KOEO Code 18? Jrichker...?

Playing around today and pulled codes just for fun. Car runs perfectly fine, but out of curiosity I wanted to see what the EEC-IV was up to. Aside from a few smog related codes (I've removed all my EGR/Thermactor stuff), I ended up with a code 18, which I understand has something to do with the SPOUT. Following this thread:

http://www.stangnet.com/mustang-forums/793738-help-w-codes-please.html

I'm struggling to understand a few things...

1. PIN 38 does not have a wire running to it on my car. The yellow/lt green wire is running to PIN 36, and when I measure resistance from it to ground, I get roughly 6000 ohms, which I assume is acceptable...?

2. The thread above says I need to check resistance from pin 38 (36?) to the "computer side" of the SPOUT connector. How do I tell which side is the computer side?

Thanks in advance to anyone who wants to help!

Update:

Tested resistance to both terminals on the SPOUT, and on one side I see an "infinite" amount of resistance, the other I'm getting ~0.3 ohms. What does this mean?

So, do my measurements sound acceptable? From what I've seen online it looks like the TFI Module could be the culprit, which makes sense because the car still has the original one and it is pushing 90K miles.

jrichker said:

The reference to pin 38 was a typo and my fault. I hope that didn't cause too many problems.

If after disconnecting the computer wiring connector you get .3 Ohms from the SPOUT connector wiring to the SPOUT wiring on the computer connector (Pin 36), that is good. The infinite open circuit between pin 36 and ground with the SPOUT out is also good.

If all the resistance checks are good, the only things left are the TFI or computer. Try the TFI first, since it is easier to work with and cheaper to replace.

Click to expand...

Thanks, man, I'll probably start with the TFI thing tomorrow. Just found out I need that special TFI wrench.

Thanks again.

Alright, update:

Pulled the TFI module out and tested the terminals' resistance (in ohms) against a chart of expected values. The numbers seemed to check out fine, and I even took a new TFI module and compared the values against that, and everything was within ~1%.

So I plugged everything back into the car, and disconnected the battery to clear the codes (at the advice of a reputable source), just in case the code 18 was a lingering code from the past that just needed to be cleared. Anyway, the next day I hooked the battery back up, drove it no less than 4 times through full operating cycles, re-ran codes and the 18 was still there.

Well then yesterday, I took the TFI module into Auto Zone and had them run it on their tester; it appears to be operating fine.

Sooooo... Where do I go from here?

Alright guys, update. I fixed the problem. Here is a run down for anybody in the future who has the same issue.

This is my understanding of a code 18 failure: When the engine is running (or attempting to start) the ECM sends a signal to the coil to fire, according to it's programmed ignition advance tables. Not only does it send a signal, but it also expects a signal back, verifying that the coil discharged when it was supposed to. If it doesn't see a signal back (and/or possibly a signal at the incorrect time, I don't know) it throws a code 18.

If there is a fault in the system itself (distributor, coil, TFI, etc), the car will run poorly or not run at all, accompanied by the code 18. If there is a fault just simply in the wiring that carries the return signal to the ECM, the car will run fine, but the computer will still show a code 18. The latter turned out to be my problem. Strange, but true.

Below is a diagram I sourced from fordfuelinjection.com



In the diagram, you can see pin "5" on the TFI connector splits into three paths; one to the tach, one to the coil negative, and one to the ECM pin "4"- the IDM, or Ignition Diagnostic Module. Notice it goes through a big, fat 22,000 ohm resistor. That's a fairly big resistor. Long story short, mine was completely fried. With that resistor burnt out and allowing no current through, the ECM wasn't seeing a return signal from the coil, and so assumed the worst, and threw a code 18. Here is a picture of that resistor:



It's pretty hard to miss. Anyway, as far as I know, Ford didn't foresee this thing ever going bad, as it is a permanent part or the wiring harness. So, replacing it means cutting it out and splicing in a new one. I went to the local electronics store, and picked up one of these:



It's a new, basic, no frills 22K ohm resistor. It's a $2 part. Try fixing a diagnostic code for less than that! I went with the 2W variety, but I'm no electrical engineer, so I can't say what the effect of going bigger or smaller would be. What little I know about wiring tells me that you probably don't want to go too small, and going bigger probably couldn't hurt (in terms of wattage. Going with a different resistance value than the original 22K ohm risks burning up wiring or even the computer). Replacing the resistor did the trick- no more code 18.

Anyway, sorry for the long winded post, but I thought I'd share my findings for the next poor guy who has the same problem. I found surprisingly little info on this subject on the internet, which tells me that it is either very uncommon, or people just convert to carb when they don't have the patience (or cognitive capacity) to actually figure it out.

No problem, I aim to please. I hope it helps someone in the future.

Also, to add to the above, I believe the resistors in the later cars are physically smaller and hidden in shrink tubing. I do not know what year exactly the resistor changed.