Calibrating speed on Dakota system?

Discussion in 'High Tech Retrofits' started by Goat, Apr 16, 2019.

  1. Goat

    Goat Veteran Member Lifetime Gold Member

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    Anyone else using the Dakota digital instrumentation? I am trying to calibrate the speedometer using the GM ecu signal - not being very successful...
     
  2. 130fe

    130fe Member

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    Are you using the GMPP harness/PCM? Do you have access to HPTuners? I just fixed mine (similiar setup as yours) in my 68.
     
  3. Goat

    Goat Veteran Member Lifetime Gold Member

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    I'm using the GM harness/ecm that they sell for the LS7 crate engine. I also have the "BIM -01-2" bus interface from Dakota that is supposed to pick up the signals directly from the ECM. It is picking up the oil, temp, and rpm signals fine, but having trouble with the speed signal...

    No HPTuners. Should not need it...
     
  4. 130fe

    130fe Member

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    I am using almost the same setup in my 68, LS3/T56 magnum with GMPP PCM/harness and Dakota Digital HDX gauges with the OBD2 BIM module. This problem only seems to happen to people that use the GMPP harness with a manual trans (other than the GM T56 super magnum). Everything online from GM says that you don't have to adjust it. Well, you do..... will it run, yes. will it drive, yes. Will everything be correct and not have any issues, not really. The GMPP PCM is the basis for what sell for the Connect and Cruise system. They have tailored that PCM around automatic transmissions. On most "factory" PCMs you can adjust the reluctor wheel count in the tune. It doesn't happen that way in the GMPP PCM.

    Transmission reluctor wheel types in commonly used swap transmissions-
    4L60E/70E/80E etc - 40 tooth
    LS T-56 (98-02)- 17 tooth
    T56 Magnum- 12 tooth
    TKO 600- 12 tooth
    GMPP T56 Super Magnum- 40 tooth

    I originally ran an LS style T56 in my car with the DD VHX gauges. Although I wasn't getting a correct VSS signal from the PCM, I was able to "adjust" the signal (through the DD VHX menu) to get the correct speed reading on my speedometer. I drove my car like this for a few years and I had some minor low speed engine symptoms like bucking and surging RPMS when coming to a stop. It wasn't all of the time, but very aggravating when you put all of this time and money and to have issues with "great" parts. I also had an intermittent check engine light at low speeds but never looked at it.

    I swapped in a T56 Magnum late last year and couldn't get the DD speedo to calibrate. It was getting enough "pulses" (or whatever the count is called) during the calibration process. Talked with DD tech support and they weren't really a big help. Found out about the different tooth count on the VSS reluctor and asked one of my other first gen buddies that had similiar issues with his GM 480HP crate and T56 magnum. He sent me this link and how it fixed his. I have HPTuners and for a while I couldn't figure out why I couldn't see the correct "box" to change my settings. You have to be in the "advanced" view of HPTuners settings. Here is the link that will help you (info is in first post).

    https://www.camaros.net/forums/56-p...0507-code-speedo-calibration-issue-fixed.html

    You use this equation to come up with your "corrected" rear axle ratio-

    rear axle ratio/ (40/what number reluctor tooth your trans has)= corrected ratio
    I am using the T56 magnum (which I have) in this example.
    mine was-
    3.89/ 3.33 (comes from 40/12) = 1.168 which I used 1.17 in the PCM table

    Car drives alot better now, especially at lower speeds. No more bucking, intermittent stalling or check engine lights like I had before. GM sells the T56 Super magnum, which is a magnum but with a 40 tooth reluctor wheel. Thats why they say it is "plug and play" with their GMPP PCMs.

    Hope this helps!
     
  5. 76z28

    76z28 Veteran Member

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    I want your engine....However; I am planning on Dakota Digital HDX as next years upgrade. How do you like them so far?
     
  6. Goat

    Goat Veteran Member Lifetime Gold Member

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    I like it a lot except for the current problem I'm having...
     
