GMT 800 Brake Conversion > Silverado HD 2500

[THEFERMANATOR]

Again, Ferm thanks for the heads up, however I had a braking issue related to the ABS calibration after I installed big tires re-calibrated the ABS & no problem. I'm not an expert by any means but the re-calibration worked for me and some other people I know after going to larger diameter tires.

So, what's your take on why such a calibration table exists?

I've never heard of such a table. When I worked at DODGE they used the same kelsey hayes system, and even when I went to school on them, the only calibration was for the speedo.

 

[Big T]

Ever test the RF wiring for a break, or?

Yep. First off, it's a brand new sender on a brand new bearing. I have tested the harness from the abs to the sender. I even temporarily swapped in a harness from the '95 wreck. Nothing worked. I put an ohm meter on it. I don't know what I'm looking for. I'm not the greatest electrical tester.

Once this is completely back together with new steering box and aligned, I will take it into the dealer to pull codes. I need to pull codes on the transfer case as the 4 lo is not working. I need to do the pump rub fix. All stuff on my to do list.

 

[FellowTraveler]

I've never heard of such a table. When I worked at DODGE they used the same kelsey hayes system, and even when I went to school on them, the only calibration was for the speedo.

More likely than not I'm not explaining it right, and perhaps it's not a table at all, how it is adjusted is via a device controller then adjusting ABS sensitivity (more/less) I believe this mode is within the ABS controller and not the usual VSSB changes. I did the VSSB changes and my brakes did not fill right until I adjusted what I believe to be the ABS sensitivity.

I did a search and found about 1.4k hits on "tire size brake calibration for ABS" so I suspect it's not well known.

 

[FellowTraveler]

Yep. First off, it's a brand new sender on a brand new bearing. I have tested the harness from the abs to the sender. I even temporarily swapped in a harness from the '95 wreck. Nothing worked. I put an ohm meter on it. I don't know what I'm looking for. I'm not the greatest electrical tester.

Once this is completely back together with new steering box and aligned, I will take it into the dealer to pull codes. I need to pull codes on the transfer case as the 4 lo is not working. I need to do the pump rub fix. All stuff on my to do list.

If its an AutoTrac t-case it's tied into the ABS too, perhaps Ferm can chime in on this one.

 

[THEFERMANATOR]

More likely than not I'm not explaining it right, and perhaps it's not a table at all, how it is adjusted is via a device controller then adjusting ABS sensitivity (more/less) I believe this mode is within the ABS controller and not the usual VSSB changes. I did the VSSB changes and my brakes did not fill right until I adjusted what I believe to be the ABS sensitivity.

I did a search and found about 1.4k hits on "tire size brake calibration for ABS" so I suspect it's not well known.

Most abs calibration is from people going to drastically larger tires, and needing to recalibrate the front abs sensors to match the rear signal. For newer GM trucks it's once you go beyond about a 34.5" tall tire you need to adjust the abs. It wasn't until the gmt-900 trucks of late 07 that GM added in functionality for the abs to control brake bias. Our 3 channel kelsey hayes systems aren't even in the loop until the abs opens the isolation valves.

 

[FellowTraveler]

Most abs calibration is from people going to drastically larger tires, and needing to recalibrate the front abs sensors to match the rear signal. For newer GM trucks it's once you go beyond about a 34.5" tall tire you need to adjust the abs. It wasn't until the gmt-900 trucks of late 07 that GM added in functionality for the abs to control brake bias. Our 3 channel kelsey hayes systems aren't even in the loop until the abs opens the isolation valves.

Now that makes me wonder what the hell my car-code device controller opened titled "GM C/K Truck under 15000 GVW and 03-04 under 8600 with no VSES" on my 99 Burb that actually made the brakes feel better, I do remember when tinkering with calibration I did get noticed the calibration was out of range or something like that.

The adjustment was in % on a circle image.

My tires are 33.5" unloaded.

 

[Big T]

So the posts here have me wondering if the ABS light is related to my transfer case and no 4 LO issue?

 

[FellowTraveler]

So the posts here have me wondering if the ABS light is related to my transfer case and no 4 LO issue?

I can't say for sure but the ABS is tied into the autotrac controller.

No 4 LO could be something else like the VSSB.

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 to terminal F-12 @ PCM


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


C-15: DK GRN vssb output to speedo

 

[FellowTraveler]

When shifting the AutoTrac transfer case to the 4 LO mode, it commands the encoder motor (9) to turn the control actuator lever shaft (8}, to move the high/low range shift fork (10). The shift fork (10) moves the high/low range collar (6), which is slip splined on the rear output shaft (7) toward the rear of the transfer case. The range collar (6) outer teeth disengage from the input shaft gear (2) inner teeth. The range collar (6) outer teeth then engage in the planetary carrier (5) teeth. The power flow is now from the input shaft gear (2) planetary teeth to the planetary gears (3) in the carrier. Rotating the planetary gears (3), which are engaged in the annulus gear (4), the planetary carrier (5) rotates. The planetary carrier (5) engaged to the range collar (6), drives the rear output shaft (7), providing a 2.72:1 reduction to the speed of the rear output shaft (7). The power flow to the front prop-shaft is the same as it is in the 4HI.


A neutral position is obtained when the range collar (6) is not engaged to the input shaft gear (2) or the planetary carrier (5). Neutral position is used for towing the vehicle.

 

[Big T]

I installed the Redhead Steering Box today, but could not get it into the alignment shop. I'll get an appointment next week and my wife will take it in.

