My 1999 Silverado pickup just did something that concerns me. It's been running fine for a long time (except for the occassional "Service 4WD" light coming on). We took a trip to Tennessee, about 800 miles round trip, drove it up to the top of the Smokey Mountains (Clingmans Dome) and it was fine. It did have some trouble deciding wheather it wanted to be in 2nd or 3rd gear when we were heading up the long steep grades, so I put it in 2nd and left it there. We headed back to Indiana the next day and it was fine (except for the "Service 4WD" light again) until I got to about 3 miles from home. I stopped at a stop sign, then pulled out again. It shifted REAL hard, actually slammed, into second gear. It was OK after that. It sounds similar to what Rocky2 was saying in this post:
I checked the trans fluid and it was not low and looked good, nice and reddish and no smell. I don't think it's related to the "Service 4WD" light coming on, since it's been doing that for a long time. Anyone else have this happen?
Re: 1999 Silverado shifting into 2nd - slam shifting
Danny, There are more “fixes†for this problem than you can shake a stick at. So whoever you talk to, it’s mechanical or it’s electrical. Everything always seems to point to the solenoid though.
There supposedly is a TSB out on it.
For the electronic side; I believe that they run an additional wire (ground or power) from the alternator to the trans. The 1-2 slam would be noticeable most under slow acceleration, just easing of the line.
I was told the tranny goes into 'protective' mode; raises the internal pressure to compensate for other problems which is why you get the slammin. (cracked valve body, internal slippage, bad solenoid) Was also told you could drive the truck like that, but not recommend it for long periods as it feels like you've been rear-ended every time.
For the mechanical side; 'Torque Converter Regulator Valve wearing out its bore in the valve body' syndrome. The ECM detects an excess slip when the converter clutch is engaged, and compensates by jacking up the pressure, hence the abrupt 1-2 shift. The SYMPTOM is an internal slip of more than the ECM is programmed to accept (usually results in a P1870 code). I’m not sure if the codes clear like the problem does when you shut it off and then start it back up, so the best bet is to either get it to a dealer or a reputable trans shop and let them try to get a code out.
-Dave
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