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OT -- GM auto transmission hunts. Why?

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  • OT -- GM auto transmission hunts. Why?

    I have a 2002 GMC Sonoma pickup with 4.3 liter V-6 engine, auto trans, 67K miles. All of a sudden the transmission has started to "hunt" between high gear lock-up and its next lowest gear. The cycles are fast, sometimes only resulting in a blip on the tachometer; sometimes they are longer duration surges in acceleration. Have any of you experienced such a thing?

    I have my suspicions; but, I'm afraid if I voiced them the power of suggestion would throw you excellent diagnosticians off track. Any ideas?

    Here are some clues. It only hunts at a leisurely throttle. I can break it out of the hunting by accelerating. That is, if it starts hunting I can accelerate and it will go in to high gear lock-up and stay there. Not downshift.

    It is tends to hunt at 55 mph or near the expected downshift speed--say 40 mph on a slow, winding road--and not at higher speeds.

    Does throttle position or mass air flow affect the downshift of the GM auto transmission? It almost acts as if there is a bad spot on the throttle position sensor that is causing a problem. If the throttle is open more than the usual granny-speed everything is all right. The same could be said for mass air flow; or, for manifold vacuum, for that matter.

    Any ideas? I don't trust repair shops and I don't like to pay for mechanics to do their guessing game on my dime. I'd like to do some research, first; so, that's why I'm asking the question on this forum of folks who are definitely a substantial cut above average.

    Best regards,

    Orrin
    So many projects. So little time.

  • #2
    Which tranny do you have? 700R4,350t??? It would help to know since each has it's own quirks.
    I just need one more tool,just one!

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    • #3
      Drive faster...Problem solved.

      Comment


      • #4
        This is a GM question. Whure's Mr. Saltmine?

        --G

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        • #5
          Leisurely throttle means high manifold vacuum so the first place I would look would be a vacuum leak somewhere.

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          • #6
            Originally posted by Orrin
            I have a 2002 GMC Sonoma pickup with 4.3 liter V-6 engine, auto trans, 67K miles. All of a sudden the transmission has started to "hunt" between high gear lock-up and its next lowest gear. The cycles are fast, sometimes only resulting in a blip on the tachometer; sometimes they are longer duration surges in acceleration. Have any of you experienced such a thing?

            I have my suspicions; but, I'm afraid if I voiced them the power of suggestion would throw you excellent diagnosticians off track. Any ideas?

            Here are some clues. It only hunts at a leisurely throttle. I can break it out of the hunting by accelerating. That is, if it starts hunting I can accelerate and it will go in to high gear lock-up and stay there. Not downshift.

            It is tends to hunt at 55 mph or near the expected downshift speed--say 40 mph on a slow, winding road--and not at higher speeds.

            Does throttle position or mass air flow affect the downshift of the GM auto transmission? It almost acts as if there is a bad spot on the throttle position sensor that is causing a problem. If the throttle is open more than the usual granny-speed everything is all right. The same could be said for mass air flow; or, for manifold vacuum, for that matter.

            Any ideas? I don't trust repair shops and I don't like to pay for mechanics to do their guessing game on my dime. I'd like to do some research, first; so, that's why I'm asking the question on this forum of folks who are definitely a substantial cut above average.

            Best regards,

            Orrin
            it sounds like a similar problem Dodge has...the tranny is momentarily unlocking the torque converter. Gives a quick "blip" on the tach, and then locks up again. On the Dodge the problem is caused by the alternator wiring inducing a "trigger" voltage on the wiring going to the engine control module. I solved mine by putting an RF filter on the wire that controls the lockup clutch. Never had it happen again.
            I'd get on a Chevrolet BB and see if anybody has a solve for the problem....you're probably not the only one that has it.
            Jim

            Comment


            • #7
              Being a 2002, the transmission in question is a 4L60e (electronically controlled 700R4)

              Since it doesn't rely on the usual inputs to control it's shifting, chances are there's something else that has changed to make the transmission "hunt".

              I've been away from this stuff for a while, so here's my guess; Possibly you have an intermittent misfire that's causing the ECM to think there's a torque drop-off, and to compensate, it drops down a gear. You also might be getting a miss from dirty injectors. The late model injection system has to be kept as clean as possible (filters) and fuel pressure must be maintained close to factory specs.

              It's difficult to do this long distance diagnosis...without "hands on".

              But, we used to get cars and trucks in the shop all of the time with alleged transmission problems, which almost always turned out to be a fuel filter or an engine misfire.

              Because it's an electronic transmission (controlled by the computer) there are no adjustments or "tricks" to fool the transmission into ignoring whatever input is making it "hunt". The trick is to find out what the computer is seeing, and look there for your cure.

              Since you probably don't have a $3000 scanner, I'd just drop by the local "AutoZone" parts store, and ask them if they have somebody who can perform a "code scan" on your truck (it's free) then, go online and look up whatever codes the guy came up with....Beware, most "VatoZones" like to use the scan tool to sell you un-needed parts. They're especially keen on selling oxygen sensors....

              Most of the time, on computer controlled transmissions, a lean miss or a bad spark plug (or wire) can have the computer trying to maintain the speed dictated by your throttle position and road speed. Many things have an indirect (but significant) effect of performance.

              At this point, make sure the transmission has sufficient fluid, that's not dirty. And check all of your filters (air & fuel especially)
              You also might open the hood at night to see if there are any stray sparks shooting around, where they're not supposed to be.
              When was the last time it had plugs? Filters? Has the transmission ever been serviced?

