OK, not to step on anyone's toes, OBD-1 is not a dumb system, it is just the first. Just like we have MS windows now, back in the eighties we used DOS. I have two trucks a 1990 Silverado 1500 and a 1988 K5 Blazer. Both have OBD-1. So, when you start the truck does the "Service Engine Soon" light even come on ? When you start the engine, the ECM or computer ( Which is on the right side just in back of the glove box.) It's silver square and it's about 3" X 15". But lets not go near it just yet. When you start the engine, the ECM takes reading from the coolant sensor, 2 wire sensor right behind the thermostat housing. That is the sensor that goes to the ECM and there is another, it is on the drivers side head right behind #1 spark plug, that goes to your gauge. With that and the signal from the O2 sensor, which if your truck is stock is in the 'Y' pipes just aft the exhaust manifolds. It needs to be hook up and working for the ECM to get a signal and the exhaust flow, and finally there is the IAC or Idle control Motor. To set the IAC, take it out and push the ' Pintal ' of the sensor back into the sensor till there is just about 1 " from the base of the motor and the very tip of the Pintal, Put a dab of silicon one the bottom treads so once tighten, it seals OK. It doesn't need any heavy tightening, it's aluminum and will strip easily. So now you have the 1.coolant sensor, 2 the oxygen sensor and the IAC idle control motor out, adjusted and your ready to go.
Did connect the negative terminal from the battery for 30 seconds and reconnect. Now start the engine see if the idle is stable and the 'Service Engine Soon' light should come on fro a 10 seconds and the go out. If it stays on the ECM has had a code set. Shut off the engine and use the paper clip and short out terminal A & B on ALDL connect under the dash and the get comfortable and turn the key to the on position. The service engine soon light will light up. It will flash 1 long flash and 2 fest times, thus you have a code 12. That is just the ECM saying hello. it will flash each code 3 times, and don't worry if you miss one it will go back to the first code 12 and start over. Write down what each code is and if you get them all, and the ECM is back to 12, shut off the ignition.
Now with the codes before you see what each code is coming from. If you don't have a code chart, just go online and you will find a OBD-1 listing. see what is in the code and that is you culprit. Between the blazer in my avatar and my pickup It took me a long, long time to find my problem. On my 1990 305 V8 we went to just about every sensor and nothing worked till we realized the only thing we hadn't changed was the ECM itself. got a Cardon rebuilt unit and installed it and that took care of the problem. Problem was a crack in it's motherboard.
On my 1988 Blazer this one was almost the one the broke the donkeys back. SES light kept coming on and coming and coming. If I had any hair I'd of pulled it all out. A technician from a Detroit GM dealership came to my aid, he said take off the distributor cap and the rotor and look at the ignition module. He said if it is white, take it out and throw it away. If it's red like a high performance module, take it out and throw it out. Go to a old time parts store or Chevrolet Dealership and get a AC Delco ignition module. Take some Acetone and wipe off the plate inside the distributor and open the packet of white heat insulator paste and apply it to the back of the ignition module. DON'T go overboard, The paste should be about the thickness of a loose-leaf page of paper, put 3 connectors on the module and put it back on, and only tighten it finger tight, then just a tad more so it snug. The 2 screws just hold it in place and have no other purpose. Did connect the battery for 30 seconds and reconnect it. I did and that was the last time I had a "Service Engine Soon" come on. The most important thing is to get AC Delco Parts. all the other crap is made in Mexico or Thailand Indonesia or. . . China. ALL the other stuff is such poor quality it a waste of your time to even try using them.
If you have any questions my name is Jim Ward.
