[fodder]

I went out and did a test with a GPS, apparently 35 actual is 45 on my speedo, so it's not as bad as what I first guessed from the eyeball test, but the difference does get worse at higher speeds. Though from what I read that's normal?

Mine is the opposite. I have the PSOM calibrated to my tire size, which has it reading accurate at highway speeds (anything north of ~45 mph), but inaccurate at around-town speeds. 30 mph indicated is actually 25 mph, 35 mph is a little over 30, 40 mph is probably 37 or 38, and then it evens out from there. I've never done a GPS test; that's all based on speed camera displays. GPS can be wildly inconsistent from day to day, in my experience.

If I could get the giant brick hurling faster than 80 mph, and had the speedo range to capture it, I'm guessing it would start reading inaccurate in the other direction.

 

[AlaskanBronco]

35 / 25 = 40% faster than actual
45 / 35 = 29% faster than actual
60 / 45 = 33% faster than actual
75 / 55 = 36% faster than actual
85 / 65 = 30% faster than actual

25 / 35 = 0.71
35 / 45 = 0.78
45 / 60 = 0.75
55 / 75 = 0.73
65 / 85 = 0.76

matter of perspective how you view it.

the faster you go when comparing them the more accurate it will be

a job of the PSOM is to convert the rear axle signal to tell the ECM how fast the truck is going. the ECM expects an 8000 pulse per mile reading which is where the 8000 in the forumla comes from.

so if you scale the PSOM constant to fool the speedometer to read correctly, now the signal to the ECM will also be scaled. this means it will shift the transmission at the wrong points. your trans would shift alot later than normal

Isn't that the point of the "programable speedometer module"... to be programable? Why would Ford include so many different cal settings if not for us to use them? I would think they wouldn't even include any calibration setting that would negatively affect the vehicle? Just trying to understand and figure all this out.

 

[Kingfish999]

ford uses the PSOM to account for tires size. not for doing a speedometer swap with a different sweep

even tho your PSOM is set alittle differetn that it should be for 32" tires, i imagine your transmission is shifting at right about the right points because the ECM is getting the correct signal. but if you use the PSOM to try and scale your Constant so the speedometer is right, then you will find the transmission will shift at the wrong time because the ECM thinks you are going a significantly different speed than your actually going

 

[Stevo440]

What if you just use the Metric scale for speed? How does that compare to your actual speeds vs the mph scale? Might not be the answer you want but it might be a workaround. Just a thought!

 

[Kingfish999]

Not sure you can actually swap between them

 

[Stevo440]

Not sure you can actually swap between them

Sorry I was incorrect in my statement, I retract what I said. What I should have said was and like @CrazyBRONCOguy stated the Bronco Speedo face 90mph vs the Lightning Speedo face 120mph are off by .75 as is the average of all the speed points on the chart the average for the 5 points checked/listed is also off by .75. So there must be something in the PSOM that accounts for the 120 mph speedo face, a constant/correction factor if you follow me. I'm not an Electrical "Engamaneer" but a Mechanical "Engamaneer".

 

[AlaskanBronco]

Sorry I was incorrect in my statement, I retract what I said. What I should have said was and like @CrazyBRONCOguy stated the Bronco Speedo face 90mph vs the Lightning Speedo face 120mph are off by .75 as is the average of all the speed points on the chart the average for the 5 points checked/listed is also off by .75. So there must be something in the PSOM that accounts for the 120 mph speedo face, a constant/correction factor if you follow me. I'm not an Electrical "Engamaneer" but a Mechanical "Engamaneer".

To be clear, the PSOM is the stock Lightning PSOM, I didn't swap it like many people do when they change clusters. I suspected my Broncos original PSOM was bad, so I sent the entire lightning cluster which I had lying around to module mechanics to get the internals inspected and tested, bulbs changed, and my broncos mileage programmed.

So far so good, no more transmission limp mode or flashing OD light, and having all the bulbs work again has been really nice. Due to the speedo issue, I haven't driven enough to know for sure if the PSOM was the problem but I'm hoping it was.

