what controls the speedo? is it a cable like on older cars or is it all regulated through the pcm. i ask because my friend messed with some stuff on my pcm last night and this morning on my way to work the speedo hit the high mph peg and never moved. could i resend the stock tune to the pcm and it be fixed or do i have bigger problems? thanks for info in advance


brian

 

I believe its measured by a sensor on the transmission and sent to the pcm.

 

The vehicle speed sensor (VSS) on the tailshaft housing of the tranny sends pulses to the PCM. The PCM converts the pulses to MPH, using programmed values for rear axle ratio and tire diameter. You can screw up the speedo reading by putting the wrong numbers into the fields for these numbers, or by making the PCM think that is has an A4 (40 pulses per DS revolution) instead of an M6 (17 pulses per DS revolution).

 

well i fixed the speedo the needle was just stuck on the face around around 160-165 mph. i took my dash apart and moved it back to where it should be. thanks for the info guys

 

Ok, I guess that is the issue my brother is having with his speedo. I thought it might be because he stuck my 93 T56 into his car.
His car was an M6, converted and programmed to an A4, and is temporarilly an M6 till the A4 is fixed.
45mph on the speedo is about 120+ in real speed.

 

What did you bury the needle on your speedo?

 

yeah but it was because of a tuning mishap