Steering box was long over due. This cleaned up all the slop in the steering. None of the issues that Fermanator had with it being harder to turn in one direction. Steering is one finger control with no slop.

I passed on doing the bushings in the upper control arms. May do that tomorrow, depending on the plans my wife has for me.

Next we'll send the spare steering box core from the '95 wreck to Redhead to get that ready for the GMT 800 brake conversion on my son's '94 Suburban.

 

[Big T]

Update, got the alignment done. Took them a bit to get it right as it was way off.

My wife took it in and there was an issue on arrival as the power steering pump "burped" about a half cup of fluid. My wife put me on the phone with the shop. They asked whether I had bled the power steering pump. I replied that I did and went through the instructions that were provided by Red-Head Steering Gear. The shop said that those instructions were fine for a gas engine and pointed out that the same steering box was used for both gas and diesel powered trucks. Their advice was to pump the brakes 3x while turning the steering wheel from stop-to-stop and pump the brakes 3x at the stop, then do the same in the other direction, 15 to 20 times. They also said to do it with wheels on the ground for resistance. So, I need to redo the power steering bleed on both my son's and my Suburbans. They said that this should also firm up the brake pedal.

I did drive the truck around the block early this AM. The steering is tight and sweet. Brakes are great and the pedal is higher and firmer than the predecessor brakes.

I have tomorrow off and will start in on the GMT 800 front brake conversion on my son's '94 Suburban. His will be easier as he has new bushings in the upper and lower control arms, so I don't have to deal with removing them. I just need to remove and replace the knuckles, then install the new brake equipment. Hope it comes out as sweet as my'99.

 

[Big T]

I mentioned it before that the GMT 800 is a thinner casting than the GMT 400 where the ball joint go through. I've attached a pic showing the side-by-side comparison. Though I used the existing ball joint to measure as I reamed the knuckles, when you go to torque down the ball joints you run out of thread and the holes for the cotter pins are beyond the castle nuts. I used two 5/8" SAE washers to take up the space. Alignment guy said it shouldn't be a problem. The alternative would be to find ball Joints with not as long of threads.

 

[Big T]

Alright, got the GMT 800 conversion done on my son's truck. Got it aligned using this method:

http://www.thetruckstop.us/forum/th...on-a-gmt400-or-any-vehicle.35698/#post-410474

We had to jury rig the brake hoses by grinding off metal until they fit within the flat on the calipers. My son will order braided stainless hoses like mine and they have banjo eye bolts for the caliper mount, which a work on these calipers. I had two sets of the GMT 800 brake hoses from the bone yard, but they take a larger fitting thread and a different retaining clip. The GMT 400 hoses have a bracket that needs to be held down to the control arm or it will rub on the tire. They're interim until he gets the stainless hoses and he doesn't have plans to drive it this week.

Brakes are good. I think mine stop a bit better, but his ABS works.

 

[Big T]

Revisited the alignment on my son's to get the steering wheel straight. Pretty quick, and the steering wheel is good an level now, steering straight as an arrow.

Also, after burning in the front pads, the brakes are stopping like a beast. Same as on my '99.

 

[Big T]

I guess I'm one of the few doing mods on these trucks anymore?

After a break to use the truck, my next project will be (drum roll) pump rub fix in the transfer case and troubleshooting my non working 4lo and ABS. New ES engine and trans mounts while I'm in there.

Down the road, that new GMT 800 rear axle is prepped and ready to install. May do that first. Flip a coin.

 

[THEFERMANATOR]

I haven't felt the need to upgrade my brakes beyond the drw pieces. I can turn the abs on at will now, don't see a reason to do anymore. I never felt even with the 2500 brakes they were that bad. The 1500's had awful brakes, but hydroboost was decent to me. And I've already modded most of the rest of her. Should be near 450-500hp to the wheels now with no smoke, thats satisfactory.

 

[Will L.]

@Big T
I think it's just your turn to mod them right now. Later I am going to get you hooked on hummer/ hmmwv some how, then you'll be modding them. Haha

Hopefully my next trip to big bear will be in my hummer and I'll have you drive it. Then you'll see why I said "haha" about getting you out of a cozy suburban.

 

[Big T]

I haven't felt the need to upgrade my brakes beyond the drw pieces. I can turn the abs on at will now, don't see a reason to do anymore. I never felt even with the 2500 brakes they were that bad. The 1500's had awful brakes, but hydroboost was decent to me. And I've already modded most of the rest of her. Should be near 450-500hp to the wheels now with no smoke, thats satisfactory.

I don't know what your freeway traffic is like, or whether you tow, but if you experienced the difference between the GMT 400 and GMT 800 brakes, it would undoubtedly be on the same level of the performance increase that you saw by swapping in the Duramax. If you ever had one of those panic stops at highway speed where you just prayed the brakes would lock it up and stop you in time, then you do this mod. Then consider all the other benefits such as long life expectancy on the pads, ease of removal for both rotors and bearing hubs, and it's a no brainer.

I will also say that I learned a hell of a lot doing this project and I'm all about the learning.

 

[Big T]

Alright, post all this work we've had time to assess the results and the braking power is great. However, the 1994 has noticeably reduced steering radius. I can see the knuckles hitting the control arm at the rear and there are marks on the control arms from this impact. The '94 has the stamped steel lower control arms. Should we grind the control arms down at this spot to increase the turning radius?

 

[Will L.]

Pics?