4L60E transmission without Tow/Haul vs. 4L60E with Tow/Haul?

c good

Original poster
Member
Dec 8, 2011
526
My Envoy has the M30 4L60E tranmission without a tow/haul feature. I'm looking at buying my friends Tahoe. Same year as my 2002 Envoy. His transmission is also an M30 4L60E but it has a "Tow/Haul" button. I understand what the tow haul does. I'm just wondering why my same year Envoy wouldn't have it and if there's an actual physical difference between the transmissions or it's just a software issue.
 

TJBaker57

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Aug 16, 2015
2,897
Colorado
My Envoy has the M30 4L60E tranmission without a tow/haul feature. I'm looking at buying my friends Tahoe. Same year as my 2002 Envoy. His transmission is also an M30 4L60E but it has a "Tow/Haul" button. I understand what the tow haul does. I'm just wondering why my same year Envoy wouldn't have it and if there's an actual physical difference between the transmissions or it's just a software issue.

I also believe it is purely software, of the PCM and for the actual switch, the BCM. My 2005 Yukon has the Tow/Haul mode. The switch is connected to the BCM. When pressed the BCM sends out the functionally addressed message "A8 3A 40 30 01" on the serial data bus. The PCM acknowledges this with the message "A8 3B 10 30 01". For the switch back the '01' in both messages is replaced by '00'. Sending the messages on a vehicle without the feature in the PCM does nothing, the message is ignored.
 

mrrsm

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Mooseman

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Dec 4, 2011
25,257
Ottawa, ON
GMT360/370 never got the tow/haul mode button. Why? Who knows. Maybe GM never thought it to be tow/haul worthy but then they did include a 7 pin trailer connector with ability to hook up a brake controller.

There was some discussion about it on the OS. The oddball PCM doesn't support it as best as I can figure.

A possible way of faking tow/haul mode is to have a second PCM with a tow/haul tune as the primary shift mapping. Tow/haul does allow shifting into 4th but less often than the regular tune and at a specific situation (flat road and light throttle) and also will downshift for engine braking if accelerating with no throttle.

Important thing to do while towing anything really heavy is to put in 3 to keep it from hunting in and out of OD and causing a lot of heat. When I towed with the TB or Saab, I just put it in 3rd. Didn't really miss tow/haul and just downshifted manually when needed for engine braking.
 
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c good

Original poster
Member
Dec 8, 2011
526
This is the reply I received from Sinister Performance.

Cameron, I own a 2004 TrailBlazer with the same engine and transmission your Envoy has. These trucks don’t have a separate TCM – they only have a PCM (that controls both the engine and transmission in these applications).



There is no way to program the PCM for a tow/haul switch, unfortunately. I’ve looked into this for my Trailblazer and there is just no way to do it as the tow/haul mode was not a factory option for these vehicles.



It is a good idea to put the shifter into “3” range so the transmission won’t be able to shift into 4th gear while towing.



I can custom tune your PCM to adjust the transmission shift points and firmness settings (as well as increase line pressure) if you so desire. But these changes will impact all aspects of driving as there is no tow/haul mode switch option.



Good call on the second PCM Mooseman. That's a great idea. Depending on price I might do that. Otherwise just keeping it in 3rd seems to be working just fine. Thanks for all the input.







Ryan Gick

Sinister Performance, LLC.

Waterloo, Indiana USA

 
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