moment, the conclusion is, something is coming into the ECM through that smaller connector, that is hosing the computer, and somehow forcing 1.5V to appear on the O2 pin. Since its hardly a computer, more of a state machine, I can see how that might happen, but I have no details on the ECM, so I have no where to check.

As mentioned, I put a scope on the O2 sensor pin, at the ECM. Without the sensor, it is a 1.5V DC voltage with the engine running. With the sensor connected, and the engine running, I can see the O2 Sensor voltage changing like its supposed to, but the waveform it generates is riding atop the 1.5VDC level, plain as day on the scope. The sensor is working, but that doesn't help me. I have to get rid of the voltage that the ECM is putting -out- on the input pin.......

My only clue is that it goes away if I pull the smaller connector, and that it does not drop instantly, but bleeds off when I pull that connector, like a cap discharging. Beyond that.... I got nothing since I have no way to know what is what inside, or even on some of the harness.

At the moment, every sensor and relay in the engine compartment, every connection I know of from the harness, and every fuse but the ECM power, is disconnected, and it still exhibits the same symptoms, so I know that I am not seeing everything there is, because all I have is the block diagram.

I am going to the boneyard tomorrow, to get the end of the harness, with the ECM connectors, and as much wire as I can and I will have to sit and tin every wire, or perhaps solder on a small in from Rat Shed, so I can create a sort of "break out box" between the existing harness connectors, and the ECM, that will allow me to pull wires in a random attempt to find the source. This I decided, after staring at it, and simply not caring enough, to start cutting and reconnecting wires :-) Using the block diagram, I probed every pin on the smaller connector while it was powered and hooked all up, hoping to get something inspirational from a reading. Only a few lines had any power. Most were zero, and no way to know if that is what to expect or not. Hence the need for better docs.


I love the truck, but nothing lasts forever, so maybe this is it. I have other vehicles that need my attention as well, and so a cooler head prevailed, and no cuts were made.

Thanks very much, especially for giving a damn enough to speak out about not wanting to insult me. When I can get help from someone out of the goodness of their heart, that is a great price, and I would take no offense at what was said. <g>

John

 

Did you say your O2 sensor is hooked up to Connector C2? I have a Chilton wiring diagram for your 1988 2.8L engine. It shows the O2 sensor on Connector C1 coming out of the ECM at Terminal D7 and it's a single Purple wire to the O2 sensor. Yes you can fix this. It just takes checking the stuff against what ought to be there.
What exactly do you need? The whole wire hookup to the ECM so you can check it against it? It'd take a while but if that's what you need we can do that. I certainly wouldn't be hacking any wires.
Reread you last post. I really just think you wiring to the ECM is not as it should be. We can fix this. Just let me know. I'll give you the hookups per W/D. Not as good as factory manual but it should do. It'll take a while though.
As far as I know, nobody has the internal workings of the ECM on their W/Ds. Even the factory books are pretty scarce on this.