180 kph speedometer (Page 1/2)
sdgdf JAN 11, 10:34 PM
Canadian Fieros came with a kph speedometer. The 120 mph speedo is miscalibrated in my car and figured out its calibrated as a setting in the ecu (3800sc). Anyone know if I could reflash the ecu and calibrate a 180 kph speedo so its accurate in mph? Back in the early days of the forum I saw some people do this with the crystal and at one point a kit was sold but that's all history now.

Right now it looks difficult even getting one of these speedos, are they that rare at this point???
theogre JAN 11, 11:46 PM
OE Fiero Speedo is Not run by ECM/PCM.
VSS goes directly to them and resistors and capacitors "program" the driver chip to move the needle. (Same thing as Fiero Tach.)

Your problem(s) is likely:
Wired speedo to ECM and done wrong.
ECM is program wrong.
Speedo gets right "data" from VSS or PCM but speedo is bad same way as Tach reads way off.

If have the 3rd problem... fixing others won't help except maybe a short time.

ECM and PCMs do have "speedo" programing because trans ratio and tires affect the trans shift points and more.
You need a scanner to see speed in the data stream and GPS to have something as reference just to see is an error or not.

But note speed in most GPS consumer units are Not very accurate so can't say GPS speed must = vehicle speed.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

sleek fiero JAN 12, 10:50 AM
Am I crazy but weren't there several optional drive gears for the VSS that were optional depending on final drive ratio ?
sdgdf JAN 12, 11:26 PM

quote
Originally posted by theogre:

OE Fiero Speedo is Not run by ECM/PCM.
VSS goes directly to them and resistors and capacitors "program" the driver chip to move the needle. (Same thing as Fiero Tach.)

Your problem(s) is likely:
Wired speedo to ECM and done wrong.
ECM is program wrong.
Speedo gets right "data" from VSS or PCM but speedo is bad same way as Tach reads way off.

If have the 3rd problem... fixing others won't help except maybe a short time.

ECM and PCMs do have "speedo" programing because trans ratio and tires affect the trans shift points and more.
You need a scanner to see speed in the data stream and GPS to have something as reference just to see is an error or not.

But note speed in most GPS consumer units are Not very accurate so can't say GPS speed must = vehicle speed.




I was under the impression that the pcm can send a signal to it that can be calibrated so it’s accurate. I’ve seen YouTube videos of calibrating the tach where you can use a scan tool to read the RPM to calibrate the tach correctly.

Currently the speedo seems to read 5 mph too fast @ 60mph. And my tach doesn’t work which hopefully I’ll figure out soon. The 3800sc has a tach signal wire that should just wire up to the tach through the c500 connector. Last I tried troubleshooting it I had signal at c500 but couldn’t tell exactly what I had at the gauge cluster. It got too late that day to go any further.

[This message has been edited by sdgdf (edited 01-12-2023).]

sanderson231 JAN 13, 12:09 AM
It depends on how the speedometer was wired for the 3800 swap. On the 3800 donor car, the VSS was wired to the PCM. There was a output from the PCM that went the speedometer. This signal can be conditioned with simple circuitry to drive the Fiero speedometer. It's likely this is how your car was wired. I have a 4.9L Cadillac in an 1988 Fiero that I wired this way. For the 1995 Pontiac Bonneville with the 3800, the speedo output wire is D15 on the white PCM plug. There is a constant in the PCM to scale the speedometer output signal.

Tell me the year of the donor car and I'll explain what it takes to change the constant in the PCM. 93 and earlier are different from 94-95 which are different from 96+.
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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

[This message has been edited by sanderson231 (edited 01-13-2023).]

theogre JAN 13, 12:34 AM
You can't assume PCM controls the Speedo.
You or someone else must follow wiring setup.

Even then PCM needs scanning to see is it to have same error.

Just having different tires vs doner vehicle can cause 5 mph.


quote
Originally posted by sleek fiero:
Am I crazy but weren't there several optional drive gears for the VSS that were optional depending on final drive ratio ?

Yes.
Even if you "read" speed on the axle itself... Total Tire Diameter can change actual road speed.
So a gear change is needed for "dumb" speedos whether cable type or VSS system w/ direct to speedo or older ECMs.

Many to all OBD2 PCM have some kind of Tire Size data because hard to change the speedo gears on most old cars and so most never even tried.
Even w/ OBD2, you often need High $ scanner and often a High $ monthly/yearly subscription to access this function or pay Dealer Price for them to change tire size. W/ engine swaps, most Dealers won't help you.
J Gunsett JAN 13, 10:07 AM
First you have a PM. You stated 5mph off at 60, how far off is it at 20. Is the needle steady or jumping around as you drive?
sanderson231 JAN 13, 10:45 AM
The VSS on the 4T60E and 4T65E that came with the 3800 S/C are not gear driven. It is a reluctor type set-up. They put out an ~22,000 pulse per mile signal that is sent to the PCM. The PCM converts the signal to a 4000 pulse per mile square wave. This can then be converted to a 4000 pulse per mile sine wave that the Fiero speedo likes. The only way to adjust for different tire size is to change a setting in the PCM.

If the transmission is a Getrag 5 speed then it may be using a gear driven VSS. However at some point GM I think about 1990) GM started using the reluctor style VSS's in the Getrag 5 speed. So every FWD Getrag I have seen uses the reluctor style.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

lou_dias JAN 16, 05:16 PM
with 5 speed, the VSS was spliced into the PCM and Fiero speedo.
With the 6 speed, the VSS only goes into the PCM and I need to use the 2000PPM out if one exists on the 4.9 PCM and covert it to a sine-wave/analog signal. I was able to do this with the 7730's 2000PPM output to make it drive the Fiero speedo WITHOUT a Dakota-Digital unit using a divisor of 8.

using 2000ppm output instead of 4000ppm is like an additional divisor by a factor of 2. Since the 6 speed outputs 60000ppm, using the 2000ppm with a divisor of 7 or 8 acts like a divisor of 16 (60000/16) which puts it close to 4000ppm... I generally run over-sized tires so 8 works better for me.

[This message has been edited by lou_dias (edited 01-16-2023).]

sdgdf JAN 16, 05:20 PM

quote
Originally posted by sanderson231:

It depends on how the speedometer was wired for the 3800 swap. On the 3800 donor car, the VSS was wired to the PCM. There was a output from the PCM that went the speedometer. This signal can be conditioned with simple circuitry to drive the Fiero speedometer. It's likely this is how your car was wired. I have a 4.9L Cadillac in an 1988 Fiero that I wired this way. For the 1995 Pontiac Bonneville with the 3800, the speedo output wire is D15 on the white PCM plug. There is a constant in the PCM to scale the speedometer output signal.

Tell me the year of the donor car and I'll explain what it takes to change the constant in the PCM. 93 and earlier are different from 94-95 which are different from 96+.



It’s from a 2000 Grand Prix GTP.