sometimes ill be cruising, and my speedometer will just drop from like 30 mph to 0 and the car will rev up by like maybe 100-200 rpms... it's not a lot, but it's noticeable. then, on it's own, the speedometer will come back to the normal reading. if i have my cruise control on and my speedometer drops, the CC disengages... what could be the problem thats causing my speedometer to drop all of the sudden and for no reason? thanks guys

 

We'll need to know the year and model before helping. Pontiacs got a VSS (no cable) speedometer in 1986, Chevys got the VSS only setup in either 1988-89, as I recall.

I would suspect you are losing the VSS signal for some reason, but the diagnosis would be different depending on the system you have.

 

i've got myself a 1990 iroc-z 5.7L tpi with a 700r4 tranny...

 

My 89 IROC has a cable speedo and it was really starting to act shady so I pulled the cable out and lubed it real good. No more noise and the speedo needle doesn't bounce around. I later found out that you can buy a brand new cable for pretty cheap. Like Vader said though, I would suspect that you do have a VSS, simply due to the corresponding RPM jumps.
Hold on....you said the speedo needle drops, what about the RPM's? Are they just jumping or are the RPM's dropping off as well?

 

they jump about 100-200 rpms till the speedo comes back... but my tach is f*cked up... it doesnt read correctly. it says my idle is at 1500 and when i redline, it goes into the black, way past 7... i think i need a new tach lol

 

you have a VSS. no cable.


its your VSS. they're like you decribe when they start to go bad...


heres a link to a lil page i made about it

http://josh.swoca.net/board/mrdude/vss/

 

If you remove the VSS, make sure the plastic gear is intact and is not slipping on the VSS thimble. Check your connections as well, since an intermittent connection will do the same thing. You should have an error code as a result of a lost VSS signal.