Need help getting 4LO with ZF6 ECM tune

JoshH

Daggum farm truck
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I have an '02 truck we are trying to get ready to do some sled pulling, and I need to get the transfer case to shift to low range. It's normally a drag truck, but we are going to try out sled pulling. The ECM knows the transmission is in park/neutral (it actually thinks the clutch is depressed), but I guess it isn't sending the signal to the TCCM telling it it is ok to shift because the 4LO button just flashes with no attempt to shift. The truck was swapped to a duraflite and tuned as a manual transmission. I'm hoping maybe someone has fought and figured out this issue and can help me get this thing working. Is there a way to make the TCCM stand alone or otherwise tricked and so it will shift whenever the button is pressed? It shifts from 2HI to 4HI no problem, but it don't change range for anything (no neutral or low). Worse case, we can unbolt the shift motor and use a wrench, but it would be nice to get it working with the switch. Thanks for any help.
 

Chevy1925

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can you engage it manually? yes i know you dont want to but just want to rule out encoder motor or t-case issues. the mode fork and collar for 4lo is the crappy one that goes bad all the time on those.

does the ecm still see speed reference from the t-case?

per gm, its just as you say. Engine running, vehicle in neutral (clutch depressed if manual), and speed under 3mph. seems more like a hard part issue??
 

JoshH

Daggum farm truck
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I haven't tested it manually yet, but it literally makes no noises trying to go from 4HI to 4LO. The light flashes, but I can't tell it's actually doing anything. It's like it's looking for something to tell it everything is good to shift.
 

Chevy1925

don't know sh!t about IFS
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i wonder if it needs to see a clutch signal or NSBU signal. ill dig a little deeper.

EDIT: see if you can read codes from the TCCM. its probably something simple going on here.
 
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JoshH

Daggum farm truck
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Feb 14, 2007
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Texas!!!
i wonder if it needs to see a clutch signal or NSBU signal. ill dig a little deeper.

EDIT: see if you can read codes from the TCCM. its probably something simple going on here.
It is getting the signal from the NSBU to what would be the clutch pedal switch. I checked already, and there are no codes and no data to show why it won't shift.
 

Mikey52

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Sep 20, 2018
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My encoder went out on my Yukon transfer case. It shifted into 4 lo, then wouldn't shift out.
 

GMC_2002_Dmax

The Still Master
All ZF6 equipped trucks had a manual TC, they were never available with a PB-Auto TC.

The Manual ZF6 ECM File with matching BCM Logic would probably work, all you would need is a 2002 ZF6 VIN to flash the BCM and then swap in a manual TC.

The only issue I see is that the wiring for a ZF6 might be required for it to all work, as the harnesses are different for sure

Good Luck
 

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