Speedo drops with RPM and won't go over 70. WTF?

[badscirocco]

Hey guys this is my first post on here. I did use the search to find an answer but came up empty handed. Can anyone provide an in depth answer? This just started and it is quite annoying. The truck in question is a 1996 GMC 1500 shortbed 2WD 5.0 vortec with the 4L60E trans. It's totally stock with the exception of a flowmaster exhaust and a suspension drop. If I hold at 2200 RPM in OD, I know I am doing approx. 70. But when I drop the hammer the speedo drops too and also will not climb above 70. Any ideas?:dunno:

[blackbodylift]

Speed sensor maybe..It's located at the back of the transfercase.

[DarkCharisma]

He doesn't have a transfer case. I don't know where the speed sensor is on a 2wd, but I'd start with that.

[xZoomerZx]

Are you saying the vehicle actually slows down beyond a certain throttle position? i.e. Hammer down? Check the O2 sensor. I had an 88 GMC van that wouldnt go over 60, slowed down if you gave it more throttle, and sucked the gas tank dry. Turns out the computer thought the engine was running lean (bad signal from the O2 sensor) and started dumping fuel to enrich the mix. No SES light, no hint at idle or when cold, only after the sensor reached operating temp (600°) did the problem present itself at highway speed.

[badscirocco]

Ok guys. Here's a new one. When getting on the freeway I had the pedal to the floor and it shifted 1-2 ok but at the top of 2nd it just went to the rev limiter without shifting into 3rd. As soon as I lifted it shifted. It pulled hard all the way to the RL but wouldn't shift. Now I tried this a couple of times and it did the same thing every time. Now if I lift just a little it will shift but not at W.O.T. Could this be related to the speedo not going over 70?

[badscirocco]

Are you saying the vehicle actually slows down beyond a certain throttle position? i.e. Hammer down? Check the O2 sensor. I had an 88 GMC van that wouldnt go over 60, slowed down if you gave it more throttle, and sucked the gas tank dry. Turns out the computer thought the engine was running lean (bad signal from the O2 sensor) and started dumping fuel to enrich the mix. No SES light, no hint at idle or when cold, only after the sensor reached operating temp (600°) did the problem present itself at highway speed.


Sorry missed your post. No the veh speeds up like usual but the speedo doesn't register the increase. So I could be doing 90 and the speedo is still reading 70. :whatever: Can you see where this might cause problems? LOL

[badscirocco]

Anybody? TTT

[badscirocco]

I guess no one has ever had this problem from the lack of response. Well thanx for the ideas anyway.