Yo Johnny,
In which year Bronco or Ford truck is is this happening?
TSB 89-02-07 Speedometer Accuracy
Publication Date: JANUARY 25, 1989
FORD: 1989 and prior ALL CAR LINES
LINCOLN-MERCURY: 1989 and prior ALL CAR LINES
LIGHT TRUCK: 1989 and prior ALL TRUCK LINES
MEDIUM/HEAVY TRUCK: 1989 and prior ALL MEDIUM/HEAVY TRUCK LINES
ISSUE: The accuracy of speedometer/odometer readings may be influenced by several vehicle components or systems. The information in this TSB article is intended to assist technicians in speedometer/odometer concern diagnosis.
ACTION: Use the following supplemental information to assist in speedometer/odometer diagnostics.
OPERATION: A mechanical analog speedometer displays vehicle speed and the odometer displays total distance traveled. The speedometer/odometer assembly is cable driven by either a transmission or a transaxle. All speedometer/odometer assemblies, except for police vehicles are the same with respect to the speed accuracy tolerance used during calibration. The odometer gear ratio is fixed so that all are identical and have no error in the speedometer head.
Electronic digital operation is similar. It could use a drive cable or a speed sensor to drive the speedometer/odometer. An electronic signal is sent from a speed sensor to the digital speedometer/odometer assembly. The speed sensor is driven by a transmission or a transaxle, similar to a cable.
Several areas of concern that may affect speedometer/odometer readings are tires, axle gear ratio and speedometer/odometer drive and driven gears.
TIRES: Improper tire rolling radius and inflation pressure, temperature and size may contribute to inaccurate system readings. System accuracy testing should be performed after the tires are set at the correct pressure as shown on the safety compliance certification label. The tire should be warmed for a short period. Best results are obtained on smooth, dry pavement while driving at a constant speed within the posted speed limit.
AXLE/TRANSAXLE RATIO: The gear ratio of the rear axle or the final drive ratio of the transaxle must be known to select or check if the proper speedometer/odometer drive and driven gears are present. Various gear ratios are available, but usually are not a concern when dealing with speedometer/odometer concerns unless the gear ratio has been changed.
WARNING: NEVER CORRECT SPEEDOMETER READINGS BY CHANGING GEARS UNLESS THE ODOMETER IS ALSO OFF.
DRIVE/DRIVEN GEARS: The speedometer/odometer drive gear is located inside the transmission, transaxle or transfer case and is not easily accessed for change. The driven gear rotates the speedometer cable. Rear wheel drive vehicles have several driven gears with various numbers of teeth available to correct input to the speedometer/odometer head. Front wheel drive vehicles generally do not offer different gears for correction.
GENERAL DESCRIPTION: The maximum allowable odometer system accuracy error is � 3.75% of the actual distance traveled. Ford Motor vehicles are well within those limits.
The speed indication is biased high, except on police vehicles with certified calibration speedometers/odometers. As a general rule, the indicated speed is equal to or greater than the actual speed. This is intended to protect the consumer against violating speed laws. Most customer concerns are related to speedometers reading too high at true speeds between 50 MPH and 65 MPH (80 - 105 Km/h). At that speed range, the worst case errors may indicate a speed that is 10% greater than true speed.
The speedometer head is an instrument which processes information sent to it by the rotating speedometer cable. If the system components send the wrong number of revolution per mile to the speedometer head, an inaccurate speed reading and amount of distanced traveled will be displayed. Since there is no error in the fixed gear ratio of the speedometer head odometer, start by checking the accuracy of the odometer even if the customer concern indicates a speed accuracy problem. Odometer accuracy can be checked by using roads established at mile increments or a known local course. If roads with mile markers are used, a five mile stretch is recommended to allow for inaccuracies. If an error is greater than 3.75%, a change to the transmission drive/driven gear selection, tire size, or tire inflation may need attention. The odometer should be checked again to verify any corrective action. If the indicated speed error exceeds 10% between 50 MPH and 60 MPH (80 - 105 Km/h), replace the speedometer/odometer assembly. Vehicles with transfer cases that have fluctuating readings may be due to slippage of drive gears, parts not splined or loose yoke nuts.
If the vehicle has speed control, the speed accuracy can be checked using the verified odometer vs. time. The formula is as follows:
3600 divded by TIME (seconds to cover one mile) = TRUE MPH(Km/h)
EXAMPLES:
60 MPH (96 Km/h) requires 60 seconds to cover one mile
55 MPH (88 Km/h) requires 65 and 3/4 seconds to cover one mile
50 MPH (80 Km/h) requires 72 seconds to cover one mile
SUPERSEDES: 84-14-06
WARRANTY STATUS: INFORMATION ONLY
