i have a c3 81 corvette , my lock up for the trans locks way to early , the speedometer cable is a bit bad .
i will replace the cable , but i was also looking for the speed sensor that is attached on the back of the speedometer.
the one i have is original 81 gm #25007217 , the only one i can find is for 82 , gm # 25007278 .
it looks the same , the connector and all , but mine is blue and the 82 is yellow.
81 have a special ecm , i believe they changed it in 82 . not sure but maybe early ones had the same as 81
anybody knows if the 82 , gm # 25007278 will work on 81 ?

here is a picture of my 81 convertible , rebuilt from coupe in california in 81 , exported to sweden in 89

 

25mph is stock I think.
yours coming in lower?

 

ok , maybe its correct then , its about that speed its locks , lots of others i saying around 40 or something.
i had a strange issue when i bought the car , the engine , charge and choke warnings was light up , but very low , and sometimes full light and then off , and it did it like that all time .
lots of people told me itwas behind the dash , ground , meters , fuses and lots of other things.
i bought the big 81 bible , gm 81 shop manual and found out that the charge signal from the alternator was feeding lots of things behind the dash.
it only sent about 12 v on the warning connector and 14 to the battery.
i changed the alternator and everything started working ok , but the lock up was not working , so i started rteadiung the bible again and found a cutted wire on the brake switch.
fixed that and now it locks somewhere around 25 mph .
but not every time , but my speedo cable is bad and the meter sometimes jumps from 0 to 25 , up and down .
i will buy a new cable , a better chip for ecm , better thermo and oxygen sensor .
have you tried any chips like "81 hypertech power chip - automatic thermo master stage 2 ?
i costs about 140 $ , but then shipment ,taxes and all to sweden is not a low price thing .....
but if the lock up is correct i will save 60 on not buying a new

 

yes, chip.
still 25mph at light throttle.
you can delay with more throttle but 25 is correct

 

really big thanks calwldlife, your answers really helped me alot .
i have asked a whole lot of people on different places about the lock up , you are the only one with a straight and good answer .