2002 GMC Sierra 2500HD transmission inquisition
Q: I have an 2002 GMC Sierra 2500HD pickup Duramax diesel and the Allison five-speed automatic transmission. I have three questions: (1) Based on the articles in your magazines and the brochures from GM, it appears the Grade Braking function in the Allison is only supposed to work in the Tow/Haul mode. On a recent trip in the mountains driving in the normal mode, the transmission downshifted on several occasions while descending on long downhill grades at 60-65 mph after the brakes were applied. Has this been changed in the newer models to work in the normal mode as well as the Tow/Haul mode? (2) It appears that, when accelerating, the torque converter locks up after the transmission has shifted to fifth gear in the normal mode and locks up when it shifts to second in the Tow/Haul mode. Is this correct? (3) Is the transmission locked out of fifth gear when in the Tow/Haul mode?
A: We contacted Allison Transmission and went over the details of the functionality of the Allison 1000 series transmission currently used on Chevy & GMC HD truck applications. The answer to your first question is "no," nothing has been changed and the Grade Braking is available only in Tow/Haul mode. However, the system's control modules look at such data as rpm, vehicle speed, and brake-pedal position to determine if you're traveling on a significant decline. It'll then automatically downshift and supply engine braking to slow the truck, a process done manually on most other automatic transmissions. But there was a glitch when GM installed the five-speed Allison 1000 as an option replacing the four-speed Hydra-Matic 4L80-E Transmissions. It stayed with the four-speed-shift quadrants along with the original Transmission Position Sensor and indicators that display only four forward gears D-3-2-1. This prevents you from manually putting the transmission into fourth gear to provide engine braking. To alleviate that problem, engineers enabled the automatic 5-4 downshift in the normal mode under certain conditions. They're working on providing another method for manual fourth-gear engagement on future models. Next question: The Lock-Up Torque Converter, which locks the engine directly to the transmission when certain conditions are met, will be applied in normal mode once the transmission is in fourth gear (not fifth) and the engine loads and vehicle speed are within certain parameters. When in the Tow/Haul mode, which provides beneficial shift quality when towing or hauling significant loads, the torque converter will be able to lock up in, as low as, second gear. This also helps with shift quality while keeping transmission oil temperature down. Last, fifth gear is fully functional in the Tow/Haul mode, but it may require higher speeds before engagement.
Reading 1997 1500 Dodge Ram trouble codes
Q: I have a 1997 1500 Dodge Ram with the 5.9L engine. It cranks for a long time (6-10 sec) before it starts. It starts up fine unless it sits for five minutes or more. The check-engine light comes on and goes off every once in a while. The codes are 12, 21, and 55. I've replaced the fuel pump, spark plugs and wires, camshaft position sensor, and the battery.
A: It's easy pulling up the trouble codes without a scanner on your Dodge Ram. Cycle the ignition key On - Off - On - Off - On within 5 sec, and the codes will start flashing from the instrument panel. The extended crank and the trouble-codes may or may not be two separate issues, but the codes are certainly the best place to start. We looked at the Dodge trouble code charts and found that the Powertrain Control Module has lost direct battery input within the past 50 times the ignition was turned on. This code will not illuminate the check-engine light and may have been set when you replaced your battery. Code 55 flashes to inform you that all the stored trouble codes in the PCM's memory have been displayed. The code 21 is the actual problem to look into concerning the oxygen-sensor circuits. Your truck uses an upstream and a downstream O2 sensor. This means one is mounted in the exhaust system before and after the catalytic converter in order to determine if the Cat is doing its job. Chances are that one of the O2s has failed, but it's going to require an experienced technician with the proper scan tool and the appropriate service information to make a without-a-doubt diagnosis. The cause can be either a sensor or a problem elsewhere in the oxygen sensor to PCM circuitry. Again, the hard-start condition is often fuel-pressure related and may have a different cause altogether.