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TSB 94-11-23 PSOM Programming by DIP Switch
Publication Date: JUNE 1, 1994

MEDIUM/HEAVY TRUCK: 1987-95 C SERIES, CARGO SERIES, F & B SERIES, L SERIES

ISSUE: The field has requested a simplified calibration procedure for electronic speedometers/odometers.

ACTION: Use the following TSB article to service all electronic speedometer/odometer calibration concerns. This TSB article supersedes both TSB 90-24-13 and TSB 93-25-16. Refer to the following procedure for service details.

SERVICE PROCEDURE
The transmission-mounted speed sensor transmits a signal to the speedometer/odometer which is dependent on:
* Rear axle ratio
* Tire revolutions per mile (RPMi) based on tire height.

REAR AXLE RATIO DETERMINATION
The axle ratio can be determined by referring to the information delivered with the vehicle or finding the metal tag attached to the differential. If the vehicle has a two speed differential, (e.g. 4.10/5.72:1), use the smaller number (4.10 in this case) in all calculations.

TIRE REVOLUTIONS PER MILE DETERMINATION
The revolutions per mile can be determined as follows:
1. Measure the tire height (ground to top of tire) in inches with the vehicle on level ground, loaded with the tires "cold" and inflated to the recommended pressure . Measure to the nearest 1/8".
2. Use the following table to convert "eighths" to decimals. If calculation is done in centimeters, divide tire height by 2.54 to convert to inches.
EIGHTHS/DECIMALS CONVERSION CHART
EIGHTHS DECIMAL EQUIVALENT
1/8 .125
1/4 .250
3/8 .375
1/2 .500
5/8 .625
3/4 .750
7/8 .875
(Example: If tire height is 36-1/8", use 36.125" in your revolutions per mile calculations.)
3. Divide 20,168 (constant) by the tire height to get revolutions per mile. This is RPMi (Example: 20,168 / 36.125 = 558.28 ).

CALIBRATION FREQUENCY DETERMINATION
Multiply the axle ratio times the revolutions per mile, times 0.267 (constant) (Example: 558.28 x 4.10 x 0.267 = 611.49). Round the 611.49 to the nearest whole number, which is 611 (on Ametek heads round to the nearest even whole number).

DETERMINE SPEEDOMETER/ODOMETER SWITCH SETTINGS
1. Determine which type speedometer/odometer is being installed. The following procedure applies for both "stand-alone" and gauge "cluster" type speedometer/odometer units. (Replacement units are either the 12-switch Beede or the 10-switch Ametek type. The 10-switch Beede has been discontinued for service.)
2. If the unit to be calibrated is a 10-switch type, determine if it is an Ametek or a Beede by looking at the type of electrical connection on the back. The Beede has four (4) "studs", while the Ametek has a single plug-in type connector. If a 10-switch Beede is replaced with a 10-switch Ametek, a simple electrical jumper wire will be included with the new speedometer.
3. There are three speedometer setting charts: Ametek 10-position, Figure 1; Beede 12-position, Figures 2-5; and Beede 10-position, Figure 6.
4. Using the correct chart, find the "test-freq" in the left hand column, and read the switch settings in the next column. A number means that switch is to be placed in the "ON" or closed position, and a period means that the switch is to be placed in the "OFF" or open position. (Example: Using the Beede 12-position chart and the 611 test frequency calculated in the example, the chart gives switch settings as: 1.3456.8.0.B). This means that switches 1,3,4,5,6,8,10 and 12 are "ON", and switches 2,7,9 and 11 are "OFF".
a. If the speedometer has 12 switches, the letters "A" and "B" are used to represent switches 11 and 12.
b. There are at least three different styles of switches with unique switch position labeling. Ignore the switch position labeling. Pushing the top of the switch in turns the switch "ON". Pushing the bottom of the switch in turns the switch "OFF".
5. Figure 7 is a "picture" of a 12-position speedometer/odometer switch with switches 1,3,4,5,6,8,10 and 12 "ON", and switches 2, 7, 9 and 11 "OFF".
NOTE: THE SWITCH SETTING IS THE SAME FOR MILES PER HOUR OR KILOMETERS PER HOUR.

OTHER APPLICABLE ARTICLES: NONE
SUPERSEDES: 90-24-13, 93-25-16
WARRANTY STATUS: INFORMATION ONLY

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