[SOLVED] ECU location in 1987 S10 Blazer

Hello all,

I'm kind of at the end of my ropes here. I recently purchased a 1987 S10 Blazer 4WD with a 2.8L TBI engine. The Service Engine Soon light keeps coming on and the code is a 13 (o2 sensor).

So far I've replaced the O2 sensor and the temperature sensor. When that didn't help I hooked up an oscilloscope to the O2 sensor to check the voltage level which was around 1.3 Volts (one point three volts) From what I can tell this reading is WAY out of range.

The next step was to hook up the laptop to it via the ALDL port and get some readings. The O2 sensor voltage reading according to the ECU was 0.455 Volts.

As far as I know, this could only mean one of two things. Either there's a problem with connectivity to the ECU (i.e. broken wire) or the sensor isn't grounded properly (1-wire sensor) and the difference between the sensor's assumed ground and the ECU's ground is .5-.6 volts.

So I was wondering where I could find the ECU in this thing to trace the purple O2 sensor wire and make sure it has continuity. Any other ideas are greatly appreciated.

Thanks, Archie

Reply to
Archibald J. Cox
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Archie,

On my OT truck I hooked a 1.5 volt AA cell up to the wires to the computer. I made a simple circuit with a few 1k resistors to roughly divide the voltage in half, then connected the ECM across one resistor. Measured the voltage on the resistor and it went up when I turned the ignition on. That told me the computer was "sourcing" some current to the O2 sensor. I can't tell you if GM uses the same scheme, but I would check the voltage at the O2 connector, with the sensor disconnected, and see if it goes up with the ignition on. That would check continuity back to the ECM.

I have no idea where the ECM is, but on most older cars and trucks it's in the passenger compartment. Usually behind one of the kick panels or on the firewall under the dash. On my newer GM, it's under the hood next to the right (Passenger's) hood hinge.

Al

Reply to
Big Al

Thanks Al, I'll give it a try

Archie

Reply to
Archibald J. Cox

the ecm on that should be under the dash on the passenger side.. kinda behind the glove box if I remember right.as far as voltage there should be

Reply to
Bobo

Bobo,

We grounded the scope to the chassis and are now looking at the ground of the exhaust to be the problem. You are correct, the ECM is accessible by removing one bolt from the underside of the dash beneath the glove box. I have to replace a fuel line before I can do any more diagnostics on the O2 sensor, but I think I should have it whooped pretty soon. Thanks a lot!

Archie

Reply to
Archibald J. Cox

When you were checking the sensor did you let it warm up and hold it at a high idle? An older truck will tend to let the O2 cool off at idle and thus it wont work....(itll read a steady .45 or so) Run it at about 2K RPM for a minute or two and then you should see it go back and forth between about .2 volts to .8 volts fairly rappedly. Also do yourself a favor and put a fuel filter on it if its been a while. If you are checking it with a scan tool (laptop or whatever) you are seeing what the ECM sees.

Reply to
Scott M

Just an update for anyone following the thread looking for handy information:

It turns out that the problem causing the code 13 was a bad ground for the O2 sensor. However, the ground issue was not at the body of the sensor as originally suspected (sensor to the exhaust, exhaust to engine, engine to ground) The ECU utilizees a wire in the harnes that ties directly to engine ground as a ground reference for the O2 sensor. That wire was not going to ground (I suspect it is broken). Once we grounded that wire, everything seems to be working normally and the O2 sensor voltage is within range.

In the harness to the ECM, the O2 sensor wire is the PURPLE (no tracer) one in the larger connector and the ground for it is the TAN wire WITHOUT ANY OTHER TRACER STRIPE (there's also a tan-white and a tan-black which I don't remember currently what they go to)

Thanks for the help all, Archie

Reply to
Archibald J. Cox

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