Driveability Corner

Volume 3 Number 3

Vehicle: ’93 Ford T-Bird 3.8LSC   
Ignition: EI Low Data Rate
Odometer: 89,600 miles
Customer complaint: “Check Engine Light on, runs rough.”

Every time I work on one of these 3.8L SC (Super Coupe) T-bird’s, I know there will be bloodshed as it has to be one of the tightest packages around due to the low hood line of this car, coupled with a blower under the hood.

With that said, my first step was to see how the thing ran.  KOER at idle, it actually ran OK, but under load it had a dead miss.  A DTC 217 was set which is a Coil 3 circuit failure.  This DTC is obviously related to this driveability problem. 

A little ignition background for this vehicle is in order.  Ford calls this system a Low Data Rate System (LDR) because the crankshaft position sensor (CKP) generates only 3 signals per crank revolution which does not offer a great deal of detail in comparison to High Data Rate Systems (HDR) which generate 35 signals per revolution.

The CKP is a 4-wire sensor: B+, ground, signal to the Ignition Control Module (ICM), and signal to the PCM.  See figure 1 for the wiring diagram.  The CKP signal sent to the ICM is for ignition control and the signal that is sent to the PCM is an RPM reference.  A Camshaft Position Sensor (CMP) or Cylinder ID (CID) is used on this system for starting and sequential fuel injection.  It is also a 4-wire sensor similar to the CKP as it generates 2 signals.  One CMP signal is sent to the ICM for coil firing sequence and one is sent to the PCM for sequential injection.

The secondary ignition is pretty straightforward, as it’s a waste spark ignition system utilizing 1 coil for 2 cylinders for a total of 3 coils. See figure 2.

OK, back to the ‘Bird.  I looked up the flow chart for the DTC 217 in the shop manual and to be brutally honest, it was terrible.  Lots of “connect special tool such and such” and “check resistance of this and that”.  No thanks.  Give me a solid system operation and a good wiring diagram and I’m off to the races!  Ford can throw away the flow chart for this DTC as far as I’m concerned…

Based on the DTC that set, there is a primary coil circuit failure.  What’s involved in the primary circuit?   An ICM which drives the coil on and off, the coil primary windings and the related wiring.  That’s about it.  With that said, I connected my low amp current probe to the coil B+ feed at the coil pack and saw this.  Each coil “ramp” indicates current flow through a coil primary circuit.  Looking at this waveform, it’s pretty obvious that a coil event or ramp is missing.  OK, I’m into this thing about 4 minutes and look what I already know.   Next, I connected channel 2 from the scope to the coil driver circuit for coil 3, which is pin 9 at the ICM.  This is what I saw.

I have a question for you.  Based on figure 4, how’s the wiring from the coil to the ICM?  If you said good, pat yourself on the back ‘cause you’re the man!  Why?  If you look back at figure 2, you’ll notice that if I have B+ at the ICM pin 9 (which I did), then the wiring MUST be good to the coil, through the primary windings and all the way to the ICM.  Do I need to check resistance from the coil to the ICM driver circuit?  I don’t think so.

	Then what’s the problem?  It has to be the ICM because it’s not driving the coil primary to ground like it should.  If you look back at figure 4, you’ll see that the driver circuit voltage is staying “high” at open circuit voltage.  The fix?  A new ICM.   Figure 5 is the waveform after the ICM was replaced.  Note how the ICM is now properly driving the coil primary on and off.

Look for more articles in the coming weeks!

Pete Vernazza

Advanced Engine Performance Instructor
petev@attbi.com