ABS Shutdown due to 4.10 Axle swap

As most of you know I did an axle swap from 3.42:1 to 4.10:1 ratio. The changes have been tuned into the PCM for shift points etc. and it runs fantastic with no drive-ability issues.

After I drive for several miles the abs system shuts down and ABS/Brake lights illuminate, also I am recording excessive mileage of about 1.4 miles to 1.

I have researched this extensively and can not find a direct answer.

My Question: Which unit do I need to focus on? Can the BCM be reprogrammed for the 4.10 axle? I know that the instrument cluster is controlled by the BCM, so that is where i suspect I need to go. However some say it may be the ABS module itself? Does anyone know the answer, or a link to where the answer may be found? I now have a way to correct the mileage so I would like to get everything back into check.

Maybe someone with a tech 2 could connect to their truck and see If the option exists to make the necessary changes to the BCM?

Any help is greatly appreciated! :tiphat:
 

HARDTRAILZ

Moderator
I think shadowgn was working on similar issue on ortb
 

Sparky

Moderator
I don't think there is a fix for the odometer/speedometer issue since yours is an 05. 06 and up can be corrected easily but there is something about 02-05 that prevents it.

I don't know about the ABS side of things where that is all controlled, or whether or not that can be corrected. I didn't know it could be an issue in the first place, but thinking about it makes sense that it can be.
 

Tiggerr

Well-Known Member
Sparky said:
I don't think there is a fix for the odometer/speedometer issue since yours is an 05. 06 and up can be corrected easily but there is something about 02-05 that prevents it.

I don't know about the ABS side of things where that is all controlled, or whether or not that can be corrected. I didn't know it could be an issue in the first place, but thinking about it makes sense that it can be.
I've always heard this as well for 02-05's. Apparently it's been tried to death and is not doable. Idk what the difference in BCM's would be for the later years. I wonder if you could swap in a later model BCM to get around that. Haven't seen where anyone tried that.

Maybe limequat would know, or PMCofNC.
 

HARDTRAILZ

Moderator
I think the 3 vs 4 channel ABS is in cahoots with the issues.
 

Sparky

Moderator
Yes, very likely since the 3ch ABS has the sensor on the tail shaft vs at each wheel. Regearing changes what the sensor sees. 4ch doesn't have that problem.
 

coolasice

Well-Known Member
Only way to fix that issue would be to reprogram the pcm with calibration from a 4.10:1 vin.

Tire size correction should be able to be done with the sps system (according to the manual anyway..)
 

HARDTRAILZ

Moderator
coolasice said:
Only way to fix that issue would be to reprogram the pcm with calibration from a 4.10:1 vin.
I think this was tried and did not work.
 

Sparky

Moderator
It is, yeah, but so far everything I've seen trying to correct it has either not worked or made it worse.

There is some weird programming juju that GM did with the 02-05.
 

Shdwdrgn

Well-Known Member
Yes the issue can be fixed permanently for the older models. If you have HPTuners, the linked file is a 2004 stock PCM tune for 4.10's with 30.6" tires (I think that was the largest available in the system). The gear adjustment corrects the issues for the ABS system so you won't get the warning lights any more. This issue frustrated me to no end, and the dealership couldn't get any help from GM... I finally found a VIN online from a salvaged trailblazer that had the 4.10 gears stock. My dealership was able to reprogram me using that VIN, and then I saved a copy of the stock tune.

A couple possible issues with this... Since nobody else has tried it, I don't know the exact details, but you will of course have to license your PCM in HPT if you haven't already, and you *might* have to get another license to read my file, since it came from a different PCM. If you try this, please let me know the results, but hopefully you do NOT need to pay for two licenses just to make this change.

Second, the VIN will be wrong (I think the VIN in this file is from the original doner PCM). After you flash the new image, HPT gives you a means of then reprogramming the VIN to match your vehicle. I don't know how critical this is, because I drove around for the first two weeks before remembering to change it, but I recommend you do it just to be safe.

Nobody seems to know exactly which parameters are chaging the gear ratio in the PCM. I sent this file over to HPT and they tried some tests but were unable to narrow it down. What I do know is that I could do a simple flash back and forth between my stock tune for 3.42s and 4.10s, and both the speedometer and ABS issues would come and go. I also have a spare PCM, which I flashed with the 4.10 file, and that worked perfectly as well. So I know for certain this does make the required changes.

