Musical Cavities and Round Connector Shells

By Richard McCuistian

Sometimes those who previously attempted repairs can throw really interesting bugs at us.

The Work Order:

1996 Grand Cherokee Laredo

4X4

4.0 Liter

Mileage: 87001

“Check Engine Light On. Both Oxygen Sensors have been replaced”

 
A Fresh Perspective

    Twenty years ago I worked as a shop foreman in the maintenance department at an offshore services company in Southeast Texas.  There was a fellow who worked in my department named Brian, and he was down at the company airfield doing a routine oil change on a Kubota tractor when he noticed some helicopter mechanics attempting to put a skid back on a chopper that belonged to our company. 

Here were two guys who were highly trained and highly motivated who simply couldn’t figure out how to get the skid installed so that all the boltholes would line up right.  Brian had no training, but from his vantage point over by the tractor, he watched them struggle until he figured out their problem.  He walked over to the crippled aircraft, had one of them hold the skid in a certain position, then had the other technician shove a bolt through a particular hole.  When the other end of the skid was raised, the holes all lined up perfectly. 

In spite of the fact that my man Brian had no Airframe and Powerplant training, he had offered them a fresh perspective, and, being the wise fellows that they were (coupled with the fact that they were totally exasperated), they welcomed his input. Sometimes a simple problem that bumfuddles a knowledgeable technician can be pretty simple to figure out when a fresh perspective is offered.
   

The dealership service department where I worked before I became a college instructor still welcomes me when I drop by, and I try to help them out while I’m there.
     Don’t get me wrong.  It’s not that I’m so much smarter than they are; the guys who are holding down my end of the service area in that department are very good at what they do.  But a fresh perspective on a tough problem can be particularly instructive, even if the guy who’s offering it isn’t a rocket scientist.  In this case, that guy would be me.

When Jeep Got “Chryslerized”

    Until 1991, Jeeps had no Malfunction Indicator Light.  How they got away with that is beyond me. But when Chrysler acquired Jeep and started building Jeeps, it didn’t take long for the old Bendix brainbox to find itself discarded in favor of a Chrysler SBEC, or “Single Board Engine Controller” and from that point on, Jeeps had an MIL like most everybody else.  

Since the advent of Jeep’s 4.0 Liter powerplant back in 1987, the Bendix system had used weird oxygen sensors that operated in a zero to five volt range with five volts indicating a lean exhaust and zero volts indicating a rich exhaust.  This sensor received a five-volt feed at all times and had the equivalent of an internal thermistor  that reacted to the temperature of the exhaust stream.  A rich exhaust is cool, while a lean exhaust is hot.  These sensors weren’t quite as reliable as the ceramic zirconia sensors used by most other manufacturers.


Same Five Volts, Different Job

    The 1996 Jeep OBDII Controller sends five volts to the sensor like the old Bendix unit did, but for a different reason.  This sensor operates in the zero to one-volt range most of us are accustomed to.

When the engine is first started on a late model Jeep, if you happen to be looking at scan tool values for the oxygen sensors (HO2S’s) you’ll see a voltage right at or just below five volts.  As the internal heater (controlled by the Powertrain Control Module) raises the temperature of the sensor, this voltage drops to within the 0-1 volt range.  As the voltage threshold drops into this range, the PCM closes the fuel feedback loop and begins operating on closed loop strategy.
   

The Mopar men were pretty wise when they put this strategy in place. If the sensor doesn’t heat up, the voltage stays high, and the PCM remains in open loop.  If the sensor wire has been disconnected or cut somehow, the voltage stays at or near five volts and the PCM remains in open loop.  The tricky thing about Chrysler Oxygen Sensors is that they don’t fail the way most other ceramic zirconia sensors do.  Instead of sending a low voltage (lean) signal when they fail, they sometimes fail at a voltage near 0.5 volts.  This can cause all sorts of strange phenomena, particularly if the sensor is partially operational but sometimes locks in at about 0.6 volts.  We discussed that in another article, but you might want to watch for it.  It can cause a serious buck/jerk concern at highway speeds as the PCM attempts to adjust fuel trim to compensate for what it sees as a rich signal that won’t respond.


Ryan’s Problem

    Ryan, my old trainee, drew the ’96 Grand Cherokee you see described above on the Work Order. The DRBIII scan tool is big on verbally defining the codes it spits out, and in this case, the ’96 Jeep tossed an “O2 Sensor Shorted to Voltage” code.  Pulling the Scan Tool PIDs up for both the front and rear oxygen sensors, (see figure 1)

Figure 1

Using wire colors from the schematic, Ryan had gone through a wire-by wire check of the harness and found that all the wires had continuity from origin to destination.   It wasn’t long afterwards that I showed up, and he called me over to have a look. 

