more info on Digital Speedo self-test please

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more info on Digital Speedo self-test please

October 21, 2004, 11:58:37 AM

Hi, I'm new to this whole message board thing and I'm looking for more information about the digital speedo self-test functions. Iknow the 1990's aerostars and rangers were capable of speedometer calibration adjustments through their own self-test modes and was hoping we might find something similar on our own cars.
any thoughts?

Re: more info on Digital Speedo self-test please

Reply #1 – October 21, 2004, 06:48:08 PM

yes the speedo has a self test feature but as far as the speedometer calibration adjustments ,, your gonna have to elaborate a tiny bit more as to what your are after here

to run a speedo self test,,(someone out there remind me of what it was i posted on the other site if i get this wrong),,,
hold the trip set button ,or maybe the trip reset button and then turn your key on.

your speedo will show a bunch of codes and you can keep hitting one of those trip buttons and it will cycle thru tests for the speedo module only.

you cant calibrate your speedo from this test that i am aware of.,, but bondocougar may have heard of this since he stumbled across some other neat eec info lately.

bondocougar, you out there?

Re: more info on Digital Speedo self-test please

Reply #2 – October 22, 2004, 04:18:15 AM

To elaborate on what I mean - on the ranger/aerostar speedo, according to info I've found elsewhere, the speedometer readings can be adjusted to compensate for rear gear ratio and/or tire size changes.
Check out ford-trucks.com
I realize that these are different units but wondered whether they might share some functional similarities. Worth looking into, I think. All those dealer-only modes got me wondering.
thanks

Re: more info on Digital Speedo self-test please

Reply #3 – October 22, 2004, 08:25:14 AM

I'm pretty sure that was a feature only available in newer models.

Re: more info on Digital Speedo self-test please

Reply #4 – October 23, 2004, 11:14:32 PM

Yup, I agree, there is no built in feature for calibration that I'm aware of for these years.

I have altered the microprocessor crystal in the cluster before, which would effect the timing, so could theoretically alter the number of pulses the speedo counts per second,as the period of time the pulses are counted would be altered.

However:
changing the crystal to a lower rate actually is causing some display problems, probably because the lower processor board communicates with the display board and changing the lower board's crystal alters the data rate between the boards, so the display board doesn't decode the data.
Even if it worked, the crystal you would need would have to be a custom cut crystal, and probably isnt worth the effort.

Here is a better idea: I could make a circuit that you feed the VSS signal to, the circuit then modifies the pulse per second count to reflect the effect of the rear end, tires etc, and then sends the signal to the cluster. I'd need to know what sort rate changes will be needed. Then it would be a wire cut at the cluster, plus wire to power the circuit. Hey, maybe I can sell these and pay for my drastically overbudget 5.0 conversion. :wtf:

I might need this circuit, as I picked up a 3.55 TC rear for my AOD, and from what I've read I think new speed sensors are only available for manual trannys (?)

Re: more info on Digital Speedo self-test please

Reply #5 – October 24, 2004, 12:24:33 AM

sounds interesting!
I've seen these things for newer stangs but they use a different vss(i think).
What would it take to get one? I'd have to do the math, but i think I'm reading fast by about 15-20% (3.73s with the closest vss gear available)

Re: more info on Digital Speedo self-test please

Reply #6 – October 25, 2004, 08:59:40 PM

Why don't you just swap in a new speedo gear in the trans? I have 3.55 gears and I swapped the driven gear and my speedo is accurate.

Re: more info on Digital Speedo self-test please

Reply #7 – October 26, 2004, 04:25:31 AM

I have swapped the speedo gear already. I'd have to check but i think it was purple (21 teeth rings a bell-not positive tho). The ford dealer said it was the closest he could get me for 3.73s since these cars never came with them in automatics. Is there another gear that will work?

Does anybody know if a T5 speedo gear will fit in an AOD?