Figured after all the research I've been doing on this, I ought to put it down somewhere just as much as reference/reminder for myself, as for anyone else. I'm only focusing on our 1991 and older body style trucks, although this is applicable to quite a few other vehicles. A lot of this will probably be helpful for folks going with other non-TBI injection setups as well. VSS is typically a hassle to run, luckily there are quite a few options for us.
My starting point now is a 1987 1227165 305 manual trans ECM/TPI setup installed on my 355, 465/205 in a 1986 K5 body. I am converting the cable drive 205 in it now to a 1991 205 which doesn't use a cable.
Up to 1989 in the "old style" trucks that we have, Vehicle Speed was sent to the cruise control module and ECM (if present be it CCC or TBI) from a buffer (yellow or light green typically) box that has a ribbon cable with an "eye" on the end of it, that is screwed into the back of the speedometer. This is a 2 pulse optical unit. For every revolution of the speedometer cable, there are 2 pulses sent to the ECM. GM speedometer cables are calibrated to turn 1000 revolutions (or 1001 depending on who you ask) per mile. So the input the ECM or cruise module sees, is 2000 pulses per mile. (PPM)
When our trucks (1990-91) went to the VSS mounted in the transfercase in 4wd apps. (241 or 205) or transmission in 2wd apps., (which means some cars and S10's/Astro's are in the SAME situation as our 1990-91 2WD trucks) the situation changed dramatically, from 2 pulse per cable revolution, to 40 pulses per OUTPUT shaft revolution. The 1990-91 VSS is also a different output than the optical unit...AC sine-wave instead of DC square-wave. However, the purpose of the DRAC is to take that AC signal, and convert it to all the other outputs needed: Cruise, ECM, ABS, and speedometer, which is what the older VSS system output originally.
The beauty of DRAC in my opinion is that you can run the '90-91 digital speedometer, is a fairly easy swap into the 1980(?)-1989 trucks, eliminates the speedometer cable, and is quite easy to manipulate to account for changes in tire size and gear ratio changes.
If there is more info people want, I'll do my best to answer any of it, but so far, this is all I can think of. It's not like I determined all of this myself, so here are all the sources if you'd like to do the reading yourself. I understand this is a very niche topic, but thats ok. At least it can be referenced for the few times it may come up. If I've made any errors, please point them out.
DRAC info I have read that the chart for the jumpers there is wrong, but the calculator is correct. FYI.
DRAC calculator from above site
Tire circumference calculator for jumper calculations
Excellent DRAC article w/pinouts
http://diy-efi.org/gmecm/ DIY-ECM site. If you search the archives, there is very little that can't be answered. ECM pinouts to discussions on various aspects of ECM operation, all here.
Thirdgen.org
Lots of good info here. Search search search. You will be amazed at how relevant the car stuff is to the truck stuff. A few people sneak truck questions in, but non-Camaro comments/questions are generally frowned upon...just don't talk about application.
Thirdgen.org will easily get you into doing your own PROM burning, it would be nice to see more of that kind of stuff going on here, so people in similar situations could talk about it, and not be afraid of talking about application. I'm awaiting a package of PROM burning stuff right now as a matter of fact, can't wait to make some needed changes.
More Info: TBI Astros that use DRAC are good donors.
First off, the connector on the tranny of at least 1989 Astro's is the SAME as the ones used in (at least) the 1991 1 ton 205 applications. I suspect all 40 tooth reluctor setups within reason use the same connector on the tranny/t-case. The TBI Astro DRAC is located in the dash, just to the right (as you sit in the drivers seat) of the instrument cluster. With the dash trim off, there is a cavity in the dash (heater vent maybe?) that you can look in, and see the white DRAC tucked up in. As you look in the cavity, it is on the left side, towards the driver. It's just clipped in place, you just pull the DRAC towards the front of the vehicle, and the clips will let go.
Wiring is very simple. Stock VSS on cable drive setup is 3 wires. 12V ignition, ground, and one wire to ECM, or without ECM, cruise control. The DRAC uses those three wires as well. As a matter of fact, the wire to the ECM is the same color on the DRAC as earlier (pre-'90) VSS setups, brown. The other two wires might be same color, no big deal, the pinout is as I posted. The other two VSS inputs are off of the t-case or tranny, same connector, and the wiring will match up with the DRAC wiring. The Astro ones I grabbed also use the RWAL pin, I just removed that wire. Cruise is on the seperate plug, no cruise, only one connector to use with the DRAC.
