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End all post for DRAC/VSS (yeah right)

Discussion in 'The Garage' started by dyeager535, Dec 21, 2004.

  1. dyeager535

    dyeager535 1 ton status Premium Member

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    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://image69.webshots.com/69/9/9/47/2327909470097469480UEQpni_fs.jpg THIS is the right wiring diagram for 1991 R/V DRAC. Others I've seen and linked here, are not correct for our trucks. This will probably work with 1990 trucks as well, I have yet to figure out what year/vehicle my DRAC came from.

    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. :)

    The wiring schematics linked appear to be incorrect in regards to PPM for cruise. At least early cruise (perhaps a difference between the stepper motor cruise and vacuum servo cruise setups) wants 2000PPM, not 4000PPM. On the DRAC pinout, you'll see the cruise pin showing 4000PPM. If you put 4000PPM to one of the older cruise modules, it obviously doubles how fast the cruise module thinks the vehicle is moving, and also doubles how fast it THINKS speed is changing, which increases the rate at which it applies throttle. Therefore you end up with the cruise engaging at around 12MPH (low limit is supposed to be 25MPH) and not engaging at anything over at most 55MPH. I was unaware there was a max MPH setting in the cruise module, but apparently there is. Not to mention how terribly jerky the setup is with the cruise constantly flooring and letting up the throttle. :) At least on my DRAC, which is referred to as a "5 out" on one of the links showing the part numbers, the cruise pin outputs 2000PPM.
     
    Last edited: Nov 13, 2007
    Bowtie70SS likes this.
  2. u2slow

    u2slow 1/2 ton status

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    So in theory you could run the DRAC without the ECM?

    I've "seen the light" and abandoned TBI altogether in favour of diesel power... :D ... but I've still got this VSS on my NP205, and the matching odometer.
     
  3. 75-K5

    75-K5 3/4 ton status

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    This is some good info, thanks. I'm trying to learn a little more about EFI and all the electronics having to do with '87-'91 trucks but there sure is a steep learning curve.

    So I could do the DRAC swap into my '87 R10 with the digital speedo and feed that signal to the ECM and it would quit being pissed off about "no VSS detected?"

    I didn't look through all the pages yet...does it describe somewhere in there about what all I would need to rob from a '92-'94 for a complete system?

    This post is getting bookmarked :D
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    Yes, since the DRAC in stock application was powered seperate of everything else in the system, you could use ONLY the speedometer output on the DRAC with the 205.
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    You can swap DRAC into anything you want, just need the right sensor/reluctor and power, and whatever you want it to run. I'm hesitant to say that the '747 ECM uses the same VSS input that my '165 uses authoritatively, since I can't find info on what the '747 looks for signal-wise, but I *seriously* doubt there is a difference in that regard to the '747/165. Someone out there knows, I'm sure. This question could probably be asked on thirdgen.org, so I may.

    the DRAC2.pdf link is what I'd look at for wiring. Basically just need the wiring connectors to mate up to the DRAC, as far as I've seen, all DRAC plugs are the same, it wasn't used very long.
     
  6. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    So... is it possible to install a 4 pulse VSS into an old style TH400? I absolutely hate my cable driven speedometer, it has given me nothing but trouble...

    I've got a couple DRACs sitting around, and for right now, I am using the 2 pulse VSS on the back of my speedo for my TPI setup, but being as sucky as it is, I don't trust it to give my ECM a good accurate signal...

    PS, about the '747 ECM VSS signal:

    1988-93 TBI and 1985-89 TPI vehicles used the 2000 pulse per mile VSS, so your '747 should need the 2 pulse per mile signal.

    1990-92 TPI's used the 4 pulse VSS with the AC sin wave sent directly to the ECM. It then has outputs to the speedometer, cruise etc on the back of the ECM.

    www.chevythunder.com has some excellent information about specifically TPI setups, but also has a lot of great information about TBI systems, and ECMs / Sensors in general.
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    Well, apparently, the 40 tooth reluctor on the 205 should "fit" the TH400 output shaft. Not sure how you'd actually do that, but the diameter apparently is close.

    jagsthatrun.com is probably the place to go with something like this. I know that in some applications gear driven VSS (4 pulse IIRC) was used. I can't recall what the 1990 454SS used for speedometer, but that was a TH400 application and obviously the right vintage for what you want.

    It stands to reason that two pulse equals two pulse, however, for instance, the '730 ECM (1990+ TPI IIRC) doesn't use a buffer or DRAC for VSS, it is directly fed into the ECM. 4 pulse I believe. This is an extreme example I guess (although it is an immensely popular ECM) but just goes to show that you can't guarantee how or what the ECM needs for signal just based on year, and I'd hate to steer someone wrong on the '747, even though it stands to reason it's exactly the same as any other 2 pulse requiring ECM. Really only need to follow the wiring diagram for which pin on the DRAC feeds VSS to ECM. If its the 2000PPM output, then the two pulse is what the '747 requires.
     
  8. u2slow

    u2slow 1/2 ton status

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    Should be a piece of cake... with the right stock parts. 88-90 C-series came this way (unless the TH700 was optioned).
     
  9. 75-K5

    75-K5 3/4 ton status

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    Dorian, do you have AIM? Your PM box is full.
     
  10. Jodean

    Jodean Registered Member

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    does anyone happen to have the pinouts and diagrams of the above links that arnt working??

    Looking to do just a speedo, a drac, off a 241 output, probralby have to make my own harness. Looking for what wires would hook up where
     
  11. y5mgisi

    y5mgisi 1 ton status

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    Good old goodle brought me back to CK5! Subscribed for future reference!
     
  12. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    I miss Dorian... Been a while since I've heard from him, hope all is good...
     
  13. cuervo

    cuervo Wears Boob Goggles! Premium Member

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    Yeah seems like a long time since he has been here.


    On a side note: Did anyone happen to download those files and happen to still have them?
     
  14. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    couple years.. it's been at least a year since I got an e-mail from him.... I'm useless on the files....
     
  15. cuervo

    cuervo Wears Boob Goggles! Premium Member

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    Well, it does say he visited the site on 8 Nov.
     
  16. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    What exactly are you needing? I recently switched from optical eye to the DRAC system and I do have the correct pinout for the DRAC and for for 89 optic eye system (pretty sure all the optic eye is the same).
     
  17. cuervo

    cuervo Wears Boob Goggles! Premium Member

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    Hey Scott,

    I was hoping for the drac.pl since someone says the numbers on the other site are not correct.

    With my DRAC set right now, it's balls on at 50 but off at 70. I figure I'll desolder and put a dip switch in and mess with it a bunch.
     
  18. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    Are you needing the DRAC wiring pin out or the chart for setting the DIP switches?
     
  19. cuervo

    cuervo Wears Boob Goggles! Premium Member

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    Na, I have both of those. I was just wondering about the drac.pl calc.

    Well on second thought, do you have the chart? I'd like to look at that and compare it to the one on tbichips to see if they are different.
     
  20. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    I'll have to take a look when I get home. I printed it off and have it somewhere on my desk and could easily scan it for you IF I can find where I put it.
     

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