Because I was sick of fighting the speedo cable & plastic gears, and since I had already swapped in an '87-96 rear axle with
ABS sensor (DY1123, F85Z-9E731-AB), I decided to convert my old '80-86 mechanical speedo to a '92-97 PSOM (at the far L).
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It also allowed me to convert to the later electronic cruise control servo, which was a big improvement over the older vacuum servo which starved itself when going uphill at high speed. This is how the newer gauges sit in the old cluster. Yes, that's a green pen cap on the SELECT button, and yes, it required drilling the cluster lens for access to them. The pen cap is just an extension - I have to use a tool to press the RESET button, but I wanted the security of knowing it wouldn't get pressed accidentally.
The following was originally posted to a BBS, and was shamelessly
plagiarized by BroncoDriver Magazine.
The "PSOM" is the (electronic) Programmable Speedometer/Odometer Module, which was used on '92-97 F-Series & Broncos. Some Econolines & Aerostars also used it. Earlier trucks use a cable-driven mechanical speedo/odo, & later ones use an electrically-driven speedo with a motor-driven mechanical odometer; both of which are calibrated by
changing the plastic gears in the transfer case tailhousing (or transmission tailhousing on 2WD). The PSOM is driven by the ABS sensor inside the 8.8" rear differential, but this is interesting...
http://www.fordf150.net/forums/viewtopic.php?t=72709The 2 methods shown in this thread actually work:
http://www.explorerforum.com/forums//showthread.php?t=300588If your PSOM fails, get one from a junkyard and swap it in using the photos in this album:
If you can't find one locally at a reasonable price, e-mail me thru my profile here or on most BBS forums dealing with Ford trucks & cars. I usually keep a few ranging from $15-80 depending on mileage.
Resetting the PSOM is EASY: it uses the "conversion constant" to calculate the vehicle speed from the sensor frequency. You can see yours by holding the Reset button while turning the key forward to Run, then release Reset. The needle will do a full sweep test, and the odometer display will show " E ## # ", meaning English units, the microprocessor revision (mine is 08 ), and the conversion constant lockout count (maximum is 6). Canadian ones might show " o " in the first position meaning "Overseas" (Metric) units. Early revisions (near 03) apparently don't have a lockout count, but according to the '92 service CD & TSB 94-16-16, they allow only 3 changes. Pressing Reset again shows " ####CAL ", which is the conversion constant (without a decimal). The constant for PSOMs in trucks originally equipped with 31x10.50's should be 9.18 . Check the original tire size on your door sticker.
If you want to change the constant, find the single-wire connector (usually marked PSOM) under the glovebox and ground the LightBlue/Yellow wire (early '92 models use a Bk/Or wire) before turning the key on (you still have to hold the Reset, as before). (The cigar lighter ground wire might reach.) Everything is the same as before, except if you press Select while viewing the conversion constant, "CAL" will flash & the constant will decrease by 0.01. Press the Select button to continue decreasing the constant by 0.01 each time. (Revisions 1-3 INcrease with each press.) When the constant reaches 5.00, it goes back to 11.00 . Larger tires or fewer teeth on the tone ring require a lower constant - R&P changes have no effect. When you have the constant you want, press the Reset button to lock it in. THIS WILL DECREASE THE LOCKOUT COUNT BY 1 - WHEN IT REACHES 0, NO FURTHER CHANGES CAN BE MADE. To abort the changes and maintain the lockout count, turn the key off before pressing Reset. When finished, turn the key off and disconnect the LB/Y wire from ground. The "PSOM Programming Enable" circuit (the LB/Y wire under the dash, #24 in the diagram below) is only used when you want to change the constant and use one of the lockouts. You can check the constant without grounding that wire.
Connector #24, lower L of this pic:
Here are some wiring diagrams involving the PSOM & ABS sensor. A failed 4WABS module can interfere with the PSOM & everything on the Gy/Bk circuit, but the RABS module won't. If the 4WABS module is suspected of interfering with the PSOM, bypass the 4WABS module & feed the ABS signal directly to the PSOM by installing jumper wires from LG/Bk to LG/Y, and from R/Pk to O/LB. If the RABS module is suspect, simply unplug it.
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When the lockout count reaches 0, it doesn't self-destruct; you just can't change it again. It keeps using that constant forever. If you want to change again, you have to replace the PSOM. But, remember: the only reason to change it is for a tire size change. After 6, you ought to have the size you like!
I think the reason a) it's not publicized, and b) there IS a lockout count, is so that people don't set it so the odometer registers fewer miles until they take it in for warranty work - they'd use up the 6 changes in 3 trips, and the techs would see the lockout going down, and void the rest of the mileage warranties. But that's just a guess.