If you have a custom tune (I have a modified PCM4Less), there's no easy way to transfer the settings back without overwriting the gear ratio. What I ended up doing was going through and copying every table that had been changed, and pasting them back into the new tune. HPT does make this easy, since you can load a referenece file to compare against your current tune, then just copy each table from one screen to the other. You will want to adjust things like your transmission shift tables down by about 19% so you're not racing the engine. Also I had to really cut back on the shift time tables. I had decreased mine to make firmer shifts, and had to increase them to keep the transmissions from ripping itself out (especially in 4LO). My current table starts with values of 0.32/0.16/0.00, and it's still a pretty firm shift without rattling your teeth.

Sorry I don't have any experience with other tuning software, but if you have any more questions I can certainly try to answer them.
 

Sparky

Moderator
OK so it can be done, but it just isn't very clear. Interesting!
 

HARDTRAILZ

Moderator
Glad the Pm worked and hopefully this helps BT with the info you worked so hard to gather.
 

Shdwdrgn

Well-Known Member
Yeah both the gear ratio and the actual tire size settings seem to be hidden or encoded in such a way that nobody can find them. You can make adjustements to the tire size with the tech II scanner, but not the gear ratio. And yet, somewhere in the block of code that HPT copies when you save a tune file (and you only have to write the calibration, not even the entire tune) all of that information is stored.
 
OP
BlazingTrails

BlazingTrails

Banned
Thanks for the direction guys. I will keep digging into it and see if I can find anything in hptuners.

I will say this though from dash command running on the obd2 port that speedometer Is correct. So I believe that the pcm is tuned correctly. It's my understanding that the signal goes from the pcm to the bcm to the cluster through can bus 2? This is a very strange predicament considering how many people have swapped axles. If I can get this resolved o will Def post my findings here.
 

Shdwdrgn

Well-Known Member
@BlazingTrails -- I think you missed the point of my post. No, you will NOT find the settings you need in HPTuners, nor in any other tuning software. You will also not find anything in a Tech II scanner. That's the problem, the settings you need are buried deep in the code and cannot simply be changed.

Regarding reading the speedo from the ODBII port -- yes there are settings you can adjust that correct PART of what the computer is reading, and the display from software can be adjusted so it shows the correct speed. However your dash speedo will still be incorrect, and the numbers that are read by systems such as the ABS will also be incorrect. Believe me, I've been through everything. I fought with it for over half a year, trying different settings in HPT, combing through years worth of forums, etc. The answer is not out there. The answer is not in your BCM, nor is it in your ABS system. What you are looking for is in the PCM only.

If you take a copy of the PCM from a truck that had the gear ratio in question (in this case, the 4.10's), and copy it to your own PCM, all of the settings you need will be blindly copied as well. This means your speedometer will read accurately, your ABS system will have accurate values to work from, and all systems will once again work exactly as they are supposed to. My speedo reads within 1/2mph at 50mph, which is more accurate than when I got the truck stock. My odometer is similarly accurate.

Seeing the new post above... I looked into those speedo recalibrators as well. They *might* correct your dash speedometer reading (I read a lot of reviews both ways, so I'm not sure they actually work on our platform), however they will NOT fix the problem with your ABS system. Your PCM still has the wrong gear ratio, and is making incorrect calculations to give to the ABS.

If your PCM is stock other than the changes made to try and correct for your new gears, you can use the HPT file that I posted directly in your truck, with the only caveat being that it was adjusted for 30.6" tires. Anyone with a Tech II scanner can make the change to tire size after the PCM has been flashed, if you have smaller tires, however even without that the system will still be close enough that your ABS system will stop misbehaving (your speedometer will read slightly faster than actual speed though).

Why mess around with chips and add-ons and other crap? Just use the PCM code that GM created for 4.10 gears and stop worrying about it.
 
OP
BlazingTrails

BlazingTrails

Banned
I misunderstood past of what you said before. I already know to do this I will need another $100 in credits, at this point I'm ready to get it done. Also my tires are 30.5" I believe so that part should be fine. Thanks for the info.
 

Shdwdrgn

Well-Known Member
That's pretty much where I ended up. The dealership exhausted every avenue they could think of, so I dropped off my truck, with the spare PCM in back in case it failed, provided them with the VIN that I had found, and told them to just flash it and not worry about if it would work or not. I had worked with their tech long enough by that point that he knew I had a good idea of what I was doing with the system anyway, so they went ahead and followed through for me... and they were surprised that it actually worked! By that point the tech was also curious about the whole matter, and he took time to calibrate the tire size and ran various tests on the system to see if everything was syncing up like it should, and didn't charge me for the extra time he spent on it. I probably spent around $300 at the dealership anyway, trying to get it fixed, plus another $100 in credits to license my space PCM, but it was so nice to have everything including the speedometer working accurately again.

