what makes the speedo work?

hi my speedo isn't working and trying to find out what controls it and where.
thx

what year is your truck?
if by chance it's a 94-01, there are two spees sensors, one on the trans, and one in the rear differentail.
it could be either one of those, or it could be a blown fuse or even a bad gauge.

its a oo

well, first off, I'd check all the fuses to make sure that they are good. in some cases, the speed sensor in the rear dif. can get covered with small metal shavings(from normal gear wear, and clutch material if you have a LSD), you may want to try pulling the sensor and cleaning it. if cleaning it dosn't work, I think those sensors are between $25-$50 from the dealership, but you may be able to pick one up cheeper at an auto parts store.

well, first off, I'd check all the fuses to make sure that they are good. in some cases, the speed sensor in the rear dif. can get covered with small metal shavings(from normal gear wear, and clutch material if you have a LSD), you may want to try pulling the sensor and cleaning it. if cleaning it dosn't work, I think those sensors are between $25-$50 from the dealership, but you may be able to pick one up cheeper at an auto parts store.

any idea onhow it comes out
thx for your help

yep, there is one bolt/stud that has to come out, then the sensor just pulls out of the dif housing.
here's how it was on my truck, nut, break line clip, nut(second nut is actuaily part of the stud), and then the sensor. just remove the first nut, and break line clip, then remove the stud. the sensor may be a little tough to pull out, I found that it helps to twist it side to side wile pulling straight up. there is an O-ring that seals the sensor to the housing, so be carefull not to damage that when you put the cleaned or new sensor back in.
hope this helps,
Keith