The E4OD & 4R70W are the only transmissions that use the PSOM signal, and they're connected to the OUTPUT, so they would be reading the converted speed. But the signal is fed directly from the PSOM to the EEC even on vehicles with mechanical automatics and manual transmissions for return-to-idle management during deceleration or manual shifting. My truck ran fine after the engine swap, but before I added the ABS rear end and PSOM, so it's not critical on trucks withOUT an electronic automatic or ABS. The rare '94-96 Bronco
overhead console uses the PSOM signal to estimate the accuracy of the outside temperature sensor.
Here are some useful formulas:
To derive a conversion constant:
ConvConst = RevsperMile x ToneRingTeeth / 8000
The '87-current 8.8" rear end has a 108-tooth tone ring; the 7.5-inch rear end has 102 teeth; the 9.75-, 10.25-, 10.5- and 11.25-inch diameters usually have 120 teeth. See the Ford PDF linked below. This TSB (94-16-16) is a complete list of Ford-authorized constants by year, tire, & axle. Broncos & F150s use the 3800-lb axle (8.8") in the 1st column. The factory tire size can be found on the door-jamb sticker.
The actual RevsPerMile for each particular size, model, & make of tire are published and available on the SPECIFICATIONS pages at
TireRack or the tire manufacturer's website. If you can't find a spec, set your air pressure where you'll be driving, find ~30' of straight, flat, empty pavement, mark a rear tire AND the ground, & roll it exactly one full rev. Be sure to stop with the mark on the tire exactly dead-center of the contact patch, just like with the first mark. (For more precision, use a bubble level to set the mark plumb below the center of the hubcap each time.) Then measure from the mark on the ground to the mark on the tire in inches (you'll need a 10' or 12' tape & an assistant).
RevsPerMile = 63360 / InchesPerRev
A rough approximation of RevsperMile can be calculated using:
RevsperMile = 20168.11439/TireHeight (in inches)
The height of metric tires (for instance P265/75R15 - 265mm wide, aspect ratio 75, rim height 15") is calculated by:
TireHeight = 2 x Width (in mm) x AspRatio x .000394 plus RimHeight (in inches)
Since metric tire width (LT265/75R15) and flotation tire height (31x10.50R15) are nominal and not the actual dimensions, using the above formulas will result in an approximate value for RevsperMile.
If you have to replace your PSOM with a used one, and the replacement has FEWER miles than you want, you can add miles. I ran my computer's sound card headphone signal out to a big Kenwood home theater receiver, then the speaker outs from THAT to the PSOM speed signal inputs. I turned the volume ALL the way down, then used GoldWave (software with a free-trial) to generate the square wave at the freq. I wanted (~2KHz IIRC), and slowly increased the receiver's volume until the needle came up. Then I just let it run - no smoke got out, and the guy who bought it said it worked fine. That was in 2003 and he hasn't complained about it, or asked for another PSOM, so I can only guess it's still working. Alternatively, you might get a junkyard ABS sensor & place it near a metal fan blade (a squirrel-cage blower would probably be best), wire it up to your PSOM, and let it count that for a few days.
There's no way to remove miles from a PSOM, for obvious reasons. IDK what happens if it rolls past 999,999.9mi.
This Ford .pdf has PSOM information on page 3.
This Ford .pdf shows the back of a digital-display PSOM that has DIP switches to adjust the conversion constant, but I've never actually seen a PSOM like that. All of the ones I've seen have an empty space where the DIP switches would go, in the lower R of this pic.
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This describes how to test all the display elements.
The instrument cluster requires special handling to avoid damaging internal components. The cluster MUST be kept FACE UP or in the in-vehicle position. Leaving the cluster face down may result in a loss of dampening fluid used in the gauges. The lost fluid could stain the cluster face or result in excessive pointer waver.
PSOM Discoloration
This PSOM's software has failed:
[Quote=DaveG on FSB]My friend has a 96 XLT with an "S" in a circle displayed on the right side of the digital odometer above the last number. I have now clue what that means. Anyone know??[/quote]
[Quote=Fordgirl on FSB]The circled "s" means at some point in time the odometer was serviced and actual mileage was not known and {left at} zero.[/quote]
A service cluster that is pre-programmed with the PSOM speed signal conversion constant and the vehicle mileage eliminates the need for "circled S" odometers.
CAUTION: The use of vinyl cleaners and similar other cleaning agents to clean the vehicle interior and/or instrument cluster lenses has resulted in damage to the instrument cluster lenses. The chemical content of these cleaning agents (o-dichlorobenzene, ethyl alcohol and/or cellosolve) has produced fogging, spotting, staining, or splotches on the lenses, either through overspray, or direct use on the lenses, Therefore, extreme caution should be taken during interior clean-up to prevent overspray of cleaning agents which contain the chemical agents mentioned from contacting the instrument cluster lenses. The instrument cluster lenses should be cleaned with Ultra-Clear Spray Glass Cleaner E4AZ-19C507-AA (ESR-M145P5-A) or equivalent commercial cleaning product, using a clean soft, lint-free cloth. The Ford Glass Cleaner has been especially formulated for cleaning windows in automotive vehicles and is approved for use in cleaning the plastic instrument cluster lenses.
