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robbiels7 · 12-14-2017, 01:11 PM

Good afternoon ladies and gentlemen. I'm having troubles with my tow/ haul mode. When this mode is selected the transmission goes between the harsh downshift as if you were trans breaking and back and forth as if it was searching and did not know what to do. I myself as a profession is an automotive technician. Unfortunately however our database shows very limited information on this particular portion of the vehicle. I don't have access to any of GMC factory information. All I can tell at the moment is there is a fuse. And the button itself sends a ground to the body control module. From there it seems the body control module seems to control everything else. I do have a cruise control brake switch code P0 573 with intermittent operation of my cruise control I have not gotten around to fixing yet. I know it's cheap and easy just have to do it. My question is has anybody else run into anything similar to this? Other than being tow haul mode the truck drives amazing no problems the transmission other then that so I don't believe that is any kind of transmission issue. Now is a cruise control confusing the breaking portion of tow haul mode? I understand that some people are having issues with the stock itself but that is usually stuck on or stuck off. If anybody has Insight I would love to hear it thank you.

robbiels7 · 12-14-2017, 05:15 PM

Just a quick update. While driving home I've tried the tow haul mode both while I can get the cruise control to come on. Also when I cannot get it to work due to the faulty intermittently working brake light switch. I'm going to replace the switch this weekend and hopefully this will fix both problems. I will post the results and hopefully will help somebody else down the road.

cherv · 12-14-2017, 06:08 PM

are you towing a trailer in tow/haul or are you in it solo? tow/haul should never be used solo...per gm

robbiels7 · 12-14-2017, 06:16 PM

Okay everybody I'm so impatient that I had to put the brake switch in tonight. Found at ACDelco brake light switch at the local Advance Auto Parts. It seems to be fixed so it took care of the tow haul mode even though the only thing that I can see that is in common with it is the control of the BCM. I hope down the road that this help somebody troubleshoot their issue good luck and happy camping!

robbiels7 · 12-14-2017, 06:44 PM

Cherv. Usually the only time I would use this operation featuring Towing. That is how I found the trouble to begin with along with my cruise control not working either. The only reason for using it not telling tonight was to verify that it had anything in common with the cruise control.

JIMNLIN · 12-14-2017, 06:49 PM

which GM transmission are you talking about ??

robbiels7 · 12-14-2017, 07:24 PM

4l90/E The basic principles should apply to all of them with that option though.

lynnmor · 12-15-2017, 06:26 AM

You can use tow/haul mode whenever you want, towing or not. It simply changes the shift point and is sometimes useful on less than ideal roads.

robbiels7 · 12-15-2017, 06:29 AM

I'd use it this morning from Clearwater to Seffner which is about a 45-minute ride. And yes it is fixed. I'm glad my intuition was correct. I just wish there was more information on this function of the vehicle when it does come up with an issue.

MrDigger · 01-01-2018, 08:20 PM

I do hope that you got your issue taken care of.
I would like to give a few pointers though....
You never stated the vehicle. Hard to give good info with no info to start with.
GM did not have a 4L90e. It would have been a 6L90e.
You said that it was controlled through the BCM? More along the lines of the ECM. BCM is body control function. Cruise would go through BCM to ECM.
GM had the 4L80 and the 4L85. The 85 series being the heavy duty variant with a 5 gear planetary. Usually in the 3/4 ton Suburbans as the Allison was to tall to fit in the tunnel. Side note, the Allison with the straight cut gears were a very noisy setup, good but noisy.
Tow/haul mode. Should not use it unless towing a trailer.
The computers will "learn" how it needs to shift. If you run it empty and then tow a trailer it just does not seem to know what to do. If you have the capability, with a scan tool to do a complete drive cycle relearn when you hook up the trailer. Trust the computers to figure it out.

2006 3/4 Suburban 8.1L. I can pass anything, except the gas station. [emoji23]
I wish I had a Duraburb.

robbiels7 · 01-01-2018, 08:25 PM

Yes sorry it is a 2008 gmc sierra 2500hd and I wish I had a duramax also.

MrDigger · 01-01-2018, 08:46 PM

Duraburb.
It is a Suburban with a Duramax finessed into it.
GM said that if it ever made a diesel Suburban it would be just like the ones that they turn out.