Change gears or go to larger dia tires and reset VSSB "NOT A GOOD IDEA"

[DA BIG ONE]

Resetting VSSB after gear/tire size change not a good idea!

It turns out that hacking VSSB on my 1999 6.5td, K2500 4wd autotrac BURB only recalibrates speedo. I've been living with shift/lock points elevating as I change out gears from 4.10 to 3.42 and reset VSSB, then changing to a larger tire diameter and resetting VSSB again resulting in even higher shift/lockup points but the speedo was right on.

I had tried a few different aftermarket speedo re-calibraters w/o success with their tech support personnel claiming the NP-246 AutoTrac t-case that was the problem preventing their units to function.

Thinking the t-case was the issue I started looking at aftermarket tranny controllers only to find there are none for my truck because of the autotrac is intergrated into the ABS too.

So, I start digging elsewhere and find that leaving the VSSB at it's settings before the gear/tire size change, in my case set for 4.10 gears, and re-calibrate before the VSSB results in everything functioning as it did before any changes. The re-calibration unit gets spliced into PPL/WHT signal wire from VSS and before pin # C-12 on VSSB.

To find change ratio for my mods I divide 4.10 into my total change "gears & tires", which is 3.09 ratio = 1.33 change ratio (example: 4.10 divided by 3.09 = 1,3268608 or 1.33) I then set micro rocker switches to 1.33 ratio.

I found ABBOTT enterprises markets a speedo re-calibrator called an electronic ratio adapter (ERA) it has the little micro rocker switches to set ratio change, easy, easy, easy, w/unlimited combos. www.atrol.com

SHIFT/LOCKUP POINTS BACK TO NORMAL.

[JohnC]

I guess if what you mean by "normal" is "the same road speeds" then yup, you got it. If on the other hand "normal" has more to do with engine speeds and torque levels, wouldn't recalibrating the VSSB be more "normal?

BTW, one thing I found when I recalibrated for different tires was that the ABS speeds no longer jived with the road speeds....

[DA BIG ONE]

I guess if what you mean by "normal" is "the same road speeds" then yup, you got it. If on the other hand "normal" has more to do with engine speeds and torque levels, wouldn't recalibrating the VSSB be more "normal?

BTW, one thing I found when I recalibrated for different tires was that the ABS speeds no longer jived with the road speeds....

Normal, as in back to the shift points and lockup points it had when 4.10's were there.

With calibrater spliced before the VSSB, the VSSB distributes to the abs, cc, speedo, & trans output to PCM, diesel VSSB does not send vehicle speed signal to PCM.

When reclibrating VSSB modual only the speedo gets the correct info.

[BrentN]

Good information to know. I was looking at upgrading to the 285 75R16's on mine and had planned on going down the VSSB route. BTW, any problems with the 285's on your 2500?

[DA BIG ONE]

Good information to know. I was looking at upgrading to the 285 75R16's on mine and had planned on going down the VSSB route. BTW, any problems with the 285's on your 2500?

I had to roll back bottom edge of fender at lower rear of wheel opening, then trim fender flare in same area leaving the flare's outside section intact.

The 4wd 2500's height was just right using factory steel, or the alcoa wheels.

I'm running a WARN bumper, factory bumper can be shimmed w/spacers (washers) forward so the tires clear back edge of bumper.

[JohnC]

Normal, as in back to the shift points and lockup points it had when 4.10's were there.

OK, I assume this means at the same road speeds as when you had 4.10's. But, from a performance (or economy) standpoint, that really doesn't make sense to me. The engine is turning a lot slower at a given road speed in a given gear than before. That's not always what you want. When I had the 3.42's with the standard, I had to rev the snot out of it to get any performance when towing. Not how I would have driven if I had 4.10's!

[More Power]

Trans shifting is controlled by the input/output speed sensors on the transmission (not the vehicle speed sensor) as well as throttle position. The trans doesn't care what the rear gearing is. Shift points will increase in speed as gearing is numerically reduced (i.e. going from 4.10 to 3.42). That's how it should work.... ;)

[DA BIG ONE]

Trans shifting is controlled by the input/output speed sensors on the transmission (not the vehicle speed sensor) as well as throttle position. The trans doesn't care what the rear gearing is. Shift points will increase in speed as gearing is numerically reduced (i.e. going from 4.10 to 3.42). That's how it should work.... ;)

Ok, Now really confused! At the very least "I need to know" how it is back to shifting the way it did before the gear change out?

Pin C-13 at VSSB transmission output speed from PCM, or am I further confused?

[DA BIG ONE]

OK, I assume this means at the same road speeds as when you had 4.10's. But, from a performance (or economy) standpoint, that really doesn't make sense to me. The engine is turning a lot slower at a given road speed in a given gear than before. That's not always what you want. When I had the 3.42's with the standard, I had to rev the snot out of it to get any performance when towing. Not how I would have driven if I had 4.10's!

Before the current calibration the trans shifting/lockup points were 25+% higher than w/4.10's od/lockup was at 60+mph, then I'm thinking the extra load on drivetrain led to loss in economy. So, now I'm thinking w/all my mods extra torque is there for lower shift/lockup points and faster shifts. I have not taken any trips, but around town, short stints on 95 it's very impressive.

[DA BIG ONE]

VSSB PIN IN/OUT 1999 GMC, 6.5TD, K2500, 4WD AUTOTRAC, BURB

Socket to VSSB has 9 pin locations, w/ 8 being used & 1 blank.

C-7: LT GRN/BLK input from vss to VSSB

C-8: BLK/WHT ground to engine

C-9: BRN ignition feed fused 10amp

C-10: WHT output to ABS

C-11: NOT USED IN DIESEL APPLICATION, otherwise it's DK BLU vss output vehicle speed signal to terminal F-13 @ PCM

C-12: PPL/WHT vss input "this is wire to which re-calibration unit spliced into"

C-13: BRN transmission output speed @ terminal F-12 @ PCM

C-14: GRN/WHT vssb output to cruse control

C-15: DK GRN vssb output to speedo

[JohnC]

This is just a WAG, but it sounds like you may have tricked the PCM with a false output shaft speed value.

Like MP said, if you go to taller gears, your shift points should go up accordingly, otherwise you're not making use of all of the available power curve.

[DA BIG ONE]

This is just a WAG, but it sounds like you may have tricked the PCM with a false output shaft speed value.

Like MP said, if you go to taller gears, your shift points should go up accordingly, otherwise you're not making use of all of the available power curve.

JC, I've been out driving all morning, the tranny no longer searches for the right gear between 45&60, shifts are crisp and fast, no lugging. However, in od and locked at highway speeds it has lost some passing performance by not downshifting at times. I have the TransGo mods which allows me to up/down shift as I want going to try that.

[16gaSxS]

When I did my gear swap wayback in about 1998 I had the dealer order a new VSSB (didn't know any different at the time) for 245-75-16 tires and 3.42 gears. Just as More power says the OD lockup happens at the SAME RPM as it did with the 3.73 gear it just lockups at about 5-6 mph faster with the 3.42 which it should.