88shortbedchevy
Newbie
Hi there. My 94 Blazer has developed a problem with the speedometer that has stumped me. I'd consider myself a capable mechanic and I have done engine swaps, an lq4 into an 88, that has required extensive wiring, but I haven't been able to figure this one out. Looking for some help.
Here's the run down.
1994 Blazer, 5.7 TBI automatic 4L60E, 234k miles.
The problem is on the highway, above 60 mph the speedometer will begin to flutter all over the place, it only ever goes down. Specifically it will jump from 70 mph to 50 and back and forth, while going 70. It only does it when it's above 85 degrees outside and typically with the AC on. The problem does not go away when the AC is off however.
I have changed the VSS on the tail shaft of the transfer case. I have checked the wiring from there to the firewall. I verified the shield on the twisted pair is grounded. I picked up several DRAC modules from the junkyard and installed one. Still did not fix it.
I have checked all the grounds between block, chassis and body.
I believe the issue is between the VSS and the DRAC module because the transmission will downshift on the highway when this occurs. This tells me the ECM is seeing this fluctuation in signal. My understanding of this circuit is that the VSS feeds the DRAC module that splits the signal to the ECM and gauge cluster.
There has been some discussion that the ABS wheel speed sensors on the front wheels could also be involved. I went over these as well, didn't appear to do anything.
The strange thing is it doesn't do it around town. The truck works perfectly when your not on the highway for any length of time.
I have done a lot of research on this prior to getting to this point and I'm hoping someone can offer some wisdom.
Thanks.
Here's the run down.
1994 Blazer, 5.7 TBI automatic 4L60E, 234k miles.
The problem is on the highway, above 60 mph the speedometer will begin to flutter all over the place, it only ever goes down. Specifically it will jump from 70 mph to 50 and back and forth, while going 70. It only does it when it's above 85 degrees outside and typically with the AC on. The problem does not go away when the AC is off however.
I have changed the VSS on the tail shaft of the transfer case. I have checked the wiring from there to the firewall. I verified the shield on the twisted pair is grounded. I picked up several DRAC modules from the junkyard and installed one. Still did not fix it.
I have checked all the grounds between block, chassis and body.
I believe the issue is between the VSS and the DRAC module because the transmission will downshift on the highway when this occurs. This tells me the ECM is seeing this fluctuation in signal. My understanding of this circuit is that the VSS feeds the DRAC module that splits the signal to the ECM and gauge cluster.
There has been some discussion that the ABS wheel speed sensors on the front wheels could also be involved. I went over these as well, didn't appear to do anything.
The strange thing is it doesn't do it around town. The truck works perfectly when your not on the highway for any length of time.
I have done a lot of research on this prior to getting to this point and I'm hoping someone can offer some wisdom.
Thanks.