              BTW, GM doesn't have that problem with their torque converter clutches, Idahojim, the TCC circuit is isolated from the charging system. On Chrysler vehicles, the alternator's voltage regulator is part of the computer's circuitboard, so there is some "crosstalk". GM's alternators are "standalone" with an internal regulator.
              Last edited by saltmine; 10-06-2010, 12:26 AM.
              No good deed goes unpunished.

              Comment


              • #8
                Saltmine, you might be onto something. We just refueled, pulled out onto the highway and within a mile the problem started. Bad fuel? Clogged fuel filter? Clogged injectors? It's a place to look.

                The engine has new spark plugs and plug wires. Routine transmission servicing is scheduled this week.

                While checking fluid levels I discovered that mice had set up housekeeping on top of the engine. It is hard telling what kind of damage they have done.

                Thank you. It's a start.

                Best regards,

                Orrin
                So many projects. So little time.

                Comment


                • #9
                  Orrin, water in the bottom of the tank will get stired up when you refuel and cause rough running for a while, or it did in my Ford until I found the problem.

                  Comment


                  • #10
                    Lots of good info esp. by Saltmine,

                    Im not a auto trans expert and certainly not a GM auto trans expert but many of the newer ECT's are very susceptible to engine output/drop off,

                    When you run across a stumper its always good to take the cheapest most easiest course of action,
                    The fact that you have had mice under the hood warrants a good check over esp. of vacuum lines and wiring,
                    If that checks out solid then I would go pick up some cheveron techtron fuel injection cleaner and mix it in with a tankful and run it through to see what happens (now that youv stated the plugs are new and such) - What your explaining (the trans acting up with the light throttle load) could indeed be either a slight vacuum leak OR injectors that are starting to gum up, Many of engine stumble is right at that transition just off idle and mild demand and it can be due to a wimpy spray pattern,
                    These deposits build up in the nozzle and nozzle components and of course build up in the area's where the injector spends most of its time operating, this means a little less (idle) or a little more demand can be fine but right inbetween the injector "dribbles" --- this does not rule out ignition as it has its own set of quirks in how a spark plug operates differently (or not at all) with different mixture and/or effective compression ratio's...

                    But ---- for about 5 bucks you can at least see if its injector gunk related.

                    I would of course check the level and quality of the trans fluid before anything - a low level or excessive foaming could create strange behavior for sure...

                    Comment


                    • #11
                      Boomer, thank you for your post. I'll make a careful inspection for mouse damage. It is more plausible, however, that the refueling might be a more likely cause than something rodent-related. We were on a trip and had logged about 500 miles. It would seem as though mouse pee or chewing would have reared its ugly head by then. That the hunting started within only a mile of taking on gasoline is something that should not be ignored.

                      Best regards,

                      Orrin
                      So many projects. So little time.

                      Comment


                      • #12
                        Yes that's very relevant ---- still - could be the injectors are borderline (or something else) and you took on a tankful of 15% ethanol and that pushed things over the edge...

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                        • #13
                          GM Trans problem

                          Hi
                          YOu will not belive this but I had a similar problem with my blazer and my son's blazer and it is the ignition switch. GM has had problems with these for years and it costs about $100 for a new one. You can un-plug from under the dash and install the new one without taking the column all apart for a test.

                          have fun
                          tom

                          Comment


                          • #14
                            Ah yes...mouse infestation. For some strange reason rodents find the insulation on automotive wiring absolutely delicious. A good course of action would be a close inspection of all the electrical harnesses and vacuum pipes (they're plastic, too.) GM did have a problem with early production injectors on these trucks, replacement parts for correcting it are available.
                            Getting gas and having the trouble start shortly afterward would make me suspicious of the quality of the fuel. High ethanol content, water, and sometimes dirt stirred up during a fuel delivery can all cause problems.
                            Due to the location of the fuel pick-up in your truck's fuel tank, you will get the water first, right off the bottom of the tank. A dose of alcohol will easily clear up this problem since alcohol will cause the gasoline and water to mix.

                            Had a similar infestation once working in a Dodge dealer. We had a small Dodge motorhome that needed an engine. A replacement was ordered, and I decided to push the van outside while waiting for the new engine. As luck would have it, the transmission was held in place with a temporary support fixture, which, unfortunately, prevented the engine cover from being put back on. About a week later, the new engine arrived, and we set about installing it. Everything went well until I went to start the thing. A flick of the ignition key, and there was quite a fireworks show under the dash. Seems that during the wait for the replacement engine, ground squirrels had discovered the opening and had eaten all of the insulation on the under dash wiring harness. Interesting start-up to say the least. Smoke and flames...nobody got hurt, and in the end, the owner got all new harnesses.
                            No good deed goes unpunished.

                            Comment


                            • #15
                              Orrin, I had a similar problem with my 2003 Ford Ranger but mine was surging and going in and out of gear. After some inspection and replacement of the speed sensor in the rear end with no help I found that the speed sensor in the top rear of the auto tranny had frayed wires but no sign of mice anywhere in the vehicle. I soldered and shrink tubed the wires and tapped it up as well and that fixed the problem. There is the possibility some road trash frayed the wires or a mouse in the past did it. I would guess the wires have been frayed for a long time and they finally shorted out.

                              I would look at all the wiring and plugs to see if there is any damage and it could also be a dirty plug connection. Since the trannys are controlled by the on board computer the problem could be there or a bad sensor.
                              It's only ink and paper

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