If you're not really familiar with electrical testing, have no fear. I'll call out the type of testing and the scale on your meter... just turn it there and place the test leads as I describe. If they're backwards, the meter won't care except that it will show a minus sign in the display. Same-same.
Most testing can be done from two of the three PCM connectors underhood, the C1 and C2 units. You'll need the digital multimeter and a load testing device (if handy) which can be nothing more complicated than a fog or headlamp bulb rigged with test leads to turn it into a 4-amp 12v test light. Load testing is critical for situations where a controller has problems sustaining operation because a high resistance power or ground feed will test with no problem with an unloaded circuit in most cases. This method would give you the opportunity to see how the circuit holds up with some serious amps running through it and bulb brightness will tell you the results without the bother of a voltage drop measurement.
If you're not familiar with PCM location, it's almost center underhood, just above the radiator fan basically. It's the 3-connector unit with three 8mm screws holding it to the bulkhead. Connector numbering goes from right to left.
We start with C1, the far-right (black/ black) connector. Since the Factory continues to alternately call these connectors A, B and C, the wire locations are shown as A1- A32, which I'll use to avoid confusion. The C2 will be the B connector with the same numbering method... Go ahead and print these two downloads; it's easier to follow that way.
This is the connector that holds all power and ground inputs to the controller and has one of the two 5v outputs to the engine/ transmission area. Since the engine is not in running condition right now, let's just jump right to the powers and grounds.
With the C1 connector disconnected, roll the key on. Do this after disconnecting, not before (some obscure hazard from disconnecting while turned on is present).
Note that the open (terminal) end of the connector is handily numbered for circuit location. It's small, but trust me... it's there.
Check your powers by connecting the load tester to battery negative, then touch the test lead to Pins A2 (ignition switch feed) and A22 (battery positive, always hot). Be careful to avoid pressing your test lead into the terminal if it's a tight fit because you sure don't want to spread one of these. Wiggle test the harness as the bulb burns, watching for changes in light intensity, giving the circuit a full minute or better to fall on its face if it's gonna do so.
Grounds are Pins 31 and 32. Do the same thing except you'll have to reverse the first test lead to battery positive.
If testing with the meter only, set it to 20vDC for the voltage tests and compare that reading to what battery voltage is at the time. A small difference in voltage is to be expected, but it shouldn't be more than a couple tenths of a volt.
Grounds can be tested using the 200 ohm scale, placing one test lead on battery negative and the other on the pin terminal. Expect to see 10 ohms or LESS if good.
The 5v circuit is next and so's the multimeter. Set it to 2K ohms.
Test Pin A17 for ANY continuity to ground. By continuity, I mean watch to see if the meter shows anything other than an open circuit. This should
be an open circuit with the C1 connector danging, as the only return path it should have is the sensor ground circuit (Pin A4). If continuity is found, start wiggling the harness, as this is the only chance for the 5v circuit to be connected to ground at this point (something's shorted). This circuit goes all the way to the round 8-way transmission connector above the left transmission pan area by the way...
Next, test between Pins A4 (sensor ground) and A17 to see what total circuit resistance is through the entire sensor group.
Low resistance in this test means there's a short within one or more sensors including MAP, TPS, cam and crank sensors. Expect no less than 400 ohms on this test if the circuit is good. It actually runs closer to 600 or so, but altitude makes a difference in MAP sensor internal resistance so it doesn't pay to split hairs. If you find overall resistance to be too low, start unplugging sensors until the resistance increases back to something acceptable. Let me know if you get a low resistance reading and can't find the source.
Leave the C1 connector off and now disconnect C2 (center of 3, black/ white).
Pin B31 is a 5v power feed to the transmission's governor pressure sensor. A short to this circuit will create problems like the A17 circuit, so we'll check it the same way. Check for continuity to ground first while doing some harness wiggling (should be open), then check between B31 and A4 for total circuit resistance. Expect to see 500 ohms or more if OK. High resistance... good.
Other tests you can do while the engine is NOT in running condition are to test your 5v feed right at one of the sensors with everything plugged in and key on. This orange wire will typically show 5.18v when healthy and about a tenth that (0.5v) when the PCM isn't feeling so good. This is done on the 20vDC scale.
The cam and crank sensors have a greater chance of shorting the 5v out, but is pretty rare on this engine. You would have to catch them in the act as well, with the 5v reading low and then disconnecting each one. Voltage will snap right back up when the short is removed. Cam is located inside the distributor and the crank sensor is the unit bolted to the right upper transmission bellhousing. Both connectors are on the left-upper part of the engine.
Instances where voltage has dropped off can mean the circuit became shorted (through a sensor or harness short) or that the 5v generator in the controller is pooping out. The way to tell the difference would be to start disconnecting the sensor group while watching output on the circuit. If you get all four disconnected with no change, turn the key off and disconnect C1 once again to open the circuit. Test the 5v circuit for continuity to ground at either location (sensor or C1), expecting to see zero continuity to ground if the sensors and harness are OK.
The B31 feed is somewhat more difficult to test thanks to its location, so stick with the C2 connector for a test point. Disconnect and repeat the test for continuity to ground through the harness and back through the A4 sensor ground circuit.
At that point if everything checks OK... it's all PCM. There's no direct way to test to see if the PCM is good or bad, you pretty much eliminate everything else and replace it by default. Welcome to my world!
Good luck, matty! Press print and take your time. Write your results right on the sheet for reference in case you need to write back.