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
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Archibald J. Cox wrote:

Its tucked up under the dash behind the glove box. There will be a screw ritgh in behind the shelf at the bottom of the dash that will let it drop down.
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Thanks!

S10
that
there's
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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
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