This describes a later odometer that spontaneously switched from miles to km:
http://www.thedieselstop.com/forums/f185/huge-odometer-problem-539602/--------------------------------------------------------------------------------
TSB 96-21-11 Programmable Speedometer/Odometer Module Pointer Waver
Publication Date: OCTOBER 7, 1996
LIGHT TRUCK:
1992-1996 AEROSTAR, BRONCO, ECONOLINE, F-150-350 SERIES
ISSUE: The speedometer needle may waver and/or a light surge may occur on some vehicles when speed control is used at highway speeds between 80-113 km/h (50-70 mph). This may be due to slight dents/chips in either the exciter ring or the Vehicle Speed Sensor (VSS) and air gaps between the VSS and the exciter ring.
ACTION: Replace the Programmable Speedometer/Odometer Module (PSOM) if required. New PSOM's come with increased immunity to system variability. Refer to the following Diagnostic Procedure for details.
DIAGNOSTIC PROCEDURE
BRONCO/F-SERIES - Perform normal PSOM diagnostics per Pinpoint Tests "H" and "J" of the 1996 F-Series/Bronco Body/Chassis Service Manual, Pages 13-01-30 through 13-01-32 and Pinpoint "B" of the 1996 Powertrain/Drivetrain Service Manual, Page 10-03-11.
ECONOLINE - Perform normal PSOM diagnostics per Pinpoint Tests "H" and "J" of the 1996 Econoline Body/Chassis Service Manual, Pages 13-01-29 through 13-01-31 and Pinpoint "B" of the 1996 Powertrain/Drivetrain Service Manual, Page 10-03-9.
AEROSTAR - Perform normal PSOM diagnostics per Pinpoint Test "K" of the 1994 Aerostar Body/Chassis Service Manual, Page 13-01B-31.
Any slight dents, chips, etc., in either the exciter ring or VSS will create needle waver. Measure air gap between the VSS and exciter ring. It should be 0.38-0.51mm (0.015-0.020").
Check exciter ring runout per the same ring gear backface runout procedure of the appropriate model 1996 Powertrain/Drivetrain Service Manual, Section 05-00. Make sure the exciter ring is mounted correctly to the ring gear. If runout is more than 0.1mm (0.004"), perform the differential runout check per the procedure in the appropriate model 1996 Powertrain/Drivetrain Service Manual, Section 05-02A or 05-02D, to find cause and repair as needed.
If all items listed above check good, replace the Instrument Cluster Assembly. Obtain the correct service part number from the Parts Catalogue and then contact the Electronic Odometer Exchange Center at (800) 259-9700 for U.S. Dealers and (800) 663-9974 for Canadian Dealers.
NOTE: DEALERSHIP MUST TELL THE ODOMETER EXCHANGE CENTER THAT YOU NEED A PSOM3 LEVEL CLUSTER FOR A SPEEDOMETER NEEDLE WAVER CONCERN .
OTHER APPLICABLE ARTICLES: NONE
WARRANTY STATUS: Eligible Under The Provisions Of Bumper To Bumper Warranty Coverage
OPERATION DESCRIPTION TIME
962111A Replace Programmable Speedometer/Odometer Module (PSOM) - F-Series, Bronco 0.6 Hr.
962111B Replace Programmable Speedometer/Odometer Module (PSOM) - Econoline 0.7 Hr.
962111C Replace Programmable Speedometer/Odometer Module (PSOM) - Aerostar 0.8 Hr.
See the 8.8" Axle, and Diagrams albums for more info.
See also:
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H1 CHECK SPEEDOMETER OPERATION
Is speedometer free from hesitation or sticking when accelerating or decelerating smoothly? This may be confirmed by performing the following test:
Press and hold the RESET button on the front of the speedometer while turning the key to RUN.
Release the RESET button.
Does the pointer smoothly prove out, or sweep from 0 to maximum speed then back to 0?
Yes GO to H2 .
No After verifying good power and ground connections, GO to G6 . REPLACE instrument cluster . VERIFY proper operation.
H2 CHECK ODOMETER DISPLAY
The trip odometer must be currently displayed for the RESET button to work.
If unsure about missing or extra segments, perform the odometer display self test as follows:
Press and hold the reset button on the front of the speedometer while turning the key to RUN.
Release the RESET button.
Press and release the select button. The odometer will begin with all zeroes and step through the display test each time the select button is pushed. Refer to PSOM Display Test Sequence below for exact display.
The first digit on the left does not always match the other digits.
Key off to exit this test.
