DRAC/VSSB Questions...

[DavidBraley]

Sorry for the length of this post. I’m sure I’m making this all harder than it needs to be…

Everything I’m writing here is based on my present understanding of information I’ve read from several forums so far. PLEASE correct me if I am wrong! I really want to understand this stuff...

My project is based on a modified 7427 PCM for MPFI use. The engine for this project is a 489 BBC with a 4L80e automatic transmission. I will be building my own intake manifold for this project inspired by the look and feel of the earlier TPI systems. It’s all getting installed in a 1948 GMC 2-Ton flatbed truck.

I need some help getting clearer in my understanding between what some call the DRAC (1992-1993?), and what some call the VSSB (1994-1995?). It’s important for me to also say my questions concern only GM OBD1, the DRAC/VSSB modules that are a little white plastic stand alone box, and NOT something that’s built into the back of the speedometer. I’m also only concerned with the model of DRAC/VSSB that receive a 40 pulses per revolution output from the VSS commonly found in the 4L60e/4L80e automatic transmissions.

It’s my understanding the earlier DRAC’s have 7 possible jumper locations on the circuit board for calibrating the speedometer with a known tire diameter and rear axle ratio. The later VSSB’s have 14 possible jumper locations on the circuit board for calibrating the speedometer with a known tire diameter and rear axle ratio.

I’ve also read the later 14 pin VSSB’s can be used in place of the earlier 7 pin DRAC’s, but not the other way around. What I don’t seem to be able to find or understand is why. I’m guessing it has something to do with the word “Buffer.” It’s possible the earlier DRAC’s do not have a buffered output? If it’s that simple, I could possible create an optically buffered output for the earlier DRAC outputs?

You might be thinking, “Why not just use a later VSSB for your project David?” That would be great! Except I’m not sure what I’m looking at when making purchases on ebay. So far all the DRAC/VSSB’s I’ve purchased are turning out to be the earlier 7 pin DRAC’s. It’s not practical for me to visit a Pull-A-Part yard because of where I live. If possible, I try to get everything I need from an automotive parts supplier, or places like ebay and Car-Parts dot com.

What I would love to understand is if the earlier 7 pin DRAC will work with the later 7427 PCM? If not, can anyone help me find a published list by GM (or?) that I can lookup the three letter codes commonly found on these DRAC/VSSB modules. That way I can get an idea of what type and year of vehicle they came out of before I hit the “Buy it Now” button.

Anywho, thanks in advance for your thoughts and help with this.

David

[dave w]

I've done several DRAC / VSSB setups. Either type, 7 pin or 14 pin, will work for 4L60E / 4L80E transmission control using the 16197427 PCM. I like the 7 pin, because I can get dip switches from Mouser: http://www.mouser.com/ProductDetail/...3aoqUyWh02E%3d Send my a Private Message (PM) if you are interested in the DRAC / VSSB I posted pictures of. dave w

[Six_Shooter]

You are correct you can use the later VSSB in place of the earlier DRAC, but not the other way around.

I discovered why this is recently...

A little background on how I came to find out why:

I have a car where I have performed an engine swap on and installed a newer trans that uses a VSS without a speedo cable drive, so I had to come up with a solution for speed readout. So far I have an Arduino with an LCD screen that currently shows me my speed. Well that output from the VSS, is a very high pulse count, I don't have an exact tooth number, because I haven't been able to find the info. Anyway, when I first set it up I was reading the VSS directly connection shared with the ECM I was using, I have since moved to a different ECM and required installing a VSSB/DRAC. I needed that 40 PPR output to input to my Arduino speedo so that I didn't have to make adjustments in the code (A bit of coder laziness there, using a hardware solution, lol). Anyway in testing I couldn't get my preferred DRAC (7 position adjustment) to work with the speedo. After a bit of playing and finaly some bench testing, I discovered the early DRAC does not have the 40 PPR output, there is no hardware support for it in the DRAC.

So in short the lack of proper 40 PPR output from the DRAC makes it incompatible in place of the later VSSB.

I believe that all code for the '7427 ($0D, $31, $85) requires this input, so you'll need to find a later VSSB to get your setup completely functional.

[dave w]

Definitions $OD, $E6, $31, & $OE require 2000 pulses per mile input to operate a 4L60E or 4L80E. Both options, DRAC or VSSB, will convert the 40 pulses per driveshaft revolution of the 4L60E or 4L80E into 2000 pulses per mile ( with correct configuration of the DRAC / VSSB jumpers). dave w

[Six_Shooter]

There's at least some that won't, that use that 40 PPR output from the VSSB for high res output. I know it's been a problem for more than a few people. When I discovered the lack out of output from teh DRAC I looked at a few wiring diagrams where that connection was definitely used.

VSSB has this output, DRAC does not.

[dave w]

There's at least some that won't, that use that 40 PPR output from the VSSB for high res output. I know it's been a problem for more than a few people. When I discovered the lack out of output from teh DRAC I looked at a few wiring diagrams where that connection was definitely used.

Is high resoulaton = 128,000 pulses per mile? Typically 128,000 pulses per mile is needed for an Anti-lock Brake Systems (ABS). I agree, for ABS applications the 14 pin VSSB is required. dave w