Discussion Starter · #1 ·
Hi, As I accelerate the 4WD light flashes on and off, the 4wd shift indicator (which is normally always lit) flashes off and on. This is often accompanied by a clunk ranging from barely felt to heavy. I can avoid the clunk by accelerating gently or by aggressively accelerating. On several occasions the heavy clunk has been followed by the VDC Off light and SLIP light coming on and staying on for one or several drive cycles before they finally go off. I have had communication trouble codes. I have built a data logger and have noticed what appear to be drop outs for some analog signals such as CAN bus ID 1F9 (data bits 3 and 4) which appear to represent engine rpm. The drop outs are a zero value showing up when, for example the value is rising from say 37 to 38 to 39 but a zero suddenly appears. I am assuming that a high priority ID such as 1F9 may be coming from the engine ECU but I need to do further testing to prove that. I am hoping that the communication problems on the CAN bus are causing all of my issues with lights and clunky shifts rather than a bad transmission. I am curious if anyone knows which values the transmission control unit reads over the CAN bus in order to smoothly regulate it's shifting - engine rpm, throttle position, engine load, or others? If I disconnect the battery overnight the truck runs smoothly for 5 to 10 minutes before problems start to occur so I am wondering whether this is learn mode for the transmission and if it runs better because the transmission control unit may be using more of it's own smarts and relying more on it's own physical inputs for information rather than putting trust in flaky or missing data on the CAN bus. Recently my engine idling has become rough. My dealer said they can't really do anything until the problem gets worse. In other words they can find something completely broken but not something intermittent. My one year warranty on this used vehicle will be up in 3 months so ironically I am hoping my truck breaks down before then so I can send it on an expensive tow to the dealer which is 1.5 hours away. The dealer says all my sensors are OK but is sure doesn't seem that way. The CAN bus resistance is also correct at 60 Ohms. It could be a flaky ECU or flaky CAN bus wiring but the dealer is not willing to use a shotgun approach and replace expensive parts or performing endless rewiring and hope for the best. I don't blame them for that attitude but this vehicle has been defective since they day I bought it and they have sneakily looked at the vehicle a few times without producing one official maintenance record. I'm going to lose my cool very shortly.