I own a '94 jeep cherokee 4.0 liter. The 27 code indicates the injector circuit is not syncing with the input signal. How do I trouble shoot this?
You need either a digital storage oscilloscope or a noid light to test signal from the PCM to the injectors. If you don't have a DSO you will also need a multi-meter.
You want to disconnect an injector, check the connector for fit and check the pins for corrosion etc...
Injectors have 2 wires. Shared power and PCM ground. Power will always be the same color/pin on the injector connector. The power wire should be battery voltage if there is < battery voltage you need to check the wire from the source in this case the ASD relay. If you have battery voltage at the ASD relay there is voltage drop. In which case you would have to replace the wire to the point where it failed, usually a splice.
Injector signal/ground is provided by the PCM. If you have power but no ground from the PCM you need to check the wiring. If the wiring passes all tests than it may be a bad injector driver in the PCM.
If you have both power and ground it's probably a safe bet to replace the injector that failed.
In this case you compare waveforms and if one is off than that's the one you concentrate on.
This could be any number of things, a bad injector, broken wire, or a loose or corroded connector.
I'd start by checking the connections to all the injectors. Make sure there's no crud in the connector and that it's making good contact.
You didn't mention whether there are any other symptoms beyond the error code.