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Alright... So as far as the loss of communications codes I was getting, I do believe I have a fix. I used my labscope and connected to the data line in the DLC. With the engine running, I found that a single module was spiking the voltage up to 8-10 volts. On this car, no module should ever go above 7 volts and if any of them do it'll start setting U codes.
After removing the "comb" in the splice pack under the dash and checking each individual module at a time I found that the PAC RP5-GM11 Radio Replacement Interface that I used with my aftermarket radio was at fault. While testing that data line alone it was an almost constant battery voltage signal after it'* been on for a few minutes.
So I removed that module and just wired up the radio differently.
After reinstalling everything, the data bus voltages on the scope went back to normal.
As far as the brake switch and cruise control and transmission go, I did find that the original brake switch sensor was bad. The reference voltage with the brake pedal in the resting position is supposed to be somewhere between .9-1.4 volts when it'* calibrated. Mine was sitting around 3 volts causing the TCC to disengage and let the torque converter slip. After installing a new brake sensor and re-calibrating it, I took the car for a test drive and the cruise control worked again as well as my instant MPG went up. I also didn't feel any of the surging or slipping from the transmission.
I will definitely monitor everything up until I replace the transmission. I also looked into the TAP Cells and found that the only one that was high was 2-3 cell 8 which was around 24psi. All of the rest of them were below 7psi. Maybe the high cell will drop as the transmission continues learning. I'm going to be taking the car on a 1500 mile each way vacation in a couple of weeks, so I'll rescan once I get back home.
I also attached screenshots of the scope before and after the voltage spikes.
Alright... So as far as the loss of communications codes I was getting, I do believe I have a fix. I used my labscope and connected to the data line in the DLC. With the engine running, I found that a single module was spiking the voltage up to 8-10 volts. On this car, no module should ever go above 7 volts and if any of them do it'll start setting U codes.
After removing the "comb" in the splice pack under the dash and checking each individual module at a time I found that the PAC RP5-GM11 Radio Replacement Interface that I used with my aftermarket radio was at fault. While testing that data line alone it was an almost constant battery voltage signal after it'* been on for a few minutes.
So I removed that module and just wired up the radio differently.
After reinstalling everything, the data bus voltages on the scope went back to normal.
As far as the brake switch and cruise control and transmission go, I did find that the original brake switch sensor was bad. The reference voltage with the brake pedal in the resting position is supposed to be somewhere between .9-1.4 volts when it'* calibrated. Mine was sitting around 3 volts causing the TCC to disengage and let the torque converter slip. After installing a new brake sensor and re-calibrating it, I took the car for a test drive and the cruise control worked again as well as my instant MPG went up. I also didn't feel any of the surging or slipping from the transmission.
I will definitely monitor everything up until I replace the transmission. I also looked into the TAP Cells and found that the only one that was high was 2-3 cell 8 which was around 24psi. All of the rest of them were below 7psi. Maybe the high cell will drop as the transmission continues learning. I'm going to be taking the car on a 1500 mile each way vacation in a couple of weeks, so I'll rescan once I get back home.
I also attached screenshots of the scope before and after the voltage spikes.
So, I have a 2005 Pontiac Grand Prix with a 3.8L and a 4T65E Transmission.
It has roughly around 200k miles.
Since I bought this car a few years ago, the transmission "surges" under light engine load.
By this I mean, if I have the accelerator at 10%, it'* fine. If I have the accelerator at 20% it surges. If I have the accelerator at 30% it'* fine.
At first I thought it was the electronic throttle actuator or accelerator pedal. I replaced the throttle assembly with an OEM GM part. (brand new)
Still happened, so I was going to replace the accelerator until I tried resetting the Trans Adapts. Problem went away.
I am ASE certified, but not in transmissions and I never rebuilt any transmissions either. I can usually diagnose them, but this one is beyond my knowledge.
All that I've done to this car is... I pulled the engine and completely re-gasketed it with a few new parts since the rear main was leaking badly.
I did a drain and fill/filter on the transmission and replaced the input shaft seal.
Watching the scanner while driving, I notice that slip speed is normal and there doesn't appear to be any problems with the torque converter itself.
The Adapts are high and I get codes for P1811 Maximum adapt and long shift, P0703 Brake switch circuit 2 (sometimes) and I have the occasional problem with a U1016 Loss of communications that kills my instrument cluster and turns off the A/C compressor during the summer while I'm driving.
Like I said, it'* pretty intermittent and if I clear the Trans Adapts it goes away completely until the transmission is finished with the relearn.
I'm planning on either rebuilding or replacing this transmission in the very near future, but I'd like to know beforehand what the likely cause is since this is something I've been slowly trying to diagnose myself.
Any help from actual transmission experts or someone that has had this problem before and solved it would be great.
Alldats shows 2005 Grandprix have a pcm recall, mine has the exact same issues as yours does and my transmission was rebuilt. I still get the long shift\max adapt code along with the brake switch code and intermittently gen L code, car has power surges at light throttle as if the map sensor is bad and sometimes it feelit as if it'* not supercharged.. everything checks out while monitoring my scan tool... my vin number is just shy of meeting the vin range of the recall but my gm dealer did tell me I need a new pcm when I took it in for the brake switch code. Randomly my cruise would kick off and the torque converter would disengage but the bcm never showed a brake pedal switch activation, the voltage of the brake switch didn't change. I took it to the dealer to confirm my pcm diagnosis on that. But now and then I look up recalls and tsb'* on all data and seen the pcm recall, so now I'm fighting with the dealer to fix it for free because I don't want to pay the $1,000 and I feel my car with these random codes and issues due to a bad pcm should be included in the recall.. gm needs to extend the VIN range of you ask me
Alldats shows 2005 Grandprix have a pcm recall, mine has the exact same issues as yours does and my transmission was rebuilt. I still get the long shift\max adapt code along with the brake switch code and intermittently gen L code, car has power surges at light throttle as if the map sensor is bad and sometimes it feelit as if it'* not supercharged.. everything checks out while monitoring my scan tool... my vin number is just shy of meeting the vin range of the recall but my gm dealer did tell me I need a new pcm when I took it in for the brake switch code. Randomly my cruise would kick off and the torque converter would disengage but the bcm never showed a brake pedal switch activation, the voltage of the brake switch didn't change. I took it to the dealer to confirm my pcm diagnosis on that. But now and then I look up recalls and tsb'* on all data and seen the pcm recall, so now I'm fighting with the dealer to fix it for free because I don't want to pay the $1,000 and I feel my car with these random codes and issues due to a bad pcm should be included in the recall.. gm needs to extend the VIN range of you ask me
I know... Worth a shot.. Car still runs fine thank God, but I've been thinking about a zzp pcm for $150.. easy to install, I can do the case relearn if I need too on my own.. but still wanted to put the recall out there, ya know... It affects other gm 2005 thru 2008 cars also. Possibly trucks too..
02-12-2020 | 10:15 PM
