I have not a clue how to reset the speedo. Can anyone guid me through this? 92 f150 4.9 with od. Thanks

 

http://bbs.off-road.com/ubbthreads/s...sb=5&o=&fpart=

 

Thanks a bunch for your help

 

steve can u explain all that technical jibber jabber better?? i understand the basic concept, but i dont know what exactly u mean. thanx

 

Exactly which "jibber-jabber" are you having trouble with?

Here it is so you don't have to go to that BBS to read it:

The "PSOM" is the (electronic) Programmable Speedometer/Odometer Module, which is on '92-97 only F-Series & Broncos. Earlier ones use a cable-driven (mechanical) speedo which is calibrated by changing the plastic gears in the transfer case tailhousing (or transmission tailhousing on 2WD). The E-series and Aerostars also used the PSOM for short periods, but I don't know the ranges. Resetting it is EASY: the PSOM 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). 31x10.50's should be 9.18 .

If you want to change the constant, find the single-wire connector under the glovebox and ground the LightBlue/Yellow wire before turning the key on (you still have to hold the Reset, as before). Everything is the same as before, except "CAL" will flash, indicating that the constant can now be changed. Press the Select button to decrease the constant by 0.01, each time. 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) 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.

Here is a partial list of constants and revs/mile for certain tire sizes:
P215/75 R12XL/AS........10.17........753
P235/75 R15XL/AS..........9.72........720
P235/75 R15XL/AT..........9.67........716
P275/60HR17XL/AS........9.32........690.5
P265/75 R15XL/AT..........9.18........680
31X10.50R15C /AT..........9.18........680

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 is the only one that uses the PSOM signal (I think), and it's connected to the OUTPUT, so it would be reading the converted speed. But it's fed directly from the PSOM to the EEC even on vehicles with mechanical automatics and manual transmissions, and I don't know why. My truck ran fine after the engine swap, but before I added the ABS rear end and PSOM, so I don't think it's used on trucks without the E4OD.

Here are some useful formulas:

ConvConst = RevsperMile x ToneRingTeeth / 8000

(The '87-current 8.8" rear end has a 108-tooth tone ring.)
What's that? You say you don't know how many revolutions your tires make per mile?????

RevsperMile = 20168.11439/TireHeight

The height of metric tires (for instance P265/75R15 - 265mm wide, aspect ratio 75, rim height 15") is calculated by:

Height = 2 x Width x (AspRatio/100) x .0394 plus RimHeight

Since metric tire width (LT265/75R15) and flotation tire height (31x10.50R15) are nominal and not the actual dimensions, using these formulas will result in an approximate value for RevsperMile. The actual numbers for each particular size, model, & make of tire are published and available at Tire Rack.

 

basically the whole thing...i get the idea of it all...push the reset button and this shows up and do this and something else and bam, reset speedo and can be done 6 times...haha

maybe u can break it down for me if i give ya my tire size and old tire size? readin all that gave me a headache big time...

 

Nope! You're gonna have to put forth some effort. Take 2 aspirin & read it again tomorrow.

 

maybe u misunderstood me...i mean break down the technical terms and stuff so i can understand it so i can figure it out...cuz i have no idea what this means:
or dont i have to worry about it?

 

Those are mathematical formulas that you plug numbers into to find the new conversion constant (which is defined in the first paragraph). Start at the bottom by going to Tire Rack to find the mfrs. Revs/mile for your particular tire, then plug that into the first equation (with 108 for "ToneRingTeeth) and voila! You get your conversion constant.

If your tire isn't listed, then you have to use the other formulas (starting at the bottom).

Every term is explained right there, so I still don't know what you're having trouble with. The formulas themselves are just algebra, using geometry & unit analysis, but you don't have to understand how they're derived to put numbers into them.

 

Steve83,
Great stuff. Thank you. This is exactly what I was looking for. Your examples are excellent and you even provided me the tooth value. The math is easy but relative to peoples experiences, I guess. I am playing with 33"s and 35's on my just lifted Bronco this weekend (Pro Comp 4" w/radius arms and new rear leafs) and will need to make the speedo adjustment after I decide which size to run. If I go 35's I'll throw in gears too and if gears, a locker! I think I got enough horses to handle the 33's without a gear change. Depends how it looks and rides. I should get it off the jacks tomorrow or saturday if I did it right. Ok, back to my shop.

 

I'm running 32s with a 4.9L and 3.55s (temporarily, but I'll soon go back to 3.08s) pushing 3 tons normally - more when I hook up the 16' trailer & load it with steel, lumber, or dirt.

 

I trailer often too. I yank a lot of old cars out of the southwest desert. That's whats making me lean toward different gears. It's also why it's still on jacks. I decided to go with drilled performance rotors for improved braking while I was under it doing the lift.
These projects tend to get a little expanded. Oh well.

 

Steve Why are going back to 3.08? My 5.0 won't pull itself w 3.08 empty nothing more than level ground. I'm thinking of changing to 3.55 for more power and prob better gas milage too, since engine will not be lugging. PM

 

Can't find the LB/Y wire. Pulled the glove box and kick panel and couldn't find the wire. Any further help?

 

wow i was just looking for this today...what a conuincidence (sp)...thanx