This is a repost of info I originally posted to other BBSs, and it was shamelessly plagiarized by BroncoDriver Magazine in 2008.
The "PSOM" is the (electronic) Programmable Speedometer/Odometer Module, which was used on '92-96 F-Series & Broncos (& '97 F-series >8500GVWR). 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).
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. 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). 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.
***NOTE*** 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.***
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.
. . . . .
This 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.
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 buy a new 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 this Ford PDF.
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 = (Width in mm x AspRatio x .000788) 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.
Here's a PSOM calculator provided by Eric "muddybronco".
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 several years ago (check the UL date on the next photo) 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.
This Ford PDF shows another style of PSOM (apparently only used on '97-? heavy trucks) 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.
This describes how to test all the display elements.
Originally Posted by DaveGTSB 96-21-11 Programmable Speedometer/Odometer Module Pointer WaverOriginally Posted by Fordgirl
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.
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 album in my signature link for more info.