Towing Diffs?

[JayArr]

My Envoy has 3.42:1 gears (GU6, G80) but I tow a 5000 lb trailer through the mountains. I do OK, I'm just slow up the hills.

It would be better if I had 3.73:1 or even 4.11:1 gears no?

Were the 4.11:1 only sold as the "tow package"?

Both Diffs are leaking at the seals so if I'm going to swap gear ratios now is the time, I don't want to replace all the seals and then crawl under 6 months or a year later and pull it all out to change it.

Dude on Craigslist is parting out a Canyon and has 8" 3.73:1 gear sets (GT4, G80)

Should I go for it or hold out for 4.11:1 to make the effort worth while?

Will I see a big difference from 3.42 to 3.73?

Will 4.11 make it hard to drive as a daily grocery getter?

Will I need to reprogram the PCM?

JayArr

 

[TJBaker57]

Will I need to reprogram the PCM?

As I understand it the 2005 4.2 PCM cannot be reprogrammed to accomodate different gearing.

I don't know if the ABS system will work in a 2005 if the gearing is altered. There may be issues with the EBCM seeing conflicting speeds wheel sensors vs the VSS signal?? Early models only had front wheel speed sensors. Sometime later rear wheel speed sensors were added.

Maybe someone makes some sort of "black box" to interface all this?

 

[JayArr]

Bummer.

I guess I'll just change all the seals and drive slowly up the hills.

 

[Mooseman]

As I understand it the 2005 4.2 PCM cannot be reprogrammed to accomodate different gearing.

That may not be entirely true. When a PCM is programmed via Tis2000 and the Tech 2, it only asks for the VIN and programs the proper gearing. Since the EBCM gets the rear wheel speed from the transfer case output shaft via the PCM, the fronts are directly from the wheels so that shouldn't need to be reprogrammed. The problem would happen when the front wheel speed didn't jive with the rear wheel speed because of the gear change.

Where it can't be reprogrammed is using tuning software like HPTuners.

What about the front diff? It's a bitch to set up a new gear set in it. Would probably be worth it to just swap the entire diff.

Personally, I would opt for 4.10 gears for towing, which were rare on this platform except for the SS. Car-part.com might have some. There are some out there, just not a lot and you'd have to get both front and rear.

BTW, if doing the swap, I'd opt for a 8.6" from an EXT/XL or a V8. Would be more robust for towing.

I have a pair of diffs from the old V8 Saab that had 3.73 and G86 LS diff. Just haven't got around to doing the swap. Rear is easy, front is a PITA.

 

[JayArr]

Can you clarify this:

When a PCM is programmed via Tis2000 and the Tech 2, it only asks for the VIN and programs the proper gearing.

Do you mean that if I enter in my VIN it will look up in a database for the correct 3.42 ratio that my VIN was built with and program the PCM for that ratio?

That's of no help at all when trying to change the ratio.

 

[Mooseman]

You would use the VIN of a truck with 3.73 gears. After the 30 minute relearn, it will work. The BCM doesn't care. There is a database of VINs with their options somewhere. Maybe even here.

 

[Mooseman]

Here is is:

VIN's with RPO's list

Thanks to @Bill Reid , here is a list of VIN's with their corresponding RPO options. This can be useful if you need to reprogram a PCM or BCM if you want to add an (or several) option(s). It is a spreadsheet (zipped) with quite a lot of VIN's with each year/model on their own worksheet. Some...

gmtnation.com

 

[JayArr]

Excellent solution Mooseman, thanks!

I've even got a spare PCM from when I bought my engine from the wrecking yard. I'll see if I can get the Tech II/TIS2000 fired up when I get home.

 

[TJBaker57]

Does anyone know a way to determine which ratio is in a PCM apart from being able to look it up in a VIN##??

I am wondering if I could feed a frequency to the PCMs VSS input from the trans and see what yhe PCM puts out for a speed?