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?

  • did you do the flashing light trick off the dash to get this code? Jan 21, 2016 at 23:53
  • I don't see your code 27 lining up with what you say it does. I think you should get your vehicle scanned for free by an auto parts store and bring the ODBII code back here and post it. It will be more defined and accurate and perhaps tell you exactly what's wrong. Jan 22, 2016 at 18:20
  • 94's are OBD1 you either need a factory scan tool or do the light flashing.
    – Ben
    Jan 22, 2016 at 23:19

2 Answers 2


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.

If you have a DSO this is a sample waveform for a working injector. enter image description here

In this case you compare waveforms and if one is off than that's the one you concentrate on.

  • thanks I think I found the problem-bad connection at a splice
    – Bob D
    Jan 24, 2016 at 1:34
  • so I checked the voltage at each injector. I have 7.1 volts at every injector. I traced the wire back to the ASD relay. Ignition switch on I have battery voltage on all pins except the one going to the injectors 7.1. Installed a new relay still no change. I stuck my voltmeter probe up in where the wire connects and moved it around with no change.How can I make this a better connection?
    – Bob D
    Feb 17, 2016 at 2:28
  • @BobD What about at the splice? The voltage drop is happening somewhere between the ASD relay and the injector splice. If all else fails you could tap into a portion of the wire where you have battery voltage and create a new splice. And in turn run new wires to each injector.
    – Ben
    Feb 17, 2016 at 22:59
  • I do not have battery voltage at the relay. I pulled the relay and checked the voltage at all four pins. I get battery voltage at the three I'm supposed to get it and I get 7.0 where there should be no voltage. If I am correct in assuming there should be no voltage at the pin going to the injectors There must be a short somewhere that the 7 volts is coming from. Am I correct?
    – Bob D
    Feb 18, 2016 at 16:39
  • well live and learn. My engine has to be running for the relay to operate. With rpms I have battery voltage and am running smooth. I appreciate your info Ben.
    – Bob D
    Feb 18, 2016 at 20:43

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.

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