I still have the code 18 on an 86 F150 4x4 with a 302. Checked the coil and TFI module according to the Haynes manual. Found no problems with either one. So, I spliced in an aftermarket tach since the code says "erratic tach signal." First, I spliced it in to the coil neg.-got accurate rpms here. Then spliced into the TFI tach wire-got accurate rpms here. Last, spliced into the tach wire on pin #4 right before the wire goes into the PCM. Got rpms here that are about 700 revs less than the reading at the coil. Tach needle jumps around also. Is my problem in the wiring/resistor from the coil to the PCM? Any ideas?

 

The book stills says it's a loss of tach signal to the computer. He says check the wire from terminal 2 of the TFI to terminal 36 of the computer(making sure the spout connector is in place). He also says while doing the KOER self-test, after the last code is received, check the timing with a timing light. It should be initial(usually about 10 degrees) plus 20 degrees, plus or minus 3 degrees. He says if the timing is right(the computer is getting a good signal to compute the timing), and you still get a code 18, then the computer may be at fault.

 

I checked the wire from terminal #2 to pin 36 and it checked out OK. I also did the timing check when the rpms went up during the test and it was about 31 degrees. So, that's when I went on to hook up the tach and found the rpms to be way off at the PCM connector. Maybe it is the computer. I cleared all the codes, drove the truck, checked the codes and got 18 AGAIN.

By the way, this is a code 18 in continuous codes(KOEO).

 

Have you been clearing the codes each time? They stay in memory even after you read them...

 

Steve83, Yes I've been clearing the codes every time. Run the truck again, and the code 18 comes back. Could it be the wiring or the in line resistor causing the erratic tach reading at the PCM? I've got good tach readings at the coil, but when I tapped into the tach wire right before it goes into the PCM, I get erratic needle actions on the tach. It also reads about 700 rpms lower here than at the coil.

 

The old TFI (mounted to the distributor) are notorious for going bad for no reason - the class-action lawsuit is still pending (I think). I'd hit the junkyards and collect a few to swap in. If it clears up, I'd buy one with a warranty.

Same for the coil. I collect that kind of part whenever I hit the junkyards just for testing, but I don't have one of those because my truck uses the fender-mounted TFI.

 

Steve, I'll try to locate a couple or three TFI modules and coils to try. Got an uncle-in-law that owns a junkyard, so parts are pretty cheap. I talked to a Ford dealer while buying a clutch and pressure plate for this truck a couple of days ago and he said when the TFI module goes bad, the truck won't run. I know this is not entirely true, because I worked on a Ranger that would run awhile and then decide to quit. Turned out to be TFI module. Dad's truck has never quit, it just runs really rough, bucks on pullout, and gas mileage is horrible. And thanks for the info.

 

Here's one more thing I forgot to mention. Dad called me last night and said when you go around a sharp left hand curve with his truck it starts missing. He said it feels just like when you have a bad plug or plug wire. When you straighten back up, the miss goes away. Going around a sharp left is the only time it does this.