Speedometer - False Vehicle Speed Reading At A Stop
Bulletin No. T 87-81File in Group 8
Number 54
Corp. Ref. No. 718305
Date March '87
SUBJECT: FALSE VEHICLE SPEED READING AT IDLE IN GEAR
MODELS AFFECTED: 1987 ALLANTE
Some 1987 Allantes may experience an incorrect vehicle speed display on the digital speedometer while at a stop, with the transaxle in gear and engine idling (correct vehicle speed display is zero). The incorrect vehicle speed may be caused by an open or shorted diode in the rectifier bridge of the generator. This condition may not set charging system related BCM Codes B409, B410 or B411 or ECM Code E016. One or all of the following symptoms may also occur when an incorrect vehicle speed is displayed:
^ High Idle - 1200 RPM in Park 900 RPM in Gear
^ Poor performance and/or erratic shifts (on deceleration to a stop, on acceleration from idle, and at actual vehicle speed of less than 3 mph).
Diagnosis
1. Start engine, place foot firmly on brake pedal and place transmission in drive. Note vehicle digital speedometer reading.
2. If the digital speedometer reading is other than 0 mph, enter diagnostics and select BCM override BS06 "Generator Disable". Refer to Section 8D in the 1987 Allante Service Information Manual for the procedure to enter diagnostics and BCM override selection. Depressing and holding the warmer button while in BCM override BS06 will disable the generator. Note digital speedometer reading with the generator disabled and proceed as follows:
FIGURE 1 - DIODE CHECKING PROCEDURE:
^ 0 mph Digital Speedometer Speed Reading - Check for a shorted or open diode in the rectifier bridge. See Figure 1 for diode checking procedure.
^ Other Than 0 mph Digital Speedometer Speed Reading - Refer to Page 8A-33 of the 1987 Allante Service Information Manual, Vehicle Speed Sensor Diagnosis.
FIGURE 1 - DIODE CHECKING PROCEDURE:
TESTING RECTIFIER BRIDGE
^ To check the bridge, all six diodes must be checked individually for opens or shorts.
^ The diode checking procedure is as follows:
Notice: Remove any loose parts from the end frame (such as brush springs) before performing diode check or inaccurate readings may result due to grounding of the insulated heat sink.
- Measure resistance (use diode test setting on DVOM) between diode copper tang (do not separate tangs) and adjacent heat sink and note reading.
- Reverse meter leads (change "direction" that meter is checking) and repeat resistance check.
- A good diode will only show continuity in one direction and an open in the other. A bad diode will show continuity in both directions or an open in both directions.
- If any diode checks bad, replace the bridge.