94 Chevy Trck question about Computer

[Old Chevy Lover]

I was wondering if there is anyway to modify the shift points on my 94 truck. I have 36" tires on it, and would like to get my speedometer closer to being correct so the torque converter will lock up sooner... Just thought I would ask Thanks!

[ThreeQuarter]

The way to correct your speedometer is to replace the Vehicle Speed Sensor Buffer (DRAC) with one made for a different gear ratio. Since the speed sensor sends an electronic signal according to how fast the gears are turning in the tranny (transfer case in a 4wd), the system requires a signal buffer to correct for gear ratios. That's what the DRAC does. If you remove your glove box liner, you'll find the DRAC is located just below the opening, sometimes stuck to the inside-backside of the dash. It's the little white plastic box with two wires going into one side.

Since you said you have big tires, I assume your speedo is reading slow, in which case you'll need a DRAC made for higher gears (lower numerically). You can play with some ratio calculators to find the best fit for your tire size and gear ratio. Good luck.!

Try the "speed calculator" on this page: http://www.clarks-garage.com/shop-manual/trans-01.htm

[Old Chevy Lover]

where are these available at? are they expensive?

[ThreeQuarter]

This would be a GM parts house or wrecking yard part. They are just a circuit board with no moving parts so I don't think they wear out. If you end up at the wrecking yard to find the DRAC you want, look inside the glovebox of the trucks to find their RPO list; it will tell you the gear ratio on the truck. Here's the RPO codes for the ratios you're likely to find:

GU4: 3.08
GU6: 3.42
GT4: 3.73
GT5: 4.10

Let me know if you have any more questions.

[Old Chevy Lover]

Thanks.. you have been really helpful.. I'm assuming that these would all the same for 2 or 4wd... what years would be compatible.. 93-?

[ThreeQuarter]

Hmm...I was going to say that I bet 2wd and 4wd are the same, but since the Vehicle Speed Sensors (VSS) are in different places I'm not sure. The VSS just reports how quickly the gear teeth pass by the sensor, so it's possible that the transfer case sensors (which work off a different gear) use a different scale altogether than the 2wd ones in the tranny, and therefore need a different buffer. Then again, they all report from a place in the driveline that is behind all the tranny and transfer gears, so they might be the same. I'd love to know. The best way to find out is to call your Chevy house and ask them if a DRAC for a 2wd truck with x.xx gears is the same part number for the same year truck with the same gears, but 4wd.

As far as years to choose from: I'd stick to 92-94 trucks, with the newer needle-type gauges. I *think* the 88-91 trucks (with the dial-type gauges) have a DRAC built in to the instrument cluster.

[Old Chevy Lover]

but didn't up to 92 trucks have a 700r4 tranny?

[ThreeQuarter]

Yeah, in '92 the automatics technically still had an ECM computer system and not a PCM like '93-up. But they're very similar computers, just that the PCM also controls the tranny shift points. I think the VSS circuit would be unchanged for the 4L60E. FWIW, if I were shopping for a replacement DRAC for my '92, I would not be afraid to grab one from a '93-'94. It might not work, but I'm betting it will. Let me know.