Active Grade Braking issue

[PJDYukonXL]

2016 Yukon XL SLT with 6L80 transmission and 97k miles.

Recently had transmission rebuilt by local shop (probably overpaid, but that's a different topic) about 1.5 months/2k miles ago.

Last night on the highway (flat Florida) at 70mph the transmission suddenly started jumping between gears. It shifted 5 times in a 15 second interval at one point. This continued all the way home (every 10 seconds or so) while either maintaining speed or coasting.

The best way to describe it is a though the vehicle is trying to downshift to slow the vehicle, like engine braking - even though I'm not trying to slow down or going downhill.

I was able to disengage the active grade braking by holding down the trailer button on the column shift and the problem did go away.

Has anyone had a similar experience/ failure? I'm taking it back to the shop for diagnosis.

Another symptom- active cruse control will not engage. It drops out as soon as I try to enable.

Not sure if/how the 2 are related?

 

[Joseph Garcia]

@NickTransmissions

 

[NickTransmissions]

Thanks, @Joseph Garcia

@PJDYukonXL sent me a private message about this issue yesterday morning. I responded shortly thereafter but have not heard back from him.

 

[PJDYukonXL]

@NickTransmissions

Took it back to the transmission shop and we determined it was not a mechanical issue or related to the rebuild. They took it to the local GMC dealer for diagnosis.

Problem solved... here's from Dealer:

SCAN ALL MODULES AND CHECK FLUIDS. FOUND TRANSMISSION UP AND DOWN SHIFTING IN 40/45 MPH RANGE WITH STEADY THROTTLE. FOUND THESE SHIFTS WERE BEING COMMANDED BY ECM. FOUND THROTTLE BODY HAD EXCESSIVE CARBON BUILD UP WITH COMPENSATION AT 64%. CLEANED THROTTLE BODY AND RELEARN IDLE. GAIN ACCESS TO CONNECTORS AT BRAKE PEDAL POSITION SENSOR, X115 AND X119. CLEAN TERMINALS AND ADD DIELECTRIC LUBE. RELEARN BRAKE PEDAL POSITION IN BOTH ECM AND BCM.

Cost me an additional $740 in labor by dealer, which I'm told was the wholesale cost charged to transmission shop, not retail rate.

 

[p1zaul]

@NickTransmissions

Took it back to the transmission shop and we determined it was not a mechanical issue or related to the rebuild. They took it to the local GMC dealer for diagnosis.

Problem solved... here's from Dealer:

SCAN ALL MODULES AND CHECK FLUIDS. FOUND TRANSMISSION UP AND DOWN SHIFTING IN 40/45 MPH RANGE WITH STEADY THROTTLE. FOUND THESE SHIFTS WERE BEING COMMANDED BY ECM. FOUND THROTTLE BODY HAD EXCESSIVE CARBON BUILD UP WITH COMPENSATION AT 64%. CLEANED THROTTLE BODY AND RELEARN IDLE. GAIN ACCESS TO CONNECTORS AT BRAKE PEDAL POSITION SENSOR, X115 AND X119. CLEAN TERMINALS AND ADD DIELECTRIC LUBE. RELEARN BRAKE PEDAL POSITION IN BOTH ECM AND BCM.

Cost me an additional $740 in labor by dealer, which I'm told was the wholesale cost charged to transmission shop, not retail rate.

Having the exact same issue, would you think the main issue is the throttle body needs to be cleaned?

 

[p1zaul]

Any update on this? Still having exact same issues after new throttle body so thinking break sensors or maybe just reset bcm? Did they replace the sensor or just clean them?

 

[p1zaul]

Solved

Update on this in case anyone having similar issues. I noticed the abrupt down shifting was completely fixed by turning off the grade brake system (holding in trailer button for 6 seconds)

In that case it was clear that was the problem.

I located the harness near the brake pedal referenced in the above post and disconnected and sprayed with MAF cleaner (as safe for electronics and evaporates quickly) added some dielectric grease and put back together.

Second I removed battery terminals and connected through a resistor to drain all energy and reset all internal computers.

Car now runs normal