Ok here goes-1991 Olds. Bravada [Blazer] My speedo will work intermittly, the needle will start moving up with my speed, will get to about 35-40mph then it will fall back to 0mph then in a few seconds it
will go right back up to the speed you are going, but in doing so it also
will cause the trans. to downshift or as it feels like its going in and out
of AWD, I did a search on speedo-sensor and nothing I have seen
covers this problem, I seen a couple threads about the signal from the
trans going to a box [white I believe] under the dash to convert for tire size on the vechical. I just want to get this fixed, because I'm sure this is not good for the transmission or tranfer case, ---and by the way it does this all the time, even at speeds of 50 at this point it will only drop to about 30 then back to 50, I don't want to go any faster for fear of tearing something up, because at the higher speeds it seems to shuttter
or thats what it feels like when the speedo drops to a lower reading,
must be sending a signal to ecm or trans. I'm lost on this one.
Someone must have had or heard of this :banghead:
Thanks, Jim

Sounds like you might have a speedometer cable gear that is partially stripped and not staying engaged when under a load.
Simple to check, just locate the speedometer cable under the truck and unscrew it from the transmission. Check the condition of the plastic gear to see if the teeth are in good shape. Replace if necessary.
If you're unsure about the condition of the gear teeth, then make sure you already have a new one to campare it to from a parts store (buy one and take it home with you, then remove the old one to do a side-by-side visual inspection. If it's undamaged, just return the new gear for a refund).
If the gear turns out to be damaged, replace with a new one and make sure to replace the speedometer cable as it may be the root cause of the damaged gear.
Not sure if that's what's going wrong with your Bravada, but that's the first thing I would check.

Does not have a cable, it has a speed sensor on the trans, but thanks anyway, I thought it had a cable also, until I looked at some other threads, I just don't want to buy something I do not need, besides if there is an answer, this forum is where I will find it, I've learned alot here.

More than likely it's your vss. They're cheap to replace, should fix your problem, if your problem continues you will need a new DRAC module.

Thanks for the reply rlith, I will try the vss, then if I need to I will replace the drac module, which I'm sure is the box up under the dash? At any rate after that I will post which fixed it.
Thanks, Jim

The drac is bolted to the ecm, its the smaller box...