Something I can remember reading about, but can't recall for sure....can't find my service manual. Anyone tell me which one it gets it reading from?

Tim

 

it gets it from the rear wheels, or in other cases, whichever wheel supplies the power... this is why is your tires are spinning on ice or just the road your speedo can read like 20mph when your not moving.

 

haha or if you're me in my old car stuck on a hill spinning on ice with the speedo reading 160 in 4th gear haha.

 

For the FD the speedo gets it's reading off a gear-driven gizmo in the tranny. Mine broke and I had to crack the damn thing open to fix it!

Beast

 

The speedo sensor is on the transmission output shaft. It has no idea what the wheels are doing, only how fast the tranny is spinning. The computer is programmed with the final drive ratio and the tire circumference, it then uses this info to turn each turn of the output shaft into a speed. Essentially the speed sensor puts out a frequency type signal. Higher frequency=higher speed.

 

Right! This is becoming a more and more common setup. A friend of mine who worked on Jags had a client who wanted to purchase an out of state XJR and get it in to California asap. You have to put at least 7500 miles on a new out of state car before you can bring it in - this prevents people from buying cars in Nevada (no CA sales tax) and bringing them into the state right out of the box. He hooked up an ocilloscope to the sensor and matched the 'wave' for 70 miles/hour, put the car on stands, and the car had not moved an inch but had 7500 miles on it by the end of the week.

Smart, very smart.

Beast

 

Too bad you can't take them back off too The frequency output also means you get miles whether you drive forwards or backwards.

I rented a truck to move a friend one time. The weekend rate was cheap but they wanted too much money for the mileage. Ohhh....I can fix that

I picked up the truck, drove it over and loaded it up. Then crawled under and unplugged the speed sensor. Drove the truck 500 miles round trip and returned it back with only about 30 miles showing on the odometer, and they give you the first 25 miles free

U-Haul is smart enough that they have tamper proof sleeves around the sensor so you can't unplug it or remove it. Those guys are smart Go to the dumpy places where they ain't so smart

 

crap, I was wondering why my 4.33 final drive and 255/40 on 17's in the rear was making my speedo seem off on the slow side. I'd do 60 mph for 1 minute and it took 62 seconds to make a mile. So that means my speedo is showing 60, but I'm really only doing 58ish.

Tim

 

If you're into electronics you can build a cct. to adjust the output appropriately. As Damon said, it's a pulse train output. Easy to mess with.