After doing some work on my '95 XJ which required removing the intake manifold, I put everything back together and the check engine light came on, OBD1 code 27: I had a misfire and a very strong exhaust smell that made my eyes burn. The wires going into one of the injector wiring plugs were missing insulation, leaving the bare wires exposed. The two injector wires shorted together, which destroyed the injector driver circuit inside the PCM. Here is how I diagnosed and fixed this issue for a FEW DOLLARS, WITHOUT getting a new PCM:

1. While the jeep is running use a mechanic's stethoscope or large screwdriver to listen to each injector for a rapid clicking sound of the solenoid opening and closing.
2. If one injector is not clicking like the others, disconnect the injector wiring plug and listen for the engine rhythm to change. If no change is heard, the injector may not be firing correctly.
3. Connect one lead of a voltmeter to the positive (NOT negative) battery terminal. While the jeep is running, use the other voltmeter lead to check the voltage on each wire in the plug of the suspected bad injector. The wire closer to the radiator (green with an orange strip on my '95) should read 0 V on the voltmeter relative to the positive battery terminal. This wire is always at 12 V, and the PCM periodically grounds the other wire to open the injector. The wire closer to the firewall should read between 1 to 2 volts (at least on my voltmeter) due to the PCM periodically grounding the wire. However, in my case for the bad injector, this wire read a full 12 V, meaning the injector was shorted to ground and stuck open. This caused the misfire and the check engine light to come on.
4. Turn off the engine. Using an ohm meter, check both wires of the bad injector's plug for shorts to ground (i.e. continuity to the negative battery terminal or engine chassis). In my case neither wire of the misfiring injector was shorted to ground, even though the driver wire was shorted to ground when the engine was running. This pointed to something wrong inside the PCM.
5. Disconnect the battery, disconnect the PCM plug by loosening the bolt, and find the pin in the PCM plug corresponding to the specific injector that is bad. This can be found using the wiring diagram in the factor service manual. Using an ohm meter, check this pin in the PCM plug for a short to ground and make sure there is continuity between the plug and the wire in injector plug. In my case there was no short to ground, again pointing to a problem in the PCM itself. At this point, I could have ordered a refurbished PCM (expensive) or pulled one from the salvage yard (hard to find the right model), but I had nothing to lose by trying to fix the PCM itself, so that is what I did.
6. Remove the PCM by loosening 3 bolts. Remove the plastic cover on the PCM using a screwdriver. For a helpful video on this step, see
   You Tube
   .
7. Remove the rubber layer covering the PCM circuit board in the locations circled in the picture below. I used a razor blade to remove the top layer of the rubber, and then a brush to gently clean the rest off. I didn't know where the injector drivers were located so I had to remove more rubber than necessary to find them.
8. Find the bad injector driver by checking for continuity between the male pin in the PCM that went to the injector (remember it is a mirror image of the female PCM plug) and the center pin of the MOSFETs. You can also check for shorts between the three wires on each of the injector driver MOSFETs (transistor). The injector driver MOSFETs are labeled 3055LE. In my case, one of the drivers had a huge hole from a chunk being blown out when it shorted, so it was easy to find! Some of the pins on the bad MOSFET were also shorted together, which lead to the injector misfire.
9. Desolder the bad injector MOSFET using copper braid solder wick. In the picture below, I have already desoldered the bad component as seen by the red circle.
10. Solder on a new 3055LE MOSFET. It is surface mount but not super tiny, so it wasn't very hard. I ordered this as RFD3055LESM9ACT-ND from Digi-Key, https://www.digikey.com/product-deta...9ACT-ND/965717, for LESS THAN A DOLLAR.
11. Ensure none of the 3 pins in the MOSFET are shorted together, and also check that the pin in the PCM that goes to the injector (it is a mirror image of the PCM plug) has continuity to the middle pin of the new MOSFET.
12. Replace the rubber that was removed from the circuit board with silicone.
13. Reinstall the PCM after the silicone cures, connect the battery, and repeat steps 1-3 above to ensure the injector is firing correctly. In my case the misfire was gone and the check engine light stayed off.
14. If necessary, add insulation to the wires going into the injector plug to prevent them from shorting together again and destroying the new injector driver. I used a blob of silicone/RTV.

Inspiration:
https://www.naxja.org/forum/showthread.php?t=953382
http://transistor-man.com/PhotoSet/i...x/P1270103.JPG

 

What does it tell you when the hotwire on the injector plug has continuity to ground? Do you have suggestions on how I can track down where the problem is?

 

It's either shorted in the harness or in the PCM. DIsconnect the PCM and check it again...

 

Nice write-up wcjeeper.

 

I disconnected control module C2 from the pcm and it didn't change anything. Oddly enough, today both the ground and hot wire in each connector have constant continuity to ground. Before it was just the hotwire. So it must be a problem within the harness?

I checked the power to the pin going to dark green/orange wire in the C3 module and there is no voltage when the ignition is on. I would've thought it should read 12v?

When I have the engine running, the hotwires on each connector are constantly ~0v and the grounds cycle between ~0.85v & 1.5v when connected to the postive battery terminal. Which sounds like correct behaviour from what you discribe.

 

The continuity stopped when I disconnected the battery so I looked farther afield and found an issue with somes wires going to the electric brake controller, of all things. Cleaned all connections and put back together and now neither of the wires in the connectors have continuity to ground. Still no 12v to the hotwire when the ignition is on but I'll chalk it up to a quirk since the 12v clearly show when the engine runs.

Evidently the short wasn't the cause of the rough idle so I'll go back to the distributor side of things.

Thanks for your write up and suggestions. Much appreciated.