Gear change affecting VSS/MPH in 7747 ECM

[PlayingWithTBI]

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.

[stew86MCSS396]

Ummm...would this help?

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

[PlayingWithTBI]

Thanks, I wish it were that easy but, unfortunately the 7747 doesn't have a DRAC or VSSB.

[jim_in_dorris]

If it is a 7747, the Speedo gear in the tranny needs to be changed. That's how I fixed mine.

[1project2many]

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.

[PlayingWithTBI]

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.

[stew86MCSS396]

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?