Re-calibrate Your Speedometer without a tuner for any tiresize or gear ratio.
02-09-2017, 10:47 PM
#1
Re-calibrate Your Speedometer without a tuner for any tiresize or gear ratio.
The PSOM:
The PSOM ‘s operation directly affects all shift scheduling! 1992 and later Ford trucks with the VSS located in
the differential are equipped with an internal microprocessor located within the speedometer cluster that uses a
liquid crystal display odometer known as the Programable Speedometer/Odometer Module. The PSOM
receives an analog signal from the VSS (RABS) in the form of AC voltage. This frequency (HERTZ) is
proportionate to road speed and is converted by the PSOM to an 8000 pulse per mile signal, that can be
deciphered by the PCM/TCM and is calibrated for each vehicles tire size and differential gear ratio. The PSOM
can be re-calibrated by the technician should tire size and/or differential gear ratio change. An electrical
overview of the PSOM system can be seen in Figure 101. Using the following diagnostic procedures, replace or
repair the faulty component.
Correction
STEP 1. The first step would be to question the customer to see if their problems began after tires and/or the
differential were changed. The problem may be as simple as over or undersized tires or a changed ratio in the
differential. If this has occurred, the situation may need to be evaluated as to whether it would be best to go back
to OE or remedy the change by reprogramming the PSOM to accommodate the modified ratio. If this is not the
case, road test the vehicle taking note of the speedometer and odometer operation for function and accuracy. If
DATA is available from a scanner, compare the speedometer to that of the scanner. If the scanner reads correct
but the speedometer is erratic and bouncy, the speedometer head is faulty. If both the DATA and speedometer
are bouncy or if DATA is not available for comparison, perform the following checks with the rear wheels off
the ground.
Locate the two wire RAB Sensor test connector (Red with a pink tracer and light green with a black tracer). For
“F” series trucks it is located in the left rear corner of engine compartment (See Figure attached)
STEP 1 (Cont'd). Set your multi-meter to hertz (HZ) and probe the RABS test connector and raise vehicle speed
to 30mph (48km/h). The multi-meter should indicate approximately 667 Hertz. If the hertz reading is correct, go
to Step 2. If the hertz reading is erratic, unplug the sensor in the differential and repeat the test at the sensor. If the
hertz reading continues to be erratic the exciter ring in the differential may be loose or, the speed sensor itself is
faulty. If a steady 667 hertz is seen directly from the sensor at 30mph, the sensor’s connector or its wiring to the
PSOM is faulty and will need to be repaired or replaced.
(EEC-V) processors (1994-95 "F" series Turbo Diesels and 1996 and later vehicles) have a 104 pin connector of
which back probe wires 33 and 58. Wire 33 is gray with a black tracer, and is known as circuit 679. Wire 58 is
pink with an orange tracer, and is known as circuit 676. Raise vehicle speed to 30mph (48km/h), the meter
should indicate approximately 67 Hertz. If the reading is correct, the PCM is faulty. If there is no reading or it is
erratic or the reading is incorrect, the problem could be faulty wiring between the PSOM and the PCM. A faulty
PSOM and/or the number 8 or 18 fuse has blown. To locate the problem, a step by step pin check of the PSOM
circuits
RE-CALIBRATING THE PSOM:
The PSOM requires re-calibration when a tire size or differential gear ratio change has occurred, or the
speedometer was serviced in some way, and/or when a loss of power to the PSOM for lengthy periods of time
has also occurred. Should any one of these occur requiring re-calibration of the PSOM, there are 3 pieces of
mandatory information needed which will be used to acquire a specific number necessary in the re-calibration
procedure called “the conversion constant.” These 3 pieces of information are as follows:
(A) The differential or (axle) capacity (gear ratio).
(B) The tooth count of the speed sensor’s exciter ring on the differential.
(C) The tire size.
