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Thread: Gear change affecting VSS/MPH in 7747 ECM

  1. #1
    Fuel Injected! PlayingWithTBI's Avatar
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    Gear change affecting VSS/MPH in 7747 ECM

    I changed the rear end gears from 3.42 to 4.10 in my 88 C1500. I was able to calibrate my speedometer display easily with my Dakota Digital dash. My problem is, while data logging in TunerPro RT the speedometer there still thinks I have 3.42 gears which throws off the parameters for TCC etc. or will I have to convert all MPH related parameters manually?

    Does anyone know of a "hack" or easy way to calibrate the speed in the ECM? I really don't want to have to go through the hassle and expense of buying a signal interface and rewiring the input.

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    Ummm...would this help?

    Quote Originally Posted by 1project2many View Post
    If you'd like to try and adjust your DRAC, here is the information to DIY:
    http://gearhead-efi.com/gearhead-efi/doc/DRAC2.pdf

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    Fuel Injected! PlayingWithTBI's Avatar
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    Thanks, I wish it were that easy but, unfortunately the 7747 doesn't have a DRAC or VSSB.

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    Fuel Injected! jim_in_dorris's Avatar
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    If it is a 7747, the Speedo gear in the tranny needs to be changed. That's how I fixed mine.
    Square body stepsides forever!!!

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    If it is a 7747, the Speedo gear in the tranny needs to be changed.
    OP said 88 C1500 which is beginning of GMT400 body style change. 88 R/V trucks still used speedo gear but C/K used 40 tooth reluctor.

    unfortunately the 7747 doesn't have a DRAC or VSSB.
    You are correct. The 7747 does not have a DRAC or VSSB built in. However, the 7747 needs a 2k ppm signal and when the vehicle transmission produces 40 pulses per driveshaft revolution the ecm has no way to change this signal. So GM has installed DRAC units in many, many 7747 equipped vehicles. In fact, the DRAC was created when GM removed the speed signal converter function from the back of the instrument cluster. The converter on the back of the cluster was introduced in model year 1988 with C and K series trucks but was removed beginning in 1991. If you look at the wiring diagram for your vehicle you'll probably see that the speed signal for the ecm comes from the speedometer.

    The solution to your problem IMO is to install the same DRAC that GM installed when they, like you, removed the speedometer with the signal converter.

    Edit: What model dash do you have? Some incorporate speed converters to provide OEM function.

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    Fuel Injected! PlayingWithTBI's Avatar
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    It's a Dakota Digital VFD3-88-PU-U there is an output on it for cruise control and/or radio volume (cruise works fine). I sent a couple emails to their "tech support" but they didn't understand what I was asking. If the output is changed when I calibrate the dash display then I figured I could just run the wire from behind the OE instrument cluster to the ECM. He kept saying that it is configurable to 4K or 2K (Duh) so, I gave up on his help.

    Jim - I was hoping I could do it electronically but, as you said, I'll have to find a gear. Ooops, I guess that's not an option. I didn't read the 1st part of !project2many's post.
    Last edited by PlayingWithTBI; 01-05-2019 at 02:35 PM.

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    Still taking me to school!!! I'm still not understanding this either. Trying to learn as I'm trying to evade expense of having a T56 tailhousing machined for a cable driven speedometer. With that said, what configuration of parts is on the OPs transmission? Is there a drive gear that turns some kind of electric transmitter?

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    Super Moderator dave w's Avatar
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    Information (40 pages) for the Dakota Digital VFD3-88-PU-U -- http://www.dakotadigital.com/pdf/VFD3-88C-PU-U.pdf

    dave w

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    Fuel Injected! PlayingWithTBI's Avatar
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    Thanks Dave, I have the manual but I can't find anywhere it says if the speed output to the cruise control is changed when you calibrate the display. I'm getting too old (and too fat) to crawl underneath it to find out. My son lives 30 miles away; I hate to waste his time on a day off for a chance it may or may not work.

    Stew, the OEM 700r4 tranny in my truck already has a 2K PPM output to the OE dash on which my Dakota Digital display is mounted. The ECM is fed from that same OEM dash. I'm just trying to tell the ECM I changed the gears by going through the new VFD3.