 

[sackman9975]

Got some info.
My buddy who's running this same 120mph speedo, is running 33 inch tires. Normally, the constant is 851 for 33s. I have his set at 877 and he states " matches my GPS absolutely perfectly".


His tire RPM is 642
642 x 108 / 7900 is 8.776

I think my math is correct and you should program yours to 901 as I stated above.

Some reason, these need to equal 7900 pulses per mile instead of 8000 to be correct.

 

[adrianspeeder]

Isn't that the point of the "programable speedometer module"... to be programable? Why would Ford include so many different cal settings if not for us to use them?

The thousand year old engineers at the time were worried that with a new fancy digital odometer people would change the constant all the time for warranty fraud. Accurate speedo be dammed they were going to keep changing the constant to make two miles read as one. Limit to six changes and you can’t keep making that trick happen.

Adrianspeeder

 

[AlaskanBronco]

Got some info.
My buddy who's running this same 120mph speedo, is running 33 inch tires. Normally, the constant is 851 for 33s. I have his set at 877 and he states " matches my GPS absolutely perfectly".


His tire RPM is 642
642 x 108 / 7900 is 8.776

I think my math is correct and you should program yours to 901 as I stated above.

Some reason, these need to equal 7900 pulses per mile instead of 8000 to be correct.

Well I finally had time to calibrate the speedo and go for another test drive, and unfortunately the new 901 setting didn't work. Here's the results:

Bronco Speedometer reading	Waze GPS Reading
30	23
35	26
40	30
45	33
50	37
55	41
60	46
75	55
85	63
90	67

Not sure what do to now. Try the regular 8000 pulse formula?

659 x 108 / 8000 gives me 8.8965 (8.91CAL)? No idea if this will actually work or send me even farther off course?

 

[sackman9975]

I just got back from testing out Rla2005s GPS method, here were my results.

Speedometer Reading	GPS Reading
35	25
45	35
60	45
75	55
85	65

Well I finally had time to calibrate the speedo and go for another test drive, and unfortunately the new 901 setting didn't work. Here's the results:

Bronco Speedometer reading	Waze GPS Reading
30	23
35	26
40	30
45	33
50	37
55	41
60	46
75	55
85	63
90	67

Not sure what do to now. Try the regular 8000 pulse formula?

659 x 108 / 8000 gives me 8.8965 (8.91CAL)? No idea if this will actually work or send me even farther off course?

Quoting both posts that have your readings.

So before with 920 and after with 901 you got the following:
35mph = 25 / 35mph = 26
45mph = 35 / 45mph = 33
60mph = 45 / 60mph = 46
75mph = 55 / 75mph = 55
85mph = 65 / 85mph = 63

So this makes no sense to me as some numbers are higher than before and some are lower than before but minimal.
Are you certain you locked in the CAL#? There should be more than 1 or 2 mph difference if you changed it to 901 specifically with 85mph.

 

[fodder]

Referring back to my earlier comment, I wouldn't put much stock in the consistency of GPS readings.

 

[AlaskanBronco]

Quoting both posts that have your readings.

So before with 920 and after with 901 you got the following:
35mph = 25 / 35mph = 26
45mph = 35 / 45mph = 33
60mph = 45 / 60mph = 46
75mph = 55 / 75mph = 55
85mph = 65 / 85mph = 63

So this makes no sense to me as some numbers are higher than before and some are lower than before but minimal.
Are you certain you locked in the CAL#? There should be more than 1 or 2 mph difference if you changed it to 901 specifically with 85mph.

Yes, I just confirmed. 901CAL with 4 remaining changes. I know it makes no sense, I can't explain it.

To fodders point, I confirmed the findings earlier today by following someone in another vehicle while they drove 35mph and 65mph. My speedometer showed 45mph and 80mph during each of those tests. I'm in the process of removing the cluster to send back to the repair shop to see if they can figure out what's wrong.

 

[Kingfish999]

have you ever contacted this company yet that rebuilt it?

 

[AlaskanBronco]

have you ever contacted this company yet that rebuilt it?

Yeah, they asked me to send back the PSOM for another look. Mailed it off yesterday, hopefully they can figure out why it's being so difficult.