Speedcal and odomter question.

[makriluc]

Speedcal and odometer question.

Ok, is the speedo and odometer locked together? In other words, if I'm going at 60mph will the odometer register the right mileage?
A little back ground. I live in Canada and bought my 96 Cobra from Florida in 2008. I had to install a speedcal as the PO changed the tranny to a TR3650. I was able to closely adjust the speedcal to correctly read the right speed (100kph = 62mph)
Last week I did my first trip back in US and alongside the different interstates, I crossed a few speed indicators. I was able to cross reference my speedo readings and what those indicators showed and it was close. Then I recorded my odometer readings with the milestones alongside the interstates and theses readings are off by 17% (the Cobra shows 10 miles as I travelled 11.7 miles). So back to my initial question: is the speedo and odometer locked together?
Thanks for your input

 

[96redcobra]

The odometer does get its reading from the speedo, how far out is your speedo? I recommend using a gps or gps phone app to get your exact speed

 

[mwolson]

The odometer motor is a stepper motor driven by the processor in the speedometer. So they are locked together.

 

[Cobra285]

The speed sensor in the trans sends a AC voltage signal to the PCM which sends a DC signal to the odomitor motor you may have a bad speed sensor gear tooth problem or the wrong gear on the sensor

 

[mwolson]

The speed sensor in the trans sends a AC voltage signal to the PCM which sends a DC signal to the odomitor motor you may have a bad speed sensor gear tooth problem or the wrong gear on the sensor

Sorry, but this is misinformation. The VSS signal goes to the PCM, Speedometer and cruise control unit. There is a custom integrated circuit in the speedometer that reads the VSS signal and drives the air-core meter movement as well as the stepper motor that drives the odometer. There is no DC signal from the PCM to the odometer. I have had an O-scope on all of these signals.

 

[Cobra285]

The stepper motor is a DC motor The VSS produces a AC signel so where the conversion take place from AC to DC

 

[mwolson]

Strictly speaking, a microcontroller uses multiple DC signals to control a stepper motor. The conversion from the AC VSS signal to the multiple DC signals for the odometer stepper motor occurs in the custom microcontroller on the little PCB inside of the speedometer. We are getting very close to arguing semantics here...

 

[Cobra285]

I was just trying to give a simple explanation but there always engineers that make things complicated. Engineers don't have to fix cars if they had to they would build them different techs have to fix cars and they do a good job so just keep it simple for all the car owners on this area

 

[mwolson]

I was just trying to give a simple explanation but there always engineers that make things complicated. Engineers don't have to fix cars if they had to they would build them different techs have to fix cars and they do a good job so just keep it simple for all the car owners on this area

Can't argue with that. I resemble that remark.