Your profile doesn't give a location, but if you're anywhere near Denver I could slap in my spare PCM and show you that it fixes everything.
 

gmcman

Well-Known Member
Sounds like the PCM swap from the donor vehicle would work. The thing about the yellow box is it would correct the faster reading from the tailshaft to coincide with the slower rotation of the front hubs. That's where I believe the issue is, the PCM is seeing the faster rear wheels but if we can slow down the signal from the rear ABS sensor in the trans, then wouldn't that work? Am I understanding this correctly or is this much deeper involved with RPM's and gears and known speeds from the RPM and gear ratios?
 

Shdwdrgn

Well-Known Member
@gmcman -- My understanding is that because the older vehicles only have three sensors for the ABS, and one of those is on the tailshaft rather than the wheels, the PCM actually does a real-time calculation against the expected gear ratio to determine what it believes the rear wheel speed is. And when you change the gears, that throws the calculation way off. Now the folks who have only changed gears by one step (going from 3.73s to 4.10s) have not had the same problems, and I believe this is because that much change still falls within the same range that you might see when simply changing tire size.

The problem with the speedo adjustment computers (like above) is that they seem to only change the rate of the signal going to your dash, so while the speedometer may read correctly, the PCM is still processing incorrect values and causing the same errors throughout the rest of the system.
 

gmcman

Well-Known Member
I guess the whole front to rear bias is out since all the front hubs are the same. When I replaced my front hubs there was no gear ratio specific front hubs.

So back to the rear speed issue, wouldn't the yellow box connected to the tailshaft sensor solve this? Not connecting to the dash for the speedo only, but the signal going to the pcm?

Does the 2005 still use the single rear abs sensor? I thought that started in 06 or 07.
 

Sparky

Moderator
Yeah actually that probably would.
 

Shdwdrgn

Well-Known Member
The single rear ABS sensor was used in 2002-2005. It was 2006 and later that they started using full wheel-mounted sensors on the rear, which is the specific reason why when changing your rear axle you have to get it from the right group of years (the later ones have the sensors mounted to the axles).

Yeah if the yellow box adjusts the tailshaft sensor also, then it likely would correct the ABS issue. I still don't know why you wouldn't choose to just get the factory programming in the PCM over a third-party solution though.
 

gmcman

Well-Known Member
Shdwdrgn said:
I still don't know why you wouldn't choose to just get the factory programming in the PCM over a third-party solution though.
I think that's the plan but the yellow box would be a 2nd alternative if all else fails.
 

Matt

Silver Supporter
Shdwdrgn said:
The single rear ABS sensor was used in 2002-2005. It was 2006 and later that they started using full wheel-mounted sensors on the rear, which is the specific reason why when changing your rear axle you have to get it from the right group of years (the later ones have the sensors mounted to the axles).
The caveat to that is swapping backwards like I did...06 to 02. Cut the wires for the ABS sensors on the rear axle and call it good.
 

Shdwdrgn

Well-Known Member
For those who were following along, we ran into a snag... HPTuners refuses to use a file from a different year of vehicle. I tried changing the VIN in the file, and it just won't let me do it.

Now like I mentioned before, you can probably flash the PCM with the code and just run it without changing the VIN, but that may or may not affect other components in the system.
 

Shdwdrgn

Well-Known Member
At the very worst you can do the same thing I did... scour the internet for 2005 vehicles that have the GT5 code, and see if any of the links lead you to a VIN. The few that I found were all from parts dealers selling used axles, and they had posted pictures of the code sticker from the glove box. Once you have a solid match, take that VIN to your local dealer, ask them to do a complete PCM reload using this VIN rather than the one on your truck (this is the same programming they would do if they got you a brand new PCM). If you are lucky, you can also convince them to adjust for your tire size while they're in there.

Once that is done, you will have a build from the correct year and gearing. Now fire up HPT, download entire, and save that to a new file. Change the VIN number in the settings, write it, and you are completely stock again and will pass any testing. At this point you can start copying over any settings you had modified from the previous PCM -- but DO NOT just load the previous file and flash it... that will overwrite your gear settings. You have to copy the previous settings by hand (copy/paste between tables). If you accidentally flash from an old file, just re-flash from the stock file you saved above. You'll see the same thing I did -- flash from a 3.42 file and your speedometer is about 20% off. Flash from the 4.10 file again, and your speedometer is accurate once more. The settings are somewhere in the block of data HPT saves, if someone can find them.
 

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