The PSOM ‘s operation directly affects all shift scheduling! 1992 and later Ford trucks with the VSS located in
the differential are equipped with an internal microprocessor located within the speedometer cluster that uses a
liquid crystal display odometer known as the Programable Speedometer/Odometer Module. The PSOM
receives an analog signal from the VSS (RABS) in the form of AC voltage. This frequency (HERTZ) is
proportionate to road speed and is converted by the PSOM to an 8000 pulse per mile signal, that can be
deciphered by the PCM/TCM and is calibrated for each vehicles tire size and differential gear ratio. The PSOM
can be re-calibrated by the technician should tire size and/or differential gear ratio change. An electrical
overview of the PSOM system can be seen in Figure 101. Using the following diagnostic procedures, replace or
repair the faulty component.
Correction
STEP 1. The first step would be to question the customer to see if their problems began after tires and/or the
differential were changed. The problem may be as simple as over or undersized tires or a changed ratio in the
differential. If this has occurred, the situation may need to be evaluated as to whether it would be best to go back
to OE or remedy the change by reprogramming the PSOM to accommodate the modified ratio. If this is not the
case, road test the vehicle taking note of the speedometer and odometer operation for function and accuracy. If
DATA is available from a scanner, compare the speedometer to that of the scanner. If the scanner reads correct
but the speedometer is erratic and bouncy, the speedometer head is faulty. If both the DATA and speedometer
are bouncy or if DATA is not available for comparison, perform the following checks with the rear wheels off
the ground.
Locate the two wire RAB Sensor test connector (Red with a pink tracer and light green with a black tracer). For
“F” series trucks it is located in the left rear corner of engine compartment (See Figure attached)
STEP 1 (Cont'd). Set your multi-meter to hertz (HZ) and probe the RABS test connector and raise vehicle speed
to 30mph (48km/h). The multi-meter should indicate approximately 667 Hertz. If the hertz reading is correct, go
to Step 2. If the hertz reading is erratic, unplug the sensor in the differential and repeat the test at the sensor. If the
hertz reading continues to be erratic the exciter ring in the differential may be loose or, the speed sensor itself is
faulty. If a steady 667 hertz is seen directly from the sensor at 30mph, the sensor’s connector or its wiring to the
PSOM is faulty and will need to be repaired or replaced.
(EEC-V) processors (1994-95 "F" series Turbo Diesels and 1996 and later vehicles) have a 104 pin connector of
which back probe wires 33 and 58. Wire 33 is gray with a black tracer, and is known as circuit 679. Wire 58 is
pink with an orange tracer, and is known as circuit 676. Raise vehicle speed to 30mph (48km/h), the meter
should indicate approximately 67 Hertz. If the reading is correct, the PCM is faulty. If there is no reading or it is
erratic or the reading is incorrect, the problem could be faulty wiring between the PSOM and the PCM. A faulty
PSOM and/or the number 8 or 18 fuse has blown. To locate the problem, a step by step pin check of the PSOM
circuits
RE-CALIBRATING THE PSOM:
The PSOM requires re-calibration when a tire size or differential gear ratio change has occurred, or the
speedometer was serviced in some way, and/or when a loss of power to the PSOM for lengthy periods of time
has also occurred. Should any one of these occur requiring re-calibration of the PSOM, there are 3 pieces of
mandatory information needed which will be used to acquire a specific number necessary in the re-calibration
procedure called “the conversion constant.” These 3 pieces of information are as follows:
(A) The differential or (axle) capacity (gear ratio).
(B) The tooth count of the speed sensor’s exciter ring on the differential.
(C) The tire size.
The following users liked this post:
97badass (02-09-2017)
02-09-2017, 10:49 PM
#2
STEP 1 - AXLE RATIO
To find the axle capacity, there is a sticker on the inside door jam that looks like the sticker shown on the top of
Figure 107. Locate at the bottom of the sticker the word “AXLE.” Under that word is a number. The example
given in Figure 107 is the number 29. Also in Figure 107, there is a cross over chart for Bronco and “F” series
trucks, in which 29 equates to a 5300 axle capacity with an axle ratio of 3.55.