The “shorted to voltage” message wouldn’t go away, and he had even tried a substitute Powertrain Control Module, only to find that it popped the same message on the screen.  As I examined the connector, I noticed that one of the metal terminals was visible at the rear of the harness connector, like it had been pushed back and wasn’t making contact with its mate in the other part of the shell.

“Look, Ryan,” I said, showing him the terminal.
 

“Oh, yeah,” he said, “I’ve noticed that, and I may need a new connector shell, but whether that wire is shoved in or not makes no difference.”  I carefully pulled on the other three wires to see how they felt, and they were all solidly anchored.

“What about the rest of the wires?  Are they in the right cavities?” 

“I checked ‘em according to the pinout in the MDS2,” he said, and they seem to be in the right place.”  We walked over to the Mopar Diagnostic System and pulled up the pinout  (see Figure 2).

Figure 2

Figure 2a

These pinout schematics can be confusing at best if a technician isn’t sure which connector he or she is looking into.  One element that contributes to this confusion is the manual publishers’ attempt at drawing the connector to match the real-world shape code. While this is a good idea in principle, it’s sometimes hard to determine what each notch and square on the illustration represents (See Figures 2, 2a and 3).

Figure 3

Correctly assuming the schematic to represent the harness connector and not the oxygen sensor connector, Ryan peered into the connector, lined up the shape codes with the illustration as best he could, and checked the wire colors. It appeared that the wires were inserted correctly. The Dark Green/Orange wire (DG/O) is a 12-volt feed from the Automatic Shutdown Relay. 

The purpose of that particular 12 volt feed is to provide power to the internal heater.  The Black/Tan wire comes from the Powertrain Control Module and provides ground when the PCM decides to power up the heater in the sensor.  Black/Orange and Black/Light Blue wires are sensor signal and sensor ground respectively.

Figure 4

Well, when I examined the wires myself, they appeared to match the drawing, but then I noticed the numbers on the back of the Oxygen Sensor connector (see Figure 5) and we discovered what had gone wrong. 

A wiring guru will use the numbers on a connector with 16, 40, 60, or 104 pins, but who’d think we were going to have problems with a 4-pin connector?  Duh!  We both felt silly about the SNAFU. There were also numbers on the harness connector, and we had failed to use those as well.

Figure 5

The Numbers Game

I mentally discarded the shape of the connector in the MDS2 illustration. Then I decided to use the connector shell of the Oxygen Sensor Ryan had removed for a tool.  In this way, it would be easy to check the oxygen sensor pinout on future vehicles with the same configuration. I labeled that connector according to the numbers on the MDS2 connector pinout screen. (see figure 6). The wires on the Oxygen Sensor pigtail don’t follow the same color code that the harness connector uses, but what we do find seems to be a standard for many Oxygen sensors. 

The two white wires on the sensor provide power and ground to the sensor heater, while the gray wire is sensor ground and the black wire is sensor signal.  (Note:A sharp tech will wire a high impedance light bulb in between the two white wires on an old Oxygen Sensor Connector pigtail and use it to check the Oxygen Sensor Heater circuits.  When plugged in, it’ll fire up the bulb when the heater is energized by the PCM.  It’s a really useful check when you’re chasing OBDII  Ox Sensor heater circuit codes!)

(Note:A sharp tech will wire a high impedance light bulb in between the two white wires on an old Oxygen Sensor Connector pigtail and use it to check the Oxygen Sensor Heater circuits.  When plugged in, it’ll fire up the bulb when the heater is energized by the PCM.  It’s a really useful check when you’re chasing OBDII  Ox Sensor heater circuit codes!)

Figure 6

When we plugged this connector into the harness and compared the color code of each wire to the connector label, we found that the wires had been swapped around by somebody who had fallen in the same trap we did.  They apparently looked at the shape code instead of using the numbers and put the wires back in the wrong order. 

How or why the wires had all been removed from the connector in the first place remains a mystery, at least for Ryan and me.  

It was time to take the connector apart and fix the wires right. Some of the connector terminals Chrysler uses are designed to be disassembled with those strange-looking Thomas-Betts tools, but for an adventurous soul, and if the connector doesn’t have too many wires, the connector can be disassembled for the purpose of playing musical terminals and whatnot (see Figure 7).

Figure 7

At this point, it was a small matter to straighten out the wire terminals, which we proceeded to do.

    With the connector reassembled and plugged in, the DRBIII reading started at the familiar level on initial startup and moved into the closed loop range after a few moments of engine burn.

    When a self-reliant mechanic (like me) burrows deep into a forest without seeing the trees, it might be a good idea to find a knowledgeable somebody with a fresh perspective on things. I never was too prone to ask others for help, and there were times when it cost me a lot of extra work. And while it might hurt our pride a bit to realize that it doesn’t take a rocket scientist to put a helicopter skid in place, there are times when we may need to accept the fact that it might just take another perspective from a different pair of eyes

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