I've searched all over TGO and google and read lots of great info on speedo calibration. Unfortunately my situation does not seem to correlate to anything I'm finding. I have an 88 GMC 1500 pickup (I know this is not a truck forum, but running the EBL, I hope to get some help). I have the 4L60 transmission with the electronic VSS mounted in the back of the tranny. I have heard the term "VSS BUFFER" as well as "DRAC" and I've searched high and low under the dash, between the trans and ECM and have come up empty handed.

I run rBOB's EBL and I have been unable to find any documentation regarding speedo calibration there so I assume there is a device somewhere that is taking the raw signal from the trans before feeding it to the ECM.

My situation is that I've just gone from 3.08 to 3.73 rear gear ratio and the speedometer is no longer accurate.

I know on a 91 1-ton I've seen jumpers on the back fo the physical speedo itself but I have not yet pulled the speedo out of the dash to see if this is the smae. Any help would be appreciated!

 

your DRAC is part of the instrument cluster.
i believe it uses a digital output to the ECM on B17 of the cluster connector. my book shows the wire to be brown with a white tracer and changes to brown with no tracer at connector C203.
the output signal to the ECM shows to be 2000 pulses per mile.

look for a piece of plastic sticking off the back of the cluster.
its called a programing clip.
it shorts certain pins to ground to calibrate the buffer to tire size and gear ratio.
sorry, i don't have any info to help you calibrate it.

 

DENN_SHAH,
This is actually extremely helpful as I did a search based on the info you provided and found a complete DIY on a site called 67-72chevytrucks.com. He covers the input ratio, pinout of dash cluster, etc. Thank you for the help!