Installing bigger tires and ABS on trucks

[turbo_bu]

Recently I had to do some work on my front wheel ABS sensor in my 2004 K2500 Silverado. While reviewing the ABS system, I realized that the ABS on my truck is only a 3 channel system. The front wheels have sensors that go to the ABS unit (EBCM I think it is called), while the rear wheels speed comes from the PCM. I believe that the speed signal that goes from the PCM to the EBCM is called a Vehicle Speed output (128K). This is not the same vehicle speed signal that the PCM sends to the rest of the vehicle (dash, cruise control, etc..). That is the lower resolution Vehicle speed output (4K).

I am hoping that my next piece of logic makes sense and holds to be true. I noticed that for the LS1B PCM's, that there is a parameter that calls out this high resolution vehicle speed output. {H0106} PCM Secondary Pulses per Mile Output Rate .... happens to be 128000. Since this is a configurable parameter, couldn't we modify it when we install bigger tires to make the ABS happy and keep it from throwing codes about speed mismatches???? My logic is this, the front wheels will measure the front wheel speed ... whatever it is calibrated for. Yes, you can change the tire size in the ABS unit with a Tech II, but there is a maximum limit (36" 's I think). So if you install 40" tires, you end up having to set the truck up as 36" 's and live with it.

BUT, what if you didn't change the ABS settings, and just adjusted the {H0106} to compensate for the larger tires? By adjusting this value, you are compensating for the larger tires and would not need to adjust anything else. Also (another guess on my part since I have not tried this yet), you could still fix your dash by adjusting the {H0101} Vehicle Speed Sensor Pulses per Mile without affecting the ABS.

If this works, my next question then becomes is the Secondary Pulses per Mile Outpue Rate available in other ECM's? I notice that in my K2500 (Duramax LB7 ECM which is similar to an LS1B), there is not a secondary output rate parameter. Looking at say an E38 also shows that the secondary pulses per mile output rate is missing as well. Did GM do away with this being configurable??? I know that adding in stuff to these "older" ECM/PCM's is not likely going to happen, but didn't know if that type of parameter was carried over into the newer Gen IV stuff.

[Gordy M]

When I changed tire sizes in my Corvette (2000) I used the bi-directional system to put in the diameters of the new tires and have not had any issues. Even when I was running Hoosiers 25.4 F/B I had no problems with all the nannies off when autocrossing or HPDE. However, the Hoosiers would trammel when driven on the expressway and I would change lanes with AH on. Not sure whether the Truck PCM would allow this.

[turbo_bu]

Guess my question, other than if this really works, is if the 128000 parameter is editable in the newer Gen IV ECM's. I think this might remove the limitation for guys running bigger tires on their trucks and wanting to get the speedometer to read correctly.