recurring code 18

[holyford86]

FIrst off the truck is an 86 with an 88 wiring harness and 90 computer (its a mutt, I know) I have been having an issue with a recurring CM code 18. I have been having issues with the truck hanging at 2500 rpm when the clutch is depressed when shifting. TPS baseline voltage is .841 volts and IAC is functioning correctly. I pulled codes today and I got KOEO 11 with the code 18 CM. And I also got KOER 11. Would a tach circut irregularity cause the running high between shifts, a high idle (1200 RPM), and occasional heavy pinging (base timing set at 10* spout removed)? Would relocating the TFI Module and not extending the shielding cause this problem? What about the IDM resistor?

 

[MAKG]

It's not that unusual for that IDM resistor to break leads off and flip IDM codes. I'd suggest a visual. It generally lives behind the PS pump inside the main harness. The stock one looks like a big brown lump. You can replace it with a carbon resistor from Radio Shack (get the highest power you can). If I remember right, it's 22 kOhm.

Perhaps not shielding a modified ignition module could be an issue. If the PCM is getting extra IDM signals, it could get very confused.

 

[holyford86]

the harnesses I extended are not tied in with any other harnesses for the lack of shielding. I would like to add it in but i am unsure of the best way to do it. My old harness was missing the IDM resistor and the truck still ran (badly) Does anyone know what kind of wattage runs through that resistor? I'm not sure how to test the circuit for current draw to find out.

 

[MAKG]

It's a 12V system with a 22K resistor.

Tiny power. The reason for the big watts is engine heat. And the fact that they are still VERY cheap.

Just solder and heat shrink a replacement. Always use a heat sink (longnose pliers and rubber band) on resistors.

 

[holyford86]

Well, I chopped out the resistor and ohm checked it, the wires to it were not broken and looked alright, the resistor checked out at ~1.3M ohms, a little high I'm guessing. I soldered in a 1/2 watt 22k ohm resistor from radio shack and was rewarded with better throttle response and absolutely no pinging, it still hangs at 2500 RPM but not for nearly as long, and it runs a load better all around. I cut the factory resistor apart and was amazed at how crappy those things are, 22k resistor with a 20% tolerance inside that rubber blob, with wires crimped onto it. Would having that code stored cause the engine to run funny even though the problem was fixed? I'm going to plug into the MODIS at work and see what I get tomorrow.

 

[MAKG]

If you have a MODIS at hand, you'll find the problem. Lucky SOB.

I supposed the fuel trims might have gotten FUBARed. Nothing a soft-reset won't fix.