STEP 2 - EXCITER RING TOOTH COUNT
Mounted on the differential ring gear is the speed sensor exciter ring (See Figure 2). There are only two different
tooth counts available at the time of this printing; 108 teeth and 120 teeth. The only way to know which one you
have is to physically remove the differential cover and count the teeth. For our example, we have a vehicle with
120 teeth.
STEP 3 - TIRE SIZE
Tire size may generally be obtained in one or two places. On the tire itself or from the sticker on the inside door
jam as seen in Figure 107. The example in Figure 107 reveals a tire size of : LT 215/85R-16D.
STEP 4 - CALCULATING THE CONVERSION CONSTANT
From the above three steps, we have an “F” series truck with an axle capacity of 5300, a speed sensor exciter ring
with a tooth count of 120, and a LT 215/85R-16D tire size. Now look at the chart for “F” series pick-ups &
Broncos in Figure 109. Find the 5300 axle capacity column where directly below you will find the 120 tooth
speed sensor exciter ring listed. Follow the column down until it lines up with the LT-215/85-R16D tire size
column to the left. Intersecting these columns you will find in the intersection the conversion constant number of
9.96. With this number, the re-calibration of the PSOM can begin.
STEP 5 - RE-CALIBRATING THE PSOM
Locate the enable circuit 567 wire connector. This is a light blue wire with a yellow tracer coming from the
PSOM connector pin 9 to a single connector. It is located under the left side of the dash, below the fuse box, near
the bulkhead connector on "E" series vans as seen in Figure 110, and under the center of the dash below the
glove box on "F" series trucks (See Figure 111). NOTE: The letters PSOM should be printed on the enable
connector. Once found, use the following procedures to reprogram the PSOM.
(1) With the ignition in the "OFF" position, ground the enable connector with a jumper wire.
(2) While pushing in on the trip odometer reset button (See Figure 112), turn ignition to the "ON" position.
Do not start the engine. Once in the ON position, release the trip odometer reset button.
(3) At this time the speedometer needle should sweep across the face of the speedometer and back again.
This sweep indicates that the PSOM has been put into the enable mode. Looking into the LCD odometer
window, you should now see the English/Metric display, the revision level number and the lockout
countdown number, which indicates how many times the PSOM can be reprogrammed (See Figure 112).
CAUTION: Each time the PSOM is re-calibrated, the number of times this can be done is reduced by
one! 1992 vehicles can be re-calibrated 3 times, while 1993 and later vehicles can be re-calibrated 6
times. If the countdown number is zero and the PSOM requires re-calibration, the instrument cluster will
require replacement.
(4) Press the odometer reset button once again. Now you will see inside of the odometer window, the
conversion constant number without the decimal point, followed by the abbreviation "CAL".
Refer to Figure 113.
(5) Press and release the select button as many times as necessary to change the conversion constant number
until the desired number is reached, which for our example is 996. Each time the select button is pressed,
the constant will decrease by 1 number. When the desired constant number is reached, press and release
the reset button once, to lock in the new conversion constant.
(6) Turn ignition to "OFF", remove jumper wire from PSOM Enable Connector, test drive and verify proper
speedometer operation.
STEP 6 - THE “WHAT IF” PROCEDURE
There may be times where one might say, “What if the differential was changed and now we don’t know what the
axle capacity or ratio is, what do we do now?” Or, “What if the tires have been changed and the dimensions of the
tire do not match up with the vehicles door jam sticker, what do we do now?” These are very difficult problems
with very involved procedures to remedy them. The easiest remedy is to get the factory specified tires and/or
axle ratio required. However, if there are brave technicians who want to go, where few technicians have gone
before, here might be some helpful methods.
For the unknown axle capacity, one method that may be employed is to see how many turns of the drive shaft it
takes to make the rear tire rotate one complete turn. If it takes slightly more that 4 turns of the drive shaft to make
the rear wheel rotate one complete revolution, you would have a 4.1 axle ratio. When comparing the charts in
Figure 107 and 108, this ratio applies to many different axle capacities. The breakdown would be like this:
For Bronco and “F” Series Trucks with a 4.1 axle ratio, the rear could have a 3800, a 5300, a 6250, a 7400 or a
8250 axle capacity. “E” Series Vans would have a 6340, a 7800,or an 8000 axle capacity.