  7. Goat

    Goat Veteran Member Lifetime Gold Member

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    130 fe, 1st thanks a ton for trying to help me. My electronics knowledge is average at best. Secondly, I am now further confused by your comments. I thought the issue was the BIM module was not picking up a signal from the OBD-ll interface, used to display a MPH reading in the dash. Why would that have anything to do with the engine not running properly? I thought we were just trying to tap into a signal, not alter anything in the tune that would change the engines' behavior? What am I not understanding?

    I just realized you are using the "HDX" system. I have the "VDX" version. Does that make a difference?

    thanks.
     
    Last edited: Apr 17, 2019
  8. 130fe

    130fe Member

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    It doesn't matter whether you have the VHX or HDX systems, they are the same but different. The differences don't matter when it comes to the issue you (and I had) are having. The OBD2 BIM module piece is awesome also. The DD stuff (like any other speedo) and just taking what ever signal you give it and makes electrons into speedo reading.

    The issue is that the cause of this particular problem is tied into what the transmission is putting out and what the PCM does with it (in its tune) and sends it to the gauge (or BIM module). The GMPP PCM, out of the box, is expecting a 40 tooth reluctor signal from the transmission. If you were to run an 4l60E or T56 Super magnum from GMPP it works like it is supposed to because both of those trans send a 40 tooth signal. The PCM take the VSS reading it gets from the trans, computes that and what tire diameter is programmed in the tune and sends that "signal" to your speedo, BIM module, whatever you are using. I connected a scanner and monitored the PCM VSS compared to a GPS speed and the PCM was only showing about half of what the GPS said.

    When I had the VHX and now HDX gauges, you have the ability to take the PCM VSS signal and adjust it from 75-125% to calibrate your speedo reading to look right. Now, knowing that when I was running the LS T56 it had a trans VSS output signal of 17x. I was able to calibrate my speedo from the VSS signal I was getting from the PCM with that trans BUT I was still having low speed engine running issues. Nothing major but just enough to be aggravating. I put 13k miles of street, highway and AutoX on my car.

    When I changed to the T56 magnum I thought this "should" be pretty easy right? No, LOL. The non Super Magnum transmissions have a 12x reluctor wheel. When I tried to drive the 2 mile calibration procedure I kept getting an error message telling me I wasn't able to do the procedure. I called the DD Tech and they asked me how many cycles I was getting on the 2 mile drive. I was only getting like 1400. They referred me to the instructions and this is what they said-

    ***If you do not receive more than 2,000 pulses during calibration the unit will error out and display a brief “pulse to low” message and revert to a default PPM setting.

    I know that gauges have been working before so they said it must be the programming in your engine PCM.

    GMPP locks the 40 VSS reluctor setting in their PCM and even if you change it, it reverts back. The only work around is to either Change the setting in your tune (new "adjusted" rear gear) or buy a GMPP T56 magnum (which is not going to happen) LOL Honestly, it takes 5 minutes to do. Not super complicated but luckily I had access to HPTuners to make the changes. If you have a Tuner in your area, they should be able to make the adjustment with ease. You don't have to do a dyno run or anything like that (unless you want to) afterwards. Once changed, you will be happy with the changes. Take your "corrected" axle ratio (your ratio divided by 3.33) and input it up to two places past the decimal point (like in the example in my previous post).

    If you are using an LS T56, you would need to take your rear axle ratio and divide it by 2.35 (40 divided by 17) and input that number.

    I will say that I was pretty aggravated that GMPP did this to their PCMs. You are also not able to use other features such as PWM fan settings and such like their other "normal" PCMs. In the end, I fixed it and my car drives totally different, like I expected it to from the beginning...... If you have any other questions, please let me know and I will try to pass on any help I can.
     
  9. Goat

    Goat Veteran Member Lifetime Gold Member

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    Thanks man. Your explanation is awesome _ starting to make some sense to me. I will need to read this over a few times to digest.

    In Dakotas' instructions, they list two other methods of getting a signal to read mph. Specifically, the ones using the "normal" signal as opposed to the "bus" signal. Are these options worth exploring?
     
  10. 130fe

    130fe Member

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    If you are taking readings off of the BIM module, select bus. I think the other one is used if you are using the supplied senders and their harnesses. Such as if you input the VSS wire into the control box, select normal.

    Have you had your car tuned or anything yet?
     

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