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Tow haul light comes on as soon transmission is shifted into gear

Tiny
LYNP639
  • MEMBER
  • 2003 GMC C1500
  • V8
  • 2WD
  • AUTOMATIC
  • 273,000 MILES
The tow haul light comes on as soon as I put the truck into first or reverse to start driving. In most cases it is also engaged, cause I can feel and hear it.
Monday, June 22nd, 2020 AT 3:41 AM

21 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good morning,

Are there any codes in the system for the transmission?

https://www.2carpros.com/articles/automatic-transmission-problems

Does it shift well while driving?

Roy
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Monday, June 22nd, 2020 AT 4:27 AM
Tiny
LYNP639
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Yes, if overdrive is not actually engaged (even tho light is on). If it's actually in overdrive, it doesn't want to work up thru the gears to accelerate.
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Monday, June 22nd, 2020 AT 3:09 PM
Tiny
ASEMASTER6371
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Okay, can you check for any codes?

There is a fault somewhere.

Roy
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Monday, June 22nd, 2020 AT 3:14 PM
Tiny
LYNP639
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I am not a mechanic, justt the owner. So no equip to test. Guess I'll have to take it in and leave it running next time it does this, so they can check it for codes while the light is on?
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Monday, June 22nd, 2020 AT 4:20 PM
Tiny
ASEMASTER6371
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A parts store will read them for free.

Once I get the codes, I can narrow the area of failure down for you.

Roy
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+1
Monday, June 22nd, 2020 AT 4:22 PM
Tiny
LYNP639
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Thank you. I will take it out at lunch tomorrow and if it's doing it, get a store to read it for me. I've had them change batteries, and headlamps but didn't know they'd check for codes. Thanks for that tip!
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Monday, June 22nd, 2020 AT 8:27 PM
Tiny
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You are welcome.

Keep us updated with the results.

Roy
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Tuesday, June 23rd, 2020 AT 4:30 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,702 POSTS
Here is a guide to yelp you get the codes. but it sounds like the tow/haul button/switch is bad have you try cycling the switch?

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Please run down this guide and report back.
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Tuesday, June 23rd, 2020 AT 11:56 AM
Tiny
LYNP639
  • MEMBER
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Thank you Ken. I have ordered a code reader. Had no idea I could do it myself! Very helpful article, and I appreciated the link to the codes in the article so I didn't have to buy a reader that tells you their description. Will be back in touch in a few days.
Also, if you will tell me how to "cycle the tow haul switch, " I will try that too.
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Thursday, June 25th, 2020 AT 2:22 PM
Tiny
KEN L
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Good work, cycle the switch mean to push it on and off many times to see if there is a change.
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Friday, June 26th, 2020 AT 11:00 AM
Tiny
ASEMASTER6371
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When you tow heavy loads, it is recommended to turn it on. It disables the clutch converter in the torque converter to put less pressure on the transmission. Otherwise, you could damage the clutches. Those clutches are the least amount in any clutch pack and can easily be damaged if they are over worked by the heavy load.

Roy
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Friday, June 26th, 2020 AT 11:04 AM
Tiny
LYNP639
  • MEMBER
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I've gotten the code reader. Used it this morning. No codes, even though tow haul light came on after cranking. Is it possible the tow haul switch wire is going bad? I saw some YouTube videos about it breaking after flexing so much during use.
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Monday, June 29th, 2020 AT 5:18 PM
Tiny
ASEMASTER6371
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Okay, what is the brand code reader you got?

Some of them will not read anything but engine codes. They will not read any other system.

A quick test you can do is this. Run it up to 55 MPH and with your foot on the gas, let the other one touch the brake pedal and see if the transmission downshift.

If it does, then it was in overdrive. If it does not downshift, it will not change at all.

Roy
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Monday, June 29th, 2020 AT 5:26 PM
Tiny
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I bought an Actron CP9125 OBD II PocketScan.
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Tuesday, June 30th, 2020 AT 4:28 AM
Tiny
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Okay, thanks. That just reads the engine system. It will not read ABS or any other module.

Did you do the test I suggested with driving?

Roy
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Tuesday, June 30th, 2020 AT 4:51 AM
Tiny
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Will do that later today.
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Tuesday, June 30th, 2020 AT 5:32 AM
Tiny
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Okay, keep me updated with the results.

Roy
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Tuesday, June 30th, 2020 AT 5:38 AM
Tiny
LYNP639
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Did the driving test you suggested several times. It did not downshift, unless I really pressed down on the brake. Also, it hasn't gone straight into tow haul mode after cranking and shifting into gear, in several weeks. It has been very warm in the mornings, like mid high 70's, so I wonder if air temperature makes a difference?
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Friday, July 24th, 2020 AT 3:37 AM
Tiny
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Are there any codes set?

Roy
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-1
Friday, July 24th, 2020 AT 3:41 AM
Tiny
LYNP639
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Will check when I leave work this afternoon.
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Friday, July 24th, 2020 AT 5:03 AM

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