The Fun Continues... (please Help With Code 18)

[5.0 CJ-7]

Code 18: loss of tachometer input / IDM circuit failure / SPOUT circuit grounded


I need some help with this one.

It has been a few months since I last posted here. I had parked my vehicle for a while to focus on some other projects, but now I hopefully have the distributor issues resolved. Unfortunately this new code that I have never previously seen appeared out of nowhere.

My observations so far:

-- randomly feels sluggish and/or jerky while driving
-- randomly idles rough
-- the engine will randomly shut off regardless of being at idle while stopped or driving at speed
-- often there is a noticeable loss of power before the engine shuts off
-- sometimes there is a backfire or two before the engine shuts off
-- the engine can always be immediately restarted after shutting off
-- the key must be turned all the way to the off position before restarting the engine
-- if the key is turned from the run position, the engine will not restart

I can tell you that the SPOUT looks clean and should be making good contact. The tachometer works, and I have not noticed any loss of input to it.

 

[TuneUpsNSuch]

Inspect the 22k ohm resistor on the ignition module harness. When this resistor goes bad you have loss of IDM (Ignition Diagnostic Monitor). Let us know.

 

[jrichker]

Code 18 - SPOUT out or wiring fault - look for short to ground in SPOUT wiring going back to the computer. Possible bad TFI or defective 22 K resistor in the IDM wiring

Use a timing light to check the timing: remove the SPOUT and observe that the timing retards at least 4 degrees. Put the SPOUT back in place and observe that the spark advances at least 4 degrees.
This code can disable spark advance and reduce power and fuel economy.

Remove the passenger side kick panel and disconnect the computer connector. There is a 10 MM bolt that holds it in place. Disconnect the TFI module connector from the TFI and the measure the resistance between the yellow/lt green wire and ground. You should see greater than 100 K (100000) ohms.
Check the resistance from Pin 4 on the computer connector (dark green/yellow) and the dark green/yellow wire on the TFI connector. You should see 20-24 K Ohms (20,000-24,0000 ohms). The resistor is located in the wiring harness about 6” from the connector. You will need solder and heat shrink to replace the resistor if it is bad.
Next measure the resistance between the yellow/lt green wire on the TFI module connector and Pin 36 on the computer connector. With the SPOUT plug in place, you should see less than 2 ohms.

The following is a view from the computer side of the computer connector.

This diagram is the wire side of the computer connector.

Diagram courtesy of Tmoss & Stang&2birds