Are all odometer digits formed correctly (no extra or missing segments)? Do the select and reset buttons work normally?
Yes GO to H3 .
No REPLACE instrument cluster . VERIFY proper operation.
H3 CHECK SPEEDOMETER POINTER
Does pointer jump or waver at a constant speed?
Yes GO to H4 .
No GO to H5 .
H4 CHECK RABS SPEED SENSOR RING AND SENSOR
Check for presence of the following conditions.
Damaged, missing or bent teeth
Metal chips on speed sensor
Open or shorted sensor
Improper gap between sensor and speed sensor ring
Refer to Section 06-09A for additional information.
Are any of the above conditions present?
Yes REPAIR or REPLACE as necessary. VERIFY proper operation.
No REPLACE the instrument cluster . VERIFY proper operation.
H5 CHECK FOR STUCK POINTER
Is pointer stuck at upper pointer stop?
Yes GO to H6.
No Speedometer normal.
H6 SPEEDOMETER POINTER PROVEOUT
Press and hold the RESET button on the front of the speedometer while turning the key to RUN.
Release the RESET button. The pointer would normally prove out, or sweep from the lower to upper pointer stop and back again. In this case, the pointer will "jump back" to where it should be in the sweep and continue to sweep normally when the pointer gets to within 180 degrees of the upper pointer stop (about 56 km/h [35 mph]).
Key off.
Does the pointer return to the normal position?
Yes Speedometer normal.
No REPLACE the instrument cluster . VERIFY proper operation.
PINPOINT TEST J: SPEED CONTROL, PCM OR ELECTRONIC TRANSMISSION DISPLAY INOPERATIVE OR ERRATIC
NOTE: Follow this procedure only after being referred here from Section 10-03, Powertrain Control/Emissions Diagnosis Manual, or electronic transmission shop manual diagnostics.
J1 CHECK ANTI-LOCK LAMP ILLUMINATION
Key to RUN.
Does anti-lock lamp on dash go out after self-test?
Yes GO to J2 .
No REFER to Section 06-09A or Section 06-09B .
J2 CHECK SPEEDOMETER SPEED OUTPUT SIGNAL
To determine if there is a short between the PSOM and the speed control module, the vehicle speed control may be used as a good indicator. If it works normally above 56 km/h (35 mph), this means that the speedometer module is at least receiving a speed input signal and the wiring and sensor can be assumed to be good. CAUTION: Before checking the continuity of any circuit, make sure there is no voltage present in the circuit prior to switching the test equipment to the resistance function to avoid damage to equipment. Refer to the test equipment user's manual for additional information. NOTE: Only wiring harness end of connector is to be probed.
Use ohmmeter to check for opens in wiring between Pin 7 (speed out) and the affected module. Verify that the terminals in PSOM wiring connector are completely seated in the connector.
Is the circuit open?
Yes SERVICE wiring between speedometer and affected module as required.
No VERIFY proper operation. GO to J3 .
J3 CHECK SPEED INPUT SIGNAL
Connect Rotunda 88 Digital Multimeter 105-00053 or equivalent to Pin 4 (speed ) and Pin 5 (speed -). If available, a frequency counter may be connected to Pin 4 (speed in ) and Pin 5 (speed in -). CAUTION: Make sure multimeter is in the voltage function and not in the resistance function to avoid damage to multimeter. NOTE: Only wiring harness end of connector is to be probed.
Does the displayed frequency of the signal increase smoothly and continuously from 0 to approximately 667 Hz at approximately 48 km/h (30 mph)? OR does the voltage increase smoothly and continuously from 0 to approximately 3.5 volts as vehicle speed increases from 0 to approximately 48 km/h (30 mph)?
Yes REPLACE instrument cluster . SERVICE wiring.
No CHECK for open wiring. If no problems are found, REFER to Section 06-09A or Section 06-09B .
B1 CHECK THAT CONDITION OCCURS ONLY WHILE USING SPEED CONTROL
Drive vehicle at the speed in which the condition occurs.
Does the speedometer needle waiver by more than ± 2 mph?
Yes Bad Programmable Speedometer/Odometer Module, RABS speed sensor, or rear axle ring gear.
No SERVICE as necessary. GO to B2 .
B2 CHECK PROGRAMMABLE SPEEDOMETER/ODOMETER MODULE OUTPUT SIGNAL
Check PSOM output signal.
Did PSOM pass output signal test?
Yes GO to B3 .
No SERVICE as required.
B3 CHECK FOR BINDING IN SPEED CONTROL ACTUATOR CABLE AND THROTTLE BODY LINKAGE
Check for binding or sticking of speed control cable or throttle linkage and throttle plate.
Make sure throttle cable bracket and speed control servo bracket are not loose.
Are components OK?
Yes REPLACE speed control servo and VERIFY condition is corrected.
No SERVICE as required.