Looking now at the axle capacities for both F and E series vehicles found in the Figure 109 charts, only a 3800
axle capacity vehicle would have an exciter ring tooth count of 108. All others would have 120. If the cover on
the differential is removed and the exciter ring has 108 teeth, this was an easy find. Now all one would have to do
is match the appropriate tire size from the left side column and intersect it with the top 3800/108 column to
obtain the conversion constant number. Once the conversion constant number is acquired, the PSOM can now
be re-calibrated.
But what if the exciter ring has 120 teeth? Now it becomes necessary to obtain the tire size. Let’s say you have an
E series van with a LT225/75R16E/A/S...689 tire size. You can find this tire size looking at the bottom chart in
Figure 107. There you will notice that all of the 120 teeth exciter ring axle capacities with this tire size has the
same 10.34 conversion constant number. You are now ready to re-calibrate the PSOM.
But what if the vehicle tire size is other than OE specified? This situation will require an involved mathematical
procedure of which there are four to choose from. Choose which ever one you are most comfortable with.
To find the axle capacity, there is a sticker on the inside door jam that looks like the sticker shown on the top of
Figure 107. Locate at the bottom of the sticker the word “AXLE.” Under that word is a number. The example
given in Figure 107 is the number 29. Also in Figure 107, there is a cross over chart for Bronco and “F” series
trucks, in which 29 equates to a 5300 axle capacity with an axle ratio of 3.55.
STEP 2 - EXCITER RING TOOTH COUNT
Mounted on the differential ring gear is the speed sensor exciter ring (See Figure 2). There are only two different
tooth counts available at the time of this printing; 108 teeth and 120 teeth. The only way to know which one you
have is to physically remove the differential cover and count the teeth. For our example, we have a vehicle with
120 teeth.
STEP 3 - TIRE SIZE
Tire size may generally be obtained in one or two places. On the tire itself or from the sticker on the inside door
jam as seen in Figure 107. The example in Figure 107 reveals a tire size of : LT 215/85R-16D.
STEP 4 - CALCULATING THE CONVERSION CONSTANT
From the above three steps, we have an “F” series truck with an axle capacity of 5300, a speed sensor exciter ring
with a tooth count of 120, and a LT 215/85R-16D tire size. Now look at the chart for “F” series pick-ups &
Broncos in Figure 109. Find the 5300 axle capacity column where directly below you will find the 120 tooth
speed sensor exciter ring listed. Follow the column down until it lines up with the LT-215/85-R16D tire size
column to the left. Intersecting these columns you will find in the intersection the conversion constant number of
9.96. With this number, the re-calibration of the PSOM can begin.
STEP 5 - RE-CALIBRATING THE PSOM
Locate the enable circuit 567 wire connector. This is a light blue wire with a yellow tracer coming from the
PSOM connector pin 9 to a single connector. It is located under the left side of the dash, below the fuse box, near
the bulkhead connector on "E" series vans as seen in Figure 110, and under the center of the dash below the
glove box on "F" series trucks (See Figure 111). NOTE: The letters PSOM should be printed on the enable
connector. Once found, use the following procedures to reprogram the PSOM.
(1) With the ignition in the "OFF" position, ground the enable connector with a jumper wire.
(2) While pushing in on the trip odometer reset button (See Figure 112), turn ignition to the "ON" position.
Do not start the engine. Once in the ON position, release the trip odometer reset button.
(3) At this time the speedometer needle should sweep across the face of the speedometer and back again.
This sweep indicates that the PSOM has been put into the enable mode. Looking into the LCD odometer
window, you should now see the English/Metric display, the revision level number and the lockout
countdown number, which indicates how many times the PSOM can be reprogrammed (See Figure 112).
CAUTION: Each time the PSOM is re-calibrated, the number of times this can be done is reduced by
one! 1992 vehicles can be re-calibrated 3 times, while 1993 and later vehicles can be re-calibrated 6
times. If the countdown number is zero and the PSOM requires re-calibration, the instrument cluster will
require replacement.
