Fuel System - Poor Driveability Upon Start Up

91saab02

CATEGORY
Engine

SECTION PAGE
2 11
ISSUE CODE
03/91-0097 248


Subject: Poor Driveability Upon Start-Up

Application: M88- 900 and 9000 Models with LH 2.4 Fuel Injection


If a car equipped with the LH 2.4 fuel injection system experiences poor driveability immediately after start-up, it may be the result of a faulty LH electronic control unit (ECU). The symptoms generally involve an inability to accelerate or a hesitation when the throttle is opened during the initial 30-60 seconds after starting. The symptoms may occur intermittently and may not be limited to specific ambient temperature ranges, unlike the more consistent behavior one might associate with a "lean calibration."

This type of driveability problem can be caused by the failure of the "hybrid circuit" within the LH ECU. The function of the hybrid circuit is to provide a reference signal for calculating the amount of Lambda regulation based on inputs from the oxygen (02) sensor and engine NTC. Upon start-up with a cold 02 sensor, the nominal voltage measured either at pin 24 of the ECU or at the single-pole connector for the 02 sensor should be 0.5 +/- 0.1 VDC. As the sensor heats up it will generate its own voltage that will be compared against the reference value and the other inputs to calculate the amount of Lambda regulation needed. While under regulation, the voltage measurement at pin 24 should oscillate between the ranges of 0 - 0.4V and 0.6 - 1.0V, indicating lean and rich mixtures respectively.

If the 02 sensor is disconnected from the single-pole connector, the signal on the ECU side of the connector should remain at the baseline of 0.5 +/- 0.1V. This baseline voltage specification is independent of engine temperature. If the voltage is higher than 0.6V, this may be an indication of a faulty hybrid circuit, requiring replacement of the ECU. Cars with bad hybrid circuits may exhibit driveability problems only during the time between start-up and when Lambda regulation takes over. The possibility of this type of fault was eliminated by the supplier effective with ECU date codes of 950 or higher.


Diagnosis





1. Park the car and set the handbrake.

2. Disconnect the single-pole connector for the oxygen sensor in the engine compartment. Connect a digital Volt-Ohm Meter (VOM) from the male terminal to a good ground.

3. Turn the ignition key to the "on" position and read the VOM. If the reading is within specification (0.5 +/- 0.1 VDC), investigate other potential sources of the complaint, such as loose grounds, etc.

4. If the voltage is higher than 0.6 VDC, replace the ECU and verify that the voltage reading is now within specification.


Warranty Information:

For warranty claims to replace the ECU, use Failure Coding 24810-67-0-01-01. Time allowance to perform the above diagnosis and replace the ECU is 0.4 hour (enter Labor Operation No. 24810 twice).