gear ratio and tire size change

American Delco GM ECUs and PCMs, ALDL, OBD 1.5.
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gear ratio and tire size change

Postby pmcquay » Fri Jan 31, 2020 4:46 am

some background: I am building a 96 gmc sierra 2wd into somewhat of an overlanding truck/fun camping vehicle, and two changes I am planning on making are the axle ratio (3.42 -> 4.10) and the tire size (~29 inch to 33).

Nowhere I have found has a definitive answer on how I actually change this in both the PCM and the ABS control unit. I know this is an older blackbox PCM, but I'd have no problem changing to an 0411, and may do that anyway. I'm really looking for more information on the ABS unit, because I have to keep that per Canadian law. I'd also like to not get screwed by insurance if I have an accident.

I know (think?) that there are some changes I could make with a Tech2 or MDI in the ABS controller, but I don't know whether I could change the tire sizes that far, and I haven't seen any information on the gear ratio, or even if I need to change it (I think I do because the rear wheels don't have sensors, its all done by the VSS in the trans, and should thus be affected by the gear ratio changing. I could be very wrong here though).

I was recommended to post this question by PCM Hammer when I messaged them on Facebook about this.

So I guess my question is, does anyone know how to change these parameters in their respective locations, or point me in the right direction?

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Re: gear ratio and tire size change

Postby pmcquay » Fri Jan 31, 2020 12:42 pm

Also to note, I am a programmer by trade, and have dabbled in some electronics work, and am willing to get into the guts of this thing if need be. If someone has experience taking something like this apart and is willing to give me some direction, I would be all ears

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Re: gear ratio and tire size change

Postby antus » Fri Jan 31, 2020 2:03 pm

I havnt done this myself, perhaps someone else can chip in if they know for sure, but I dont think you'd need to change the ABS. I dont think it needs to know the speed, only if one of the wheels is slipping, which it can still do regardless of tyre diameter. As for the pcm, there is a setting PPK or PPM (pulses per km / miles) you can adjust by the percentage of difference to keep the speed reading correctly in the PCM. Where your dash gets its speed signal is another question. If you do have a problem with this and cant fix the dash you can use a hardware speedo corrector inline with the speed sensor. pcmhammer cant currently flash black box computers. there is a tool by dimented24x7 which can with an avt interface, but swapping to an 0411 would give you a much more widely known and supported platform. Auto trans you could test and see. Most the aussies do not change these tables in the earlier pcms but you would be able to if those parameters are defined in the definitions you have for the operating system id you plan to run.
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Re: gear ratio and tire size change

Postby delcowizzid » Fri Jan 31, 2020 8:32 pm

The older chevy trucks had a vss buffer that devided the vss signal to make the same dash work with varying ratios and wheel sizes I know a lot of the USA guys used to get the right one for there applications from pickapart or upullit
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Re: gear ratio and tire size change

Postby pmcquay » Fri Jan 31, 2020 8:56 pm

@Antus, I think the reason I believe it needs changed is because the front sensors are on the wheels and the rear is just using the vss in the trans, which is ahead of the rear diff, so if I change the diff, the ratio between the front and rear sensors changes. I dont know if the abs is smart enough to pick that up. I have considered making or buying a solution that would alter the speed sensor signals, it is an option for me for sure.

@delcowizzid, I think my truck may have something like this, but I know that the 88-95 ones had a box with dip switches on it, and that mine doesnt. I will do more research into this.

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Re: gear ratio and tire size change

Postby pmcquay » Fri Jan 31, 2020 10:32 pm

hmm.. preliminary research suggests that the VSS buffer was only on the 95 and older trucks. http://67-72chevytrucks.com/vboard/show ... p?t=322002 96 and up apparently the vss just goes to the PCM.

Also, per the pinout for the PCM here https://troubleshootmyvehicle.com/gm/4. ... 997-page-1, looks like the PCM passes the VSS signal to the ABS on pin 15 of the C1 connector. There appear to be no more connections to wheel sensors on the PCM which makes sense, it doesn't need to know what the front wheels are doing.

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Re: gear ratio and tire size change

Postby pmcquay » Fri Jan 31, 2020 11:16 pm

I wonder of the PCM does the conversion for the rear sensor so that the ABS doesnt have to know about the gear ratio. I could potentially test that with an oscilloscope on one of the front sensors and the vss out from the PCM.

Does anyone know if the 0411 PCM does it that way?

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Re: gear ratio and tire size change

Postby pmcquay » Fri Feb 28, 2020 10:05 pm

Took a while, but I think I found a solution. Looks like a 411 swap is in my future, because at least one tuning solution (HPTuners) appears to have a setting for doing exactly what I want. https://forum.hptuners.com/showthread.p ... post281353

Does anyone know if this setting is unique to HPTuners? I would imagine it isn't, but information on this is frustratingly vague.

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Re: gear ratio and tire size change

Postby B52Bombardier1 » Sat Feb 29, 2020 9:47 am

This is not unique to HP Tuners. These things can be changed in the OS using a program called Tuner Pro RT using several simple calculations within the speedometer tab. Please see attached pictures.

Rick

Tire Diameter.png
Tire Diameter.png (23.65 KiB) Viewed 5032 times
Gear Ratio.png
Gear Ratio.png (20 KiB) Viewed 5032 times

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Re: gear ratio and tire size change

Postby pmcquay » Mon Mar 02, 2020 11:48 pm

@B52Bombardier1 You are a hero! That is exactly what I have been hoping to find!

From the looks of your screenshot and some googling, it seems H0106 is the abs output I am looking for?

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