$42 gear ratio

**Six_Shooter** · 11-07-2022

I think I'm losing my mind....

I have checked several $42 XDFs along with some disassemblies, and can not find the parameter for rear end gear ratio.

I need to set up a chip for someone I know that had an '8747 go bad, and will be replacing it with a '7747, and need to make sure that everything is set correctly for the vehicle, and a big one being the gear ratio so that vehicle speed is correct. I'm sure I've had this parameter before, but unless it's named something odd, that I don't remember (It has been a bunch of years since I've worked with anything '7747), so am I missing something here or is it just not there, and requires every speed/gear based parameter to be set individually?

**ralmo94** · 11-07-2022

Isn't it set by pulse per mile?
(Tire revs per mile * axle ratio ) * vss teeth
I can't find it there either, or even a sensor tooth count

Tire specs on a tire website is usually an easy place to get revs per mile for tire size.

Is the swap vehicle using a. Drac? I think that is the only was as drac gets vss and sends to ecm?

**PlayingWithTBI** · 11-07-2022

There's a VSSB on the back of the instrument cluster with jumpers to set gear ratio and tire size. Then it sends the signal to the ECM, CC, and Speedometer. Here's a write up.

https://67-72chevytrucks.com/vboard/showthread.php?t=313815

Edit: Some people solder in dip switches to make it easier for future mods.

**Six_Shooter** · 11-07-2022

Yes, I had a brain fart with this, I completely forgot about the DRAC, and that it sets the PPM for the ECM (and cruise, etc). I was reminded about the DRAC when I mentioned it to someone I do EFI and electrical work with.

I can continue on now without worry. :)

**tayto** · 11-07-2022

747 era would have the "green box" buffer, like what's in my squarebody. i seem to remember a parameter to choose between optical and speed sensor?? DRAC needs a 40 pulse per revolution signal.

**1project2many** · 11-07-2022

The 7747 expects 2,000 PPM square wave on the Optical Sensor input or 4,000 PPM sine wave on the "Magnetic VSS" input. Signal type is selectable in software.

7747 used in square body trucks with speedometer cable received two pulses per cable revolution from an optical sensor on the back of the mechanical speedometer. These trucks had a circuit housed in a green box that provided the correct voltage to the speed sensor and ECM.

7747 used in 2WD square body trucks with electronic speedometer (88-91) received 2,000 PPM speed signal from yellow "4 OUT" buffer box. 4 OUT buffer box converted 4,000 PPM A/C signal from sensor to produce 1:2 square wave.

7747 used in 4WD square body trucks from 88-91 received 2,000 PPM speed signal from DRAC. Signal was sent to to "Optical" VSS pin.

7747 used in early GMT400 trucks, 88-90, received a speed signal created by the ancestor to the DRAC which was found on the reverse of the speedometer. There was a removable clip on the back of the speedometer that calibrated the speed divisor to create a 2,000 PPM output to the ECM.

7747 used in 91-92 GMT400 trucks received a speed signal from DRAC. DRAC was calibrated to produce a 2,000 PPM signal for ECM.

To the best of my memory (less than perfect for sure) no OE applications using the 7747 used the 4,000 PPM magnetic input.

Edited for specificity.

HTH

**stew86MCSS396** · 11-08-2022

Your statement would be true if the qualifier was "as it applies to the 7747," but one of my very first projects was with the 7730. One of the changes that I learned to do was to change the mag to optical which is what my G-body has behind the speedometer head.