can I trick computer

[knez]

I have 04 gmc 2500hd cc sb 8.1L that im having 4.56 gears put in.My question is when i lifted truck and put on 35in tires i had to reprogram computer so speedo was correct plus my abs light would come on.So i bought hypertech to change tire size which was a mistake anyhow untill I get my westers tune. Can I trick computer by putting in smaller tire size so no abs and wrong speedo reading?Example I have 3.73 r&p ratio and 35 in tires now. I'll have 4.56 r&p ratio and 35in tires after gear change but computer still thinks I have 3.73 so if i put in say 31 in tires will that work?Thanks

[Butch 02 Denali]

Yes, that should work. Keep going down in tire size till the ABS light doesn't come on anymore. Make sure when you order your Westers tune that you mention the ABS light coming on!

[GetMore]

No, that won't work.

You need a taller tire to offset the lower gears.

 

Here's a comparison of engine speeds would be at 60 mph with a .75 overdrive ratio. Shown are: rpm, tire diameter, gear ratio.

2225 31" 4.56

1971 35" 4.56

1612 35" 3.73

1852 30.5" 3.73 (stock tire size)

1623 42.5" 4.56

 

So, as you can see, you need a 42.5" tall tire with 4.56 gears to turn the same rpm as you do with 35" tires and 3.73 gears.

 

As for the ABS issue, I think once the speedometer is calibrated you shouldn't have the light, but I don't know where the pickups are and what the computer looks for.

[mmmikkke]

TMobile did the math for you, but his conclusion is wrong. Your stock setup gave 1852 rpm at 60mph. Your current 35"/4.56 setup gives 1971 rpm at 60mph. That's close enough, you're only off by 5%. It's as if you kept your 3.73 and put 215's on your truck, which is exactly the size that comes on a dually. So the computer should have no problem with it, provided you can return the computer to the stock setup, and that doesn't sound like a problem.

[knez]

Nice thats what I was hoping.Hopefully this will really improve my accelertion/pulling power. Thanks

[Butch 02 Denali]

I know this is confusing guys, but sava's intent is to fool the computer, not actually change his tire size. Now his ABS system is outside it's calibration range because of the 4:56 gears. Adding any gear over 4:10's will usually confuse the ABS computer because it is not seeing the correct wheel revolutions for what the speedometer is reporting. Tricking the computer by inputing a smaller tire size will get the speedometer reading faster than he is going and get within the calibration range of the ABS computer.

[mmmikkke]

Ok, I admit I'm confused.

 

Knez, good luck with that, dude. Let us know if it worked out.

[silverado3500]

If I'm understanding the problem right, changing the tire size won't fix the problem. There are 2 wheel speed sensors (WSS one at each front wheel) and the vehicle speed sensor (VSS) that measures the speed of the rear wheels. The VSS mounts in the transmission and counts axle revs.

 

The problem you will have is that the VSS in the tranny (transfer case if 4x4) is affected by the axle ratio. The sensors on the front wheels are not. (they measure the speed from the hub). The reason that you are getting the abs light is that you are getting 2 different speed inputs, one from the VSS, and the other from both Wheel speed sensors. So the computer assumes that you have a faulty speed sensor. Changing the tire size won't fix the problem, as the computer will still receive 2 different signals.

 

Hope this is of some help.

[Butch 02 Denali]

You're right 3500, I wasn't thinking of the VSS sensor!

 

Then the only option left Sava is to get the tune and the ABS module will be recalibrated for that 4:56 gear ratio.

[knez]

Bummer.When i first had truck lifted and put on 35's i got abs light at about 100 miles.Got the hypertech changed tires size to 35in no problems.I was hoping I could use it to change rear end ratio in computer but hypertech can't because they cant get in to allisons computer only on the 4.8,5.3,and 6.0's engines oh well looks like i'll have to get westers tune sooner then later.Any body know how long it takes to get westers tune once you place order?Thanks Knez

[Mark_in_Tulsa]

Ok correct me if I'm wrong, but it dosn't matter what gears you put in the truck. Isn't the speed sensor on the wheel?

[Haulin' 8.1]

Each front hub has a speed sensor but the rear wheels share a speed sensor that is located in the tailshaft housing of the trans*. So the ABS signal is not effected by gearing on the front wheels but is on the rear since it is calculated from driveshaft speed. If you can't reprogram for gearing changes (like with the Allison TCM) then the three won't match at equal wheel speeds. If it had a speed sensor on the rear axle or each rear wheel it woudn't matter what you did as long as all your tires were the same size.

 

Vernon

[Butch 02 Denali]

Bummer.When i first had truck lifted and put on 35's i got abs light at about 100 miles.Got the hypertech changed tires size to 35in no problems.I was hoping I could use it to change rear end ratio in computer but hypertech can't because they cant get in to allisons computer only on the 4.8,5.3,and 6.0's engines  oh well looks like i'll have to get westers tune sooner then later.Any body know how long it takes to get westers tune once you place order?Thanks Knez

 

<{POST_SNAPBACK}>

 

 

One to two days from me!

[Haulin' 8.1]

Somebody PLEASE correct me if I'm wrong because I'm in the same boat but from what I understand NO ONE has the ability outside of Allison and GM to reprogram the Allison TCM. They will only program for gear ratios availiable when new. So even the usual PCM tuners can't help us yet.

 

I've been thinking about this today and I drove home with a scanner observing ABS wheel speeds. I'm wondering if I can use a speedo correction device to intercept the rear wheel speed signal going to the ABS module. If so, I wonder if one could be used to match wheel speed signals up at the ABS module while the PCM and TCM use the existing signal for actual MPH. I'm going to study it.

 

Vernon

[Butch 02 Denali]

We do these all the time Vernon. But you're correct, no one can reprogram the Allison TCM, but the ABS light problem isn't fixed in there. Lyndon cam also program shift chartistics without getting into the TCM too.