Sometimes those who previously attempted repairs can throw interesting bugs at us.
Twenty years ago, I worked as a shop foreman in the maintenance department of an offshore service company in Southeast Texas. One day, Brian, a fellow in my department, was at the company airfield doing a routine oil change on a Kubota tractor when he noticed some helicopter mechanics struggling to put a skid back on a chopper. Here were two highly trained, highly-motivated guys who simply couldn't figure out how to get the skid installed so that all the boltholes would line up.
Brian had no training, but from his vantage point by the tractor, he figured out their problem. He walked over to the crippled aircraft, had one of them hold the skid in a certain position and 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. He had offered them a fresh perspective, and they welcomed his input. Sometimes a simple problem that befuddles a knowledgeable technician can be simple to figure out when a fresh perspective is offered.
Although I don't work there any more, my old dealership's service department still welcomes me when I drop by, and I try to help them out while I'm there. It's not that I'm so much smarter than they are; the guys who are holding down my end of the service area 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 Mal-function Indicator Light (MIL). How they got away with that is beyond me. But when Chrysler acquired and started building Jeeps, it didn't take long for the old Bendix brainbox to find itself discarded in favor of a Chrysler Single Board Engine Controller (SBEC). From that point on, Jeeps had an MIL like most other vehicles. 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 5-volt range, with 5 volts indicating a lean exhaust and zero volts indicating a rich exhaust. This sensor received a 5-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 5 volts, different jobThe 1996 Jeep OBDII controller sends 5 volts to the sensor like the old Bendix unit did, but for a different reason. This sensor operates in the zero- to 1-volt range to which most technicians are accustomed. The difference is that when the engine is first started on a late model Jeep and you look at scan tool values for the oxygen sensors (HO2S), you'll see a voltage right at or just below 5 volts. As the internal heater, controlled by the Powertrain Control Module (PCM), raises the temperature of the sensor, this voltage drops to within the zero- to 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 5 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 (See Motor Age Garage, May 2001), 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 interprets as a rich signal that won't respond.
Ryan's problemRyan, my former trainee, drew the '96 Grand Cherokee 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, I saw voltages near the 5-volt range, and they wouldn't pull down after the engine was started.
Ryan told me he had the rear oxygen sensor unplugged, so I ignored the voltage reading on that sensor. The front sensor, however, was connected, and the voltage was definitely too high to be normal.
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. The "shorted to voltage" message wouldn't go away. He had even tried a substitute PCM, 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, as if it had been pushed back and wasn't making contact with its mate in the other part of the shell.
I showed Ryan the terminal. "Oh, yeah," he said, "I've noticed that. I may need a new connector shell, but it makes no difference whether that wire is shoved in or not."
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 asked.
"I checked them according to the pinout in the MDS2," he said, "and they seem to be in the right place," Ryan responded. We walked over to the MDS2 (Mopar Diagnostic System) and pulled up the pinout.
Correctly assuming the schematic represented 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 (DG/O) wire 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 PCM and provides ground when the PCM decides to power up the heater in the sensor. The black/orange and black/light blue wires are sensor signal and sensor ground respectively.
When I examined the wires myself, they appeared to match the drawing. Then I noticed the numbers on the back of the oxygen sensor connector, and we discovered what had gone wrong. Any good 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 four-pin connector? Duh! We both felt silly about the oversight. There were also numbers on the harness connector, and we had failed to use those as well.
The numbers gameI 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 instead of simply relying on the shape of the connector.
The wires on the oxygen sensor pigtail don't follow the same color code as the harness connector, but what we did 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.
A sharp tech will wire a high impedance light bulb 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 will fire up the bulb when the heater is energized by the PCM. It's a really useful check when you're chasing OBDII oxygen sensor heater circuit codes.
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 someone who had fallen in the same trap we did. He or she 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. It was time to take the connector apart and fix the wires right. Some of the connector terminals that Chrysler uses are designed to be disassembled with those strange-looking Thomas-Betts tools. However, for an adventurous soul, if the connector doesn't have too many wires, the connector can be disassembled for the purpose of playing musical terminals and causing follow up technicians some stress.
At this point, it was a small matter to straighten out the wire terminals.
Happy endingWith the connector reassembled and plugged in, the DRBIII reading started at the familiar 5-volt level on initial startup and moved into the closed loop range after a few moments of engine burn.
While it's good to be a self-reliant mechanic, 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. 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.