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Thread: DRAC/VSSB Questions...

  1. #1
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    DRAC/VSSB Questions...

    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
    Last edited by DavidBraley; 12-09-2016 at 03:38 AM.

  2. #2
    Super Moderator dave w's Avatar
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    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
    Last edited by dave w; 12-09-2016 at 05:00 AM.

  3. #3
    Super Moderator Six_Shooter's Avatar
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    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.
    The man who says something is impossible, is usually interrupted by the man doing it.

  4. #4
    Super Moderator dave w's Avatar
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    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

  5. #5
    Super Moderator Six_Shooter's Avatar
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    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.
    Last edited by Six_Shooter; 12-09-2016 at 06:46 AM.
    The man who says something is impossible, is usually interrupted by the man doing it.

  6. #6
    Super Moderator dave w's Avatar
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    Quote Originally Posted by Six_Shooter View Post
    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

  7. #7
    Super Moderator Six_Shooter's Avatar
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    No, not for ABS, this is an input to the PCM, pin 13 of the DRAC (labled "40 PPR" and "Vehicle speed to ECM/PCM") to pin F12 of the PCM. My notes say C and Q series only but C series covers a lot of applications on it's own.
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  8. #8
    Super Moderator dave w's Avatar
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    Quote Originally Posted by Six_Shooter View Post
    No, not for ABS, this is an input to the PCM, pin 13 of the DRAC (labled "40 PPR" and "Vehicle speed to ECM/PCM") to pin F12 of the PCM. My notes say C and Q series only but C series covers a lot of applications on it's own.
    Can you post a schematic? dave w

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    Oh man, do I appreciate the two of you! Thank you so much for your replies!

    I've read somewhere the 94-95 years are sort of unique on their own. Transition years possibly. GM was getting ready for the release of the OBDII systems, and it might explain why some of the PCM's like the 6395's and 7427's have more features than the 7747. I might have read that on the Moates site maybe? I've read so much it's becoming all a blur now. I need to start a notebook!

    In my searching, I'm finding most DRAC/VSSB three letter codes that starts with either a Q or a P. I found only a couple that start with a C, and an A. I'm assuming that's what you are talking about Six_Shooter when you said, "My notes say C and Q series only but C series covers a lot of applications on it's own." In some listings, the DRAC/VSSB modules that start with a P are typically from 94-95 trucks, and Q are from 92-93 trucks. I haven't found any complete enough listings to show what a C series DRAC/VSSB comes out of. All the DRAC's I have here are Q series and they are all 7 pin. Unfortunately, most DRAC/VSSB listings say they will work with ECM/PCM's from years 92-95.

    So, Maybe I'm not going crazy! HA!

  10. #10
    Super Moderator dave w's Avatar
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    Quote Originally Posted by DavidBraley View Post
    Oh man, do I appreciate the two of you! Thank you so much for your replies! I've read somewhere the 94-95 years are sort of unique on their own. Transition years possibly. GM was getting ready for the release of the OBDII systems, and it might explain why some of the PCM's like the 6395's and 7427's have more features than the 7747. I might have read that on the Moates site maybe? I've read so much it's becoming all a blur now. I need to start a notebook! In my searching, I'm finding most DRAC/VSSB three letter codes that starts with either a Q or a P. I found only a couple that start with a C, and an A. I'm assuming that's what you are talking about Six_Shooter when you said, "My notes say C and Q series only but C series covers a lot of applications on it's own." In some listings, the DRAC/VSSB modules that start with a P are typically from 94-95 trucks, and Q are from 92-93 trucks. I haven't found any complete enough listings to show what a C series DRAC/VSSB comes out of. All the DRAC's I have here are Q series and they are all 7 pin. Unfortunately, most DRAC/VSSB listings say they will work with ECM/PCM's from years 92-95. So, Maybe I'm not going crazy! HA!
    I'm curious, what are you needing? 40 pulses per drive shaft revolution to 2000 pulses per mile conversion? dave w

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    Dave,

    I am embarrassed to say I have no idea what I need. I need what will work with a 4L80e and a 7427 PCM. The only thing I do know is the VSS at the rear of the 4L80e outputs a 40 pulse per driveshaft revolution. I will go back and read some of the DRAC articles I have collected... I think they mention what kind of signal is needed for the PCM.

    Worse case, I can use and microprocessor to do this all for me. But I would like to avoid that...
    Last edited by DavidBraley; 12-09-2016 at 08:00 AM.

  12. #12
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    Thanks for the help. I'm going to reach out to some of the sellers and see if I can get them to open up the plastic case these things come in and let me know if the unit they have for sale is the 7 pin (DRAC) or 14 pin (VSSB). I could even send them some images so they are sure.

    Thanks again and take care.

    David

  13. #13
    Super Moderator dave w's Avatar
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    I've attached some better schematic pictures. I have successfully used both the 7 pin and the 14 pin to convert a 40 pulses per drive shaft revolution signal into 2000 pulses per mile signal. The 16197427 requires the 2000 pulses per mile signal to operate either a 4l60E or 4L80E. I have never used a DRAC (sometimes known as the 7 pin) or VSSB (sometimes know as the 14 pin) to supply signals to a speedometer, cruise control or an anti-lock brake system. Rear Wheel Anti-Lock = (RWAL) in the schematics. dave w

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    Thanks for that information Dave!

    I managed to find a 14 pin VSSB this morning from a 95 C1500.

    Turns out, I will need to do something with a microprocessor for my speedometer signal anyway. The truck as an Eaton 2-Speed rear axle. The idea kicking around at the moment is to use some known signal from the DRAC/VSSB (probably the 40 pulses per rev), and then come up with a way in software to send the correct count going to the speedometer when the rear differential is in low or high.

    I'm trying to collect "two" of everything for this project. The hope is to create some kind of bench testing area with the second set of parts. I'm hoping it helps me learn more about the way these systems work, how to get TunerPro talking to the 7427, and a way to sniff some of these signals.

    Of course I will share everything I learn!

    David

  15. #15
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    Quote Originally Posted by DavidBraley View Post
    Thanks for that information Dave!I managed to find a 14 pin VSSB this morning from a 95 C1500.Turns out, I will need to do something with a microprocessor for my speedometer signal anyway. The truck as an Eaton 2-Speed rear axle. The idea kicking around at the moment is to use some known signal from the DRAC/VSSB (probably the 40 pulses per rev), and then come up with a way in software to send the correct count going to the speedometer when the rear differential is in low or high.I'm trying to collect "two" of everything for this project. The hope is to create some kind of bench testing area with the second set of parts. I'm hoping it helps me learn more about the way these systems work, how to get TunerPro talking to the 7427, and a way to sniff some of these signals.Of course I will share everything I learn!David
    Might be possible to use 4low ability of 7427 and drac/vssb to use when 2 speed in low..
    6395, BHDF, 7.4 BBC lightly modded now 6395 BMHM back to BHDF

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