A search for tach calibration turned up this procedure http://www.fieros.de/en/articles/tach.html where a resistance is changed on the tach board, thereby calibrating the air-core behavior of the tachometer movement.
The Fiero speedo is also an air-core device as seen here: http://www.fieros.de/en/articles/speedo.html
From all I've read and not been able to find, there is no fine tune adjustment for the speedo other than a Dakota Digital device that is less than perfect by almost all accounts.
Programming the IPD and VSS in a 7730 ECM gets you close, but not spot on if you have non stock tires and/or a magnetic 24,000 puls per mile VSS as is my case.
I have spent an insane amount of time trying/reading/searching/trying to get my speedo correct, all to no avail - still low by 10%.
Now, since both speedo and tach are air-core devices, can a resistance on the speedo board be modified to tune it the same way as the tach?
In theory, it should be able to be done? Anybody done it?
I don't want to lay up my car for this during our short summer, so maybe I'll have to get a test mule speedo and make a signal generator to try it out.....
Any thoughts from anyone more electronically educated than me?


Edit: Credit must go to Oliver Scholz for the above links. Check out his site for many other interesting documents: http://www.fieros.de/en/main.html

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Daviero - 88 N* GT

[This message has been edited by Daviero (edited 07-25-2010).]

Pull off the needle and set it to the correct spot.

quote Originally posted by phonedawgz: Pull off the needle and set it to the correct spot.

If it was only that simple........this type of change is just an offset by however much you move the needle.
A true calibration is proportional, and is what is required when the reading is out by a given %, in my case, about 10%.
So at 30 mph, my speed reads 3 mph slow, at 60 it is 6 mph slow and so on....
I suspect this is the case for other's calibration requirements too.

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Daviero - 88 N* GT

Good morning Dave, ;-)


unfortunately it's not that simple. The Tach is a standard circuit using a (back in the 1980's) common air core driver integrated circuit. Basically this is an analog circuit using a resistor/capacitor for calibration. Since variable resistors are more common than variable capacitors (at least in the range required), that is what GM used and also what I described in my webpage. (GM used laser trimmed resistors, I described the variable resistor method). When going from 6->8 cylinders, you can also replace the capacitor, but the resistor method is more accurate.

Now to the speedo. You're correct, this is also an air core instrument, but it uses a combination of two ICs for control of the needle. Things are complicated by the fact that GM used different chip sets, but the following is essentially true for all Fiero speedos I've seen.

The coils are driven by a digital circuit controlling coil polarity and a series of pulses driving the coils (possible PWM, I don't remember for sure, it's been some time). Anyway, this driver IC is controlled by what appears to be a small microprocessor, measuring the 2000/4000ppm signal, and generating the proper control sequence for the driver IC. Both of these run off of 12V though, so maybe it's not a microprocessor but a custom CMOS IC. In any case, contrary to the tach, it is most likely custom GM (if anyone knows different, I'd be glad to stand corrected). By changing the crystal on the board, you can calibrate the speedo somewhat, but only within a certain range (depending on the chip set), and using standard crystal frequencies, you probably need to make your own gauge faces.

Additionally there are the two trip/odometer counters. They are run off of a small 8-pin stepper driver, which also runs off of the 2000/4000ppm signal. These are unaffected by any crystal changes.

If you change to a different VSS, you have to either divide/multiply the signal to get in the 4000ppm AC range (which is what the dakota box apparently does), or design your own circuit board. If your VSS is from a GM car and that car has a speedo which is not controlled by the ECM, you might make the circuit board from that car's speedo work in the Fiero, but I highly doubt that. If the dakota digital box does what you need, I'd say go for it.

Best regards,

Oliver

You can adjust the PPM value in the ecm and that will also change the speed. While you have 24000 VSS, I had mine set to 25600 in the 7730 to dial in the ecm speed to my GPS. If your ECM speed is correct, but the instrument panel off, then raising/lowering the 24000 ppm value should allow you to fine tune the instrument cluster speed at the expense of the ecm speed being off slightly (which isn't a big issue).

The higher the #, the slower the displayed speed will be, the lower the number the higher the displayed speed.

quote Originally posted by fieroluke: Good morning Dave, ;-) If the dakota digital box does what you need, I'd say go for it. Best regards, Oliver

Thanks for your explanation Oliver. So it's not that simple, and explains why this struggle still exists. I was trying hard to avoid the Dakota solution due to many dissatisfied comments I have read about it, but perhaps it is the least of all evils......
Dave.

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Daviero - 88 N* GT

I know you've been struggling with this for some time now. If you're not averse to changing out the speedo for a different unit, and you have some spare change lying around, many of the modern aftermarket "analog" speedos are actually stand-alone GPS units. Nothing except power and a ground to be hooked up and during the first drive, you calibrate it according to a set procdure. Voila... well... until US government decides to cut us off from the high resolution signal.

quote Originally posted by fieroguru: You can adjust the PPM value in the ecm and that will also change the speed. While you have 24000 VSS, I had mine set to 25600 in the 7730 to dial in the ecm speed to my GPS. If your ECM speed is correct, but the instrument panel off, then raising/lowering the 24000 ppm value should allow you to fine tune the instrument cluster speed at the expense of the ecm speed being off slightly (which isn't a big issue). The higher the #, the slower the displayed speed will be, the lower the number the higher the displayed speed.