(4) Press the odometer reset button once again. Now you will see inside of the odometer window, the
conversion constant number without the decimal point, followed by the abbreviation "CAL".
Refer to Figure 113.
(5) Press and release the select button as many times as necessary to change the conversion constant number
until the desired number is reached, which for our example is 996. Each time the select button is pressed,
the constant will decrease by 1 number. When the desired constant number is reached, press and release
the reset button once, to lock in the new conversion constant.
(6) Turn ignition to "OFF", remove jumper wire from PSOM Enable Connector, test drive and verify proper
speedometer operation.
STEP 6 - THE “WHAT IF” PROCEDURE
There may be times where one might say, “What if the differential was changed and now we don’t know what the
axle capacity or ratio is, what do we do now?” Or, “What if the tires have been changed and the dimensions of the
tire do not match up with the vehicles door jam sticker, what do we do now?” These are very difficult problems
with very involved procedures to remedy them. The easiest remedy is to get the factory specified tires and/or
axle ratio required. However, if there are brave technicians who want to go, where few technicians have gone
before, here might be some helpful methods.
For the unknown axle capacity, one method that may be employed is to see how many turns of the drive shaft it
takes to make the rear tire rotate one complete turn. If it takes slightly more that 4 turns of the drive shaft to make
the rear wheel rotate one complete revolution, you would have a 4.1 axle ratio. When comparing the charts in
Figure 107 and 108, this ratio applies to many different axle capacities. The breakdown would be like this:
For Bronco and “F” Series Trucks with a 4.1 axle ratio, the rear could have a 3800, a 5300, a 6250, a 7400 or a
8250 axle capacity. “E” Series Vans would have a 6340, a 7800,or an 8000 axle capacity.
Looking now at the axle capacities for both F and E series vehicles found in the Figure 109 charts, only a 3800
axle capacity vehicle would have an exciter ring tooth count of 108. All others would have 120. If the cover on
the differential is removed and the exciter ring has 108 teeth, this was an easy find. Now all one would have to do
is match the appropriate tire size from the left side column and intersect it with the top 3800/108 column to
obtain the conversion constant number. Once the conversion constant number is acquired, the PSOM can now
be re-calibrated.
But what if the exciter ring has 120 teeth? Now it becomes necessary to obtain the tire size. Let’s say you have an
E series van with a LT225/75R16E/A/S...689 tire size. You can find this tire size looking at the bottom chart in
Figure 107. There you will notice that all of the 120 teeth exciter ring axle capacities with this tire size has the
same 10.34 conversion constant number. You are now ready to re-calibrate the PSOM.
But what if the vehicle tire size is other than OE specified? This situation will require an involved mathematical
procedure of which there are four to choose from. Choose which ever one you are most comfortable with.
The following users liked this post:
97badass (02-09-2017)
02-09-2017, 10:55 PM
#4
__ x 3.14 = _____ ÷ 63360 = ______ x ___ = _______ ÷ 8000 = CC
A 31-10.50R-15 tire will be used for this example:
Overall Tire Height / Multiplied by / Pi / equals / Overall Tire Circumference / Inches per Mile / Divided
31 x 3.14 = 97.34 63360 ÷
by the Overall Tire Circumference / equals / Revolutions per Mile / Multiplied by
97.34 = 650.91 x
RABS Exciter Ring Tooth Count / equals / _______ / Divided by / PSOM Pulses per Mile / equals
108 = 70298.3 ÷ 8000 =
/ Conversion Constant
8.79
A 31-10.50R-15 tire will be used for this example:
Overall Tire Height / Multiplied by / Pi / equals / Overall Tire Circumference / Inches per Mile / Divided
31 x 3.14 = 97.34 63360 ÷
by the Overall Tire Circumference / equals / Revolutions per Mile / Multiplied by
97.34 = 650.91 x
RABS Exciter Ring Tooth Count / equals / _______ / Divided by / PSOM Pulses per Mile / equals
108 = 70298.3 ÷ 8000 =
/ Conversion Constant
8.79
