tow haul causes eratic shifting

Water Dog · November 11, 2020

My 08 3500 HD when the tow haul button is activated instead of shifting down it will drop from 5th gear to 2nd or shifts up & down back & forth. You do not have to hit the brakes it will start when you let off the gas. The trans was replaced last year with a rebuilt. The rebuild co. has replaced the valve body & internal computer. This did not change the problem any. The truck does not have a problem when tow haul is in off position. It does not matter if I am pulling my mini Ex or if I am driving bobtail. The problem is there regardless. My local service center is at a loss. Any info would be appreciated.

steelhead_steve · November 11, 2020

It is the second break switch on break peddle telling the system you have break pedal pushed.

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newdude · November 11, 2020

I wonder if your stop lamp switch is seeing the brake pedal is active even though your foot isn't touching it.

Does your cruise control work if you have cruise?

Water Dog · November 12, 2020

You may be on to it. I replaced my tail lights in my service bed about a month ago with leds. At night I can see low voltage is going to the tail lights. They have a dim glow in the center diodes. Newdude, as for the cruse control, it has been hit or miss for about a year. Steelhead_steve I didn't know that it had a secondary brake light switch. I will research & follow up. Hopefully this will resolve the problem I appreciate yall taking the time to help me out!