Auto Service World
Feature   January 1, 2000   by Craig Cook

Unraveling a no-start

Things aren't quite what they appear


After a recent conversation with a friend regarding a problem he had with a Ford Aerostar Ignition System, it brought back some memories of a problem I had a few years ago.

A 1992 4.0L Ford Aerostar came into the shop on the hook. The customer’s complaint was that it quit on the highway with no prior indication that there was a problem.

It backfired as I tried to start the vehicle. Sometimes it would start, then stall.

A quick check of the battery, wiring harness connections, and the engine compartment revealed nothing out of place (or so I thought).

Next, fuel pressure and spark were checked. Fuel pressure was a healthy 45 PSI.

The spark was checked at the plug wires on two different coil towers, sometimes there would be a very weak spark, and sometimes no spark at all.

The pulse width at two injectors was then checked using a noid light, with similar test results being achieved as the spark test.

Next, trouble codes were checked in the EEC-IV module. Code 211 was present. This code represents “Profile Ignition Pickup (PIP) circuit failure”.

A brief description of PIP and VRS signals. The EDIS module monitors the input signal from the Variable Reluctance Sensor (the crankshaft position sensor).

The VRS sensor is mounted at the front of the engine.Upon receiving the VRS signal the EDIS module then decodes the signal by counting the teeth on the crankshaft pulley.

There are 36-1 teeth on the crankshaft pulley. The EDIS module looks for the missing tooth. Once the missing tooth has been identified, the module is said to be “synchronized.”

On the 4.0L engine the missing tooth indicates when cylinder No. l is 60 degrees before TDC.

Synchronization is essential to the EDIS module in order to track the angular position of the crankshaft relative to a fixed reference.

The missing tooth is indexed to TDC positioning of a cylinder pair. By identifying the missing tooth, the EDIS module knows when a specific cylinder pair will be at TDC, and when it should fire the proper coil.

The profile ignition pickup signal (PIP) is an output synthesized from the EDIS module using the input from the VRS signal. The PIP signal generated by the EDIS module is then sent to the EEC-IV module.

The PIP signal is used by the EEC IV module to indicate engine speed and crankshaft position.

This is one of the most important signals Ford uses.

If the PIP signal is missing there will be no pulse width, spark and no rpm reading while observing the scan tool. Seeing as I was familiar with the EDIS system that Ford uses on this vehicle I grabbed a wiring diagram and began my diagnosis.

First the power and ground circuits for the EDIS module were checked. The connector and module pins were clean and tight. Voltage drop tests on the power and ground circuits showed them to be okay; 0.09 volts was the maximum reading obtained, acceptable for this system.

A couple of weeks prior to seeing this vehicle I purchased my first lab scope. This was the perfect scenario for putting it to the test. I hooked the scope up to pin 1 at the EDIS Module harness. This is the PIP signal wire that runs from the EDIS module to the EEC- IV Module. See Figure 1 for results.

Figure 1.

As you can see from figure 1, the waveform was breaking down while the vehicle was being cranked.

Suspecting that the VRS Sensor may be faulty, I unplugged the EDIS module and checked the VRS Signal at the Harness. See Figure 2. For results.

Figure 2.

The VRS Signal looked perfect; the peak to peak voltage, shape and reference pulse all appeared to be okay.

Just in case something was missed, the resistance between harness pins 5 & 6 with the VRS sensor still plugged in was checked.

A reading of 1500 Ohms was obtained, acceptable for this system.

A check for continuity to ground was also performed. The wiring harness was then grabbed, and wiggled to simulate vibration. Resistance stayed the same with no apparent drop-outs in the reading.

Next I focused my attention on the EDIS module. It is in my opinion that these EDIS modules are very reliable. I have heard of only a few failures on these units, but have never experienced any.

Having already checked the power and ground circuits for the EDIS module, the PIP signal wire from the EDIS module to the EEC-IV module was checked to make sure it wasn’t grounding out. It appeared to be okay. Lastly the rest of the wires were checked for continuity and shorts to ground. Everything checked out satisfactory. My conclusion up to this point was that the EDIS module wasn’t generating the PIP signal properly.

Seeing as the PIP signal is used by the EEC-IV module to fire the fuel injectors, this would explain the intermittent or no pulse width condition. Having verified that the VRS signal to the EDIS module was good and taking into consideration that the PIP signal was erratic, I called for the EDIS module to be replaced.

Big $$ and a new EDIS module later, the problem was still there.

Time to break out the Tylenol.

After another careful examination of the wiring diagram it suddenly occurred to me, that maybe shield wire was shorting to the VRS signal. But I had already checked the VRS signal several times.

This is where my inexperience let me down. When I was checking the VRS signal at pins 5 & 6,at the ignition module, I had the module unplugged.

Although my lab-scope was used for the diagnosis of the problem, a little more thought and experience using the scope would have saved all these headaches.

If only I had used the scope in dual-trace mode, checking the VRS signal and the PIP signal at the same time, I would have seen the VRS signal dropping out as the shield wire was shorting to the VRS signal wire.

So why wasn’t the problem detected while checking the VRS signals at pins 5 & 6 at the ignition module?

By unplugging the EDIS module to check the VRS signal, I was not grounding the shield wire.

The purpose of the shield wire is to shield the VRS signal wires from EMI (electro-magnetic interference).

On some vehicles the shield wire has an eyelet attached to it, and is bolted to a ground somewhere on the vehicle. When Ford designed the EDIS system they incorporated the shield ground wire right into the main ignition harness plug.

As the EDIS wiring diagram shows, the shield wire runs from the VRS sensor up to the EDIS module plug. Pins 4, 7 and 9 are all connected internally in the module.

Murphy’s Law

The Problem

As it turned out, the wiring harness had come loose from its locating tab, and was intermittently rubbing on the back of the serpentine belt adjuster.

The adjuster had rubbed right through part of the shield wire and part way through one of the VRS sensor signal wires.

As the engine was being turned over the serpentine belt adjuster was flexing and making contact with the damaged harness.

Seeing as the adjuster is metal, it was the perfect conductor.

The pulley would intermittently make a bridge between the damaged shield wire and the damaged VRS sensor wire to ground.

The end result, intermittent PIP signal, intermittent pulse width and intermittent spark.

Conclusion

The shield wire is connected to the EDIS module at pin 7, and pin 7 is internally connected to pins 4 and 9 inside the ignition module.

When checking the VRS signal, the EDIS module plug was disconnected. That was a mistake due to inexperience.

Because pins 7, 4 and 9 are connected internally when the EDIS module is plugged in, one of the VRS signal wires was then being shorted to the shield wire, which is in turn grounded when the serpentine belt pulley would rub on the wiring harness.

The end result, code 211 PIP signal fault.

If I had to do it all over again , there are two things I’d keep in mind

When checking the signals at the EDIS module, keep it plugged in.

I’d use my lab scope on dual trace mode. See figure 3.for test results.

Figure 3 clearly shows the VRS signal breaking down. By using duel trace mode on the Lab Scope, it is easy to see that the EDIS module was not defective.


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