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    It's a Dakota Digital VFD3-88-PU-U there is an output on it for cruise control and/or radio volume (cruise works fine).
    I actually had that manual open when I posted the question. The manual describes using the original dash circuit board and connecting the speed input from the new dash to one of two locations on the original board. It also talks about several options in the speed menu, and it talks about an auto-calibration procedure. There is one output from the DD dash control box for speed, and there are three outputs from the original dash circuit (anti-lock brakes, cruise, and ecm). Since one output cannot provide signals for all three of these systems, and since at least two of the systems are receiving speed signals (ecm and cruise) I'm going to guess that the DD speed output is not used, and that the original speed converter is still in use on the back of the original dash. Can you confirm the original dash circuit board is still installed? If so my recommendation is to make your speed adjustment in the original speed converter using the DRAC instructions. This will correct the speed signal to the cruise, the ABS (if still working), and to the ECM.

    Again, please confirm that the original dash circuit board is still in use and that the DD speed signal is sourced from that dash.

    As a side note, the cruise doesn't actually know what speed the truck is travelling at, it only knows that it's been asked to maintain a speed. It will check to make sure the speed signal is not below the minimum speed and not above the maximum speed. A change from 3.42 to 4.10 is small enough that the cruise will probably work at highway speed. The question is will it engage below 35 mph and will it stay engaged to the max speed, about 85 moh IIRC. My hunch is that the speed signal is not correct for 4.10 and that cruise will engage around 20 mph and will cut out lower than 85.

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    Still taking me to school!!! I'm still not understanding this either. Trying to learn as I'm trying to evade expense of having a T56 tailhousing machined for a cable driven speedometer. With that said, what configuration of parts is on the OPs transmission? Is there a drive gear that turns some kind of electric transmitter?
    OP's truck from GM should have come with a 40 pulse wheel on the transmission output shaft which generates a 40 pulse per driveshaft revolution a/c signal as it turns beneath a fixed speed sensor. The signal was sent to the dash where it was converted to 2k pulses per mile square wave signal for the ecm and the cruise control, and 128k pulse per mile for the RWAL system.

    IIRC the T56 is a bit of an oddball because it used a different wheel which produced 17 pulses per mile. It may be possible to configure a DRAC to convert this signal to 2000 pulses per mile. You would need to follow the instructions in the DRAC2 paper to come up with a divide ratio then see if the ratio you calculated is one of the available ratios for the DRAC.

    Stew, the OEM 700r4 tranny in my truck already has a 2K PPM output to the OE dash on which my Dakota Digital display is mounted. The ECM is fed from that same OEM dash. I'm just trying to tell the ECM I changed the gears by going through the new VFD3.
    It took me so long to make sure my last reply was correct that you managed to sneak in another post. ;) So if you are using the original dash circuit then the best answer is to make or modify the "programming clip" on the back of the OEM circuit. The only other option I can think of is to transfer the ECM speed wire from the OEM circuit to the speed output on the DD and make sure it's set to 2k pulses. I would opt to maintain the integrity of the OEM design as much as possible.

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    Fuel Injected! PlayingWithTBI's Avatar
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    This is good stuff!!! That's what I was hoping for was moving the ECM wire from the OEM circuit to the DD speed output, as long as it is calibrated to the same speed as the DD display and not just a pass-through from the OEM circuit.

    I'll get my son to come over for a steak on the barbie and rope him into helping me play with it. After all, he's the one that installed the DD in the 1st place.

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    Hmm... The DD speed output option is not gauranteed to work. Changing the programming clip might turn out to be less work and is the best solution. It's just a plug-in device in the back of the OE circuit board.

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    Changing the programming clip might turn out to be less work and is the best solution. It's just a plug-in device in the back of the OE circuit board.
    What will that involve, changing jumpers or a different device? What does this device look like?

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    Yeah, as 1project mentioned the Drac for a 747 was in the instrument cluster itself. I moved some jumpers around in mine way back when, when I put 4.88 gears in mine in one of the previous iterations of this truck. But since you are Dakota digital gauges I guess that won't be an option!!!

    I wonder if it would be possible to wire in a seperate drac module like the PCMs used and calibrate it that way???? THere may be an easier way I don't know, but I'd think it WOULD be possible to address this issue this way.... If not, maybe time to consider upgrading from a ECM to PCM so you can easily use the DRAC, along with the many other advantages it offers.
    Last edited by CDeeZ; 01-05-2019 at 11:04 PM.

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