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Thread: DRAC/VSSB calibration

  1. #1
    Carb and Points!
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    DRAC/VSSB calibration

    Hello everyone, I have a 1987 Pontiac Bonneville that I swapped a 4t65-e into. I am controlling the trans via a 16147609 TCM.

    I'm looking for some insight into using a DRAC/VSSB to convert the VSS signals to the correct ratio so my ECM, cruise and speedometer all work properly. Correct my calculations as needed please!

    I'm using the following formula: Input Ratio=(63360 x axle ratio x VSS pulse per rev) / (rolling circumference x 128000). Since the 4t65-e has a 30 pulse per revolution VSS signal and no axle ratio to worry about, the equation becomes (63360 x 30) / (79.5979 x 128000) which equals a .18656271083 divide ratio.

    I'm not sure if this is correct and I cannot find a jumper setting to make the VSSB achieve this ratio.

    Help please!
    Last edited by beretta3400; 06-26-2014 at 07:24 PM. Reason: misinformation

  2. #2
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    The first thing to know is whether or not your trans is using a 40 pulse per revolution speed sensor

  3. #3
    Carb and Points!
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    No, it's a 30 pulse per rev sensor.
    Last edited by beretta3400; 06-27-2014 at 05:12 AM.

  4. #4
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    Sorry... guilty of not reading the post completely.

    The DRAC is designed for the 40 pulse sensor and may not have the correct divide ratio for your 30 pulse unit. Can you install a 40 tooth reluctor on the output shaft? If not, you're limited to the ratios available.

  5. #5
    Carb and Points!
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    I'm still unsure if that added reluctor would work, the expected revs per mile would still be 3 1/2 times what my unit would produce since I don't have a differential to go through; My reluctor is mounted to the final drive and spins at axle speed and with my current tires it will yield 23880 pulse per mile.

    Since this doesn't look like it is going anywhere, I think my other alternative is to use a ECM from a vehicle that uses a 30 PPR sensor and can accept the AC signal, wire it parallel to the existing TCM, in place of the VSSB and only use it as a buffer. I'll burn a PROM that will have the correct divide ratio for my car to make my speedometer work.

    It will just sit there with B+, ignition, grounds and the VSS inputs and the 4000ppm output.

  6. #6
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    That works but I would feel wrong implementing it.

    What engine / ECM is in the Bonneville?

  7. #7
    Carb and Points!
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    It's an LG3 3.8 SFI. I believe it's an 1227783 ECM.

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