1994 G-20, 4.3L automatic, no frills TelCo van.
30 x 9.5 tires.
Pulled stock 3.73 gears & installed 3.08 gears w/ Auburn rear posi.

According to http://www.tbichips.com/drac/ replacing the jumpers with dip-switches should allow me to set the speedo correctly w/ the new gear ratio but after installing the dip switches there is no effect. Regardless of switch setting the speedo remains off by approximately 20%. (If the speedo says 45mph I'm traveling closer to 60mph.)

Q: Do I need to swap the speed sensor out from the tranny? On an old cable driven speedo I'd just swap in the proper gear and be done... with this set-up it was my understanding that I needed to mod the VSSB only but obviously I'm missing something.

My VSSB has two connectors - one of which has a matching pigtail which must be plugged in for the gages to work (inc speedo) but the other small (4-pin) connector does not have a pigtail that I can find beneath the dash.
Q: Can anyone tell me what that smaller connector is supposed to go to?

Any input would be appreciated! (I get reimbursed at .50 per mile so this speedo error is killing my wallet! )

 

Bumpity bump.

 

I had a similar problem that was rectified when I installed tires 1" larger. BUT!!!!!!!! It was a cable driven speedo. With an electronic speed sensor I d think you d have to reprogram the TCM to work with the lower rear end gear ratio. Even still, at what speed does your van do into OD?

 

I have had the VSSB modified to correct the speedo - instructions said nothing about reprogramming the TCM. According to the website: http://www.tbichips.com/drac/ I should be able to calibrate the speedo for ANY gear ratio with the dip switches installed...

I added the tire size to first post but tires are almost identical to original "D" range tires (no longer manufactured). Speedo was dead on with them prior to gear swap.

With the 3.08's OD slips in so gently that ya have to pay attention... but with the speedo error no way to say what the MPH is.

Any ideas people?

 

Response from the website that hosts the VBBS mod above indicated that the 'spare' connector "MAY" be for the heavy duty tranny option and that not all vehicles use it.

Playing with dip switches got speedo to read fast now... I'm thinking a harness issue since I had to pull & replace the box a few times to change dip settings again.

Will update as I learn more in case someone else needs the info.

 

Doing the VBBS (or DRAC) mod does work. Calculated settings for the dip switches / jumpers didn't match up in my case for unknown reasons but was able to home in on correct settings by trying "high" then "low" then "split the difference" repeated until speedo ended up dead on compared to GPS.

 

Well, a wiring harness issue wouldnt necessarily make the speedo go faster. I d think it d make it go slower or inop. High resistance from corrosion or bent pins.

I think the piggy tail is for special models like a Cadillac with a viscous clutch torque converter or HD Transmission with a high stall rpm Torque converter(maybe a 2nd ground for lockup solenoid on the VC or an additional input for 2 gear start?)

Until you get it figured out Im going to stick with live action TCM reprogram.

 

My speedo is dead on now. All systems go.