Right....I understood what you are saying to be correct too, BUT....
My RSC is set to 22140 and TunerPro displays the same speed as my GPS.
My IPD is set to 128 and this makes my speedo about 10% slow compared to the GPS and TunerPro/ECM - by this I mean 3mph slow at 30, 5mph slow at 50 and so on.
I have a US car with a MPH speedo, forged Quad4 diff and magnetic 24,000 ppm VSS (both from a Canadian car in KPH if that makes a difference?), and a 7730 ECM.
Now here's the dig:
I change RSC to 11070 (half of original rsc) and leave IPD the same, then what? TunerPro displays double the GPS speed, and the speedo stays unchanged!
So the RCS seems to factor the VSS signal to the ECM, and the IPD factors the ECM raw signal to the speedo, and the RSC does not factor the signal to the speedo.
I have also right clicked on IPD in Tunerpro and experimented with the formula in the the Conversion tab of the "Edit in XDF" option. This changes the "calculated value" of the IPD while the hex value stays the same. No changes to the formula have any effect on the speedo.
If the IPD could be a whole number (1.8182 in my case) then all would be golden, but it seems that the divisor has to be an integer number.
Is there no other way to fix this other than a Dakota Digital converter? What does everyone else with a 7730 and non-stock tire size do other than have an "off" speedo or use the apparently bouncy Dakota converter?

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Daviero - 88 N* GT

Call Speed Hut and ask for Wade, (he's the technial guru there)
You can probably use one of their programmable speedometers and fit it into the OEM Fiero cluster to look the same as stock.
Most all of their speedos are programmable from 500 - 200K pulses per mile.

With mine all I had to do after installation was push the button, drive a measured 2 miles, push the button again and it was calibrated.
Checking it out against a handheld GPS then verified it was accurate.


http://www.speedhut.com/gau...-auto-revolution.htm

Speedhut inc.
165 North 1330 West B2
Orem Utah
84057

(801) 221-1460 (10am - 5pm MST)

(801) 221-1468 FAX


service@speedhut.com
(All technical,installation questions etc. Pre-sales technical)
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[This message has been edited by randye (edited 07-26-2010).]

Just to double check. How is your car wired? Do you have the VSS wires going directly to the ECM? Then from the ECM's output wire run through a signal converter and then connected to the speedo? Some ECM's use a programmed tire size for speed calculation also. This is usually one of the things that is changed to calibrate the speedo.

The reason I ask is because if you did not re-route the VSS wires then they may still be feeding the speedo directly. This would display your speed based entirely on VSS gear tooth count bypassing any calibrations from the ECM. The VSS tooth count is how GM calibrated the speedo in fieros, gears were changed to adjust for different tire sizes. Any changes in ECM calibration would leave your speedo unchanged like you said was happening.

This is how I wired mine and it is dead on (double checked daily with raydar speed sign on my commute). Fiero VSS wires to PCM inputs (x2), PCM output (x1) to signal converter then off to speedo. Basically the signal converter just adds +12v bias voltage the fiero speedo needs to display anything coming directly from a newer ECM/PCM. The calibrations are done entirely in the PCM by adjusting tire size. PPM output was set to the proper value when the memcal was first programmed. BTW I'm running 225/55-16's. 1.5" increase in tire diameter over original tires and it's still dead on.

(4.9l 2240 PCM. Originally uses a 24000ppm VSS also.)

Try this....

And another explaination that may be of help...
http://www.fieroaddiction.com/speedo.html

[This message has been edited by Fieroseverywhere (edited 07-26-2010).]

quote Originally posted by Fieroseverywhere: Just to double check. How is your car wired? Do you have the VSS wires going directly to the ECM? Then from the ECM's output wire run through a signal converter and then connected to the speedo? Some ECM's use a programmed tire size for speed calculation also. This is usually one of the things that is changed to calibrate the speedo.

Yup, that sums up how mine is wired - VSS output to 7730 ECM, speedo output of 7730 ECM T'd to both the input of the speedo buffer circuit and the digital cruise input, and the speedo buffer output going to the speedo (purple I think) and the ground from the speedo (green) grounded.
I run TunerPro on a laptop on the passenger seat and change values on the go with a Moates emulator - I have tried all combinations of things in the program.
The fact that changes to the IPD in TunerPRo alters the speedo display confirms that the speedo signal comes from initially from the ECM. It seems that the IPD just has to be an interger (a non-decimal number) divisor and the IPD that would set mine spot on is 1.8182.
I think this is either a programming limitation of the 7730, or I just have not figured out a way to do it.

quote Originally posted by randye: Call Speed Hut and ask for Wade, (he's the technial guru there) You can probably use one of their programmable speedometers and fit it into the OEM Fiero cluster to look the same as stock. Most all of their speedos are programmable from 500 - 200K pulses per mile.

Hey thanks for the suggestion....this sounds good, especially if it looks stock. I'll check it out.

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Daviero - 88 N* GT

Can you or have you adjusted the tire size and overall gear ratio in the ECM yet? IF not can you change VSS gears to onw with a higher tooth count?

You could try contacting Darth fiero. He'd know how to do it.

[This message has been edited by Fieroseverywhere (edited 07-27-2010).]

Hey Daviero,

Did you ever get your speedo issue resolved? What was your ultimate solution?

I'm trying to find one that won't require it to go through the ECM (since I don't have one at all right now). Did Speedhut get you squared away? Did you build another system? Did you go the ECM alteration route?

Thanks!!

I believe he ended up using an IPD of 36 as referenced in this thread https://www.fiero.nl/forum/A...110502-2-100722.html
I used it on my car and its off, although we are both using 24000 PPM VSS. Might be due to my tires though..
Don't see any reason why the correct speed couldn't be achiever with the IPD though.

10% should be within the range of just changing the VSS gear?

I do most of my driving between 50-70 mph. I just pull the needle off, drive the average (60 mph) with GPS and stick the needle on 60. Its accurate in the range im driving. Not that important to me if its a few miles off at 25mph or 120mph. I guess if your really anal about it though............