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On 1993-'97 Chrysler, LH Body:
Posted 11/13/2000
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“On the Jeep truck engine controller (1996 and newer) this 5-volt reference is also used to run the heater test.”
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Vehicles:
1993 through 1997 Chrysler, LH body.
Engine:
3.3 and 3.5 L.
Symptom/Problem:
5 volts seen on the O2 sensor circuit accompanied by a code 21 (O2 sensor shorted to voltage). The powertrain control module (PCM) has detected an open circuit in the O2 sensor.
Test and Fix:
Measure the voltage on all four wires at the O2 sensor. There should be two ground wires that have no more than 50 mv with the vehicle running and the sensor connected; one wire with battery voltage for the heater, and the signal wire, which should be switching from approximately 0.2 to 0.8 volts. If the signal wire is at 5 volts, there is an open in the sensor and it needs to be replaced.
Background:
In 1990, Chrysler changed the design of its PCM and went to a single board engine controller (SBEC). When this transformation took place, one change noticed was that the O2 sensor went to 5 volts when open-circuited. The purpose of this voltage is to monitor an open circuit or a heater failure. If, when running with the engine coolant temperature sensor at approximately 176 degrees F and the O2 sensor voltage above 1.2 volts for more than 2.5 seconds, a code 21 (O2 sensor shorted to voltage) will set. This is somewhat deceiving since the voltage will go high open circuit or with a heater or ground failure. The voltage is really low current. Normally, when monitoring the circuit if the sensor is disconnected, the voltage will immediately go to 0.5 volts and then will slowly climb to nearly 5 volts either on a scan tool or DVOM. This is a normal condition and not a problem with the PCM.
Note: On the Jeep truck engine controller (1996 and newer) this 5-volt reference is also used to run the heater test. With the engine at idle immediately after a cold start, and with engine coolant temperature sensor less than 147 degrees F and battery temperature sensor within 27 degrees F of the coolant temperature sensor reading, if the O2 sensor voltage stays above 3 volts for 30 to 90 seconds, the heater failure code will set.
 | Tom Vadnais is an Identifix DaimlerChrysler, Mitsubishi and Hyundai specialist. He is ASE master and L1 certified with 29 years of diagnostic and repair experience.. |
On '93 Subaru Imprezza, 1.81 MPFI:
Throttle Body Cooling Off Causes High, Inermittent Idle
Posted 11/13/2000
By Todd Erickson
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“ After several days of driving, a pattern emerged.”
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1993 Subaru Imprezza, 1.81 MPFI.
Symptom:
High, intermittent idle when coming off the highway. Talking to the owner of the car did not reveal a pattern. Checking for codes revealed no problems seen by the powertrain control module (PCM).
The customer did not want the car back until the shop found the cause of the high idle. After several days of driving, a pattern emerged. The engine would idle at normal speeds all day long in the shop. Driving through town did not cause any higher-than-normal idle speeds. And when the technician took it out on the highway, nothing unusual happened for several miles. Then, pulling up to a stop light after about 5 miles of steady cruise, the idle showed 1100 rpm. By the time the technician got the car off on the shoulder of the road, it was at normal idle again.
The technician test drove, test drove and test drove until he could get it to repeat the high idle. Finally he saw more patterns. If he drove more than 5 miles at a steady speed above 50 mph and if the outside temperature was below 40 degrees F, the engine would idle higher than normal when coming to a stop. Within minutes of slowing down or stopping, the idle speed returned to normal.
Here's what was happening: The cold idle compensator for this engine package is a wax pellet-type plunger that physically opens the throttle. By using a temperature probe on the compensator, the technician was able to see that it was cooling down when cruising on the highway. Under certain conditions the throttle body cooled off enough to cause the idle compensator to start pushing the throttle back open. The PCM did not see this as an abnormal condition, so no codes. Flushing the compensator pushed out a piece of plastic wrap that was restricting the coolant flow. Road testing on the next cold day proved the compensator was staying up to operating temperature again.
 | Todd Erickson is an IDENTIFIX Asian specialist. Erickson is an Accredited Automotive Manager (AAM) with 16 years of diagnostic experience. He is a certified Subaru senior master technician, as well as ASE master certified, L1 and alternate fuels. |
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