The 1990 S-10 pickup and Blazer -- as well as its cousins from GMC -- used one of three different engines: a 2.5-liter inline-four, as well as the 2.8-liter and 4.3-liter V-6s. Ignition module and coil testing are fairly basic procedures once you know how to do them, and can save you big bucks compared to simply making assumptions and throwing parts at the thing.
Identify the wiring harnesses going into your distributor. You'll see two of them; a smaller one with two wires that supplies power to the ignition module, and a larger one with four wires that acts as the coil's input/output. Identify the wires. From the lefthand -- black wire -- to right, they are "ground," "bypass," "reference," and "EST," or Electronic Spark Timing.
Turn your ignition key to the "on" position, but do not start the truck. Poke the needle probes from your DMM into the two power wires in the smaller harness; you should get a constant reading of between about 5 volts. If voltage is significantly lower or nonexistent, then your coil module isn't getting power from the chassis.
Stick your black sensor probe into the "ground" wire on the larger harness, and your red probe into the "bypass" wire; these are the two left-hand wires on the harness. You should see a constant voltage in the bypass wire when the engine is off or running at under 400 rpm. Start the truck, if possible, and re-check the bypass wire. You should see no voltage in the bypass. If you do, then power isn't flowing through the module, and the module is likely bad.
Leave the black probe in the ground wire, and probe the "reference" wire -- the second one to the right. This wire supplies a constant 5 volts from the electronic control module. If you don't get a reading on this wire or the voltage is low, then you have a problem either in the computer or in the wiring between the computer and module.
Leave the engine running. Touch the black DMM probe to your negative battery terminal, and probe the EST wire on the far right. You should see a varying voltage that settles somewhere around 5 volts. No voltage output in this wire definitely indicates a bad ignition module.
Trace the thick wire coming from your distributor to the ignition coil, and unplug it from the coil output. Unplug the wiring harness that supplies your ignition coil with power.
Set your DMM to read in ohms of resistance, and touch the sensor probes to the power terminals on the coil, where the harness is plugged in; this is the "primary" circuit. You should get a reading of between 0.10 and 2.0 ohms. A reading of "0" or anything higher than 2.0 indicates a bad coil.
Lift one of the probes from your power terminals -- it doesn't matter which one -- and touch it to the coil output, where the distributor wire is plugged in. You should get a reading of between 6,000 and 30,000 ohms; any higher or lower than that and you've got a bad coil.
