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Truck going in and out of overdrive???

need help with overdrive? Lock up?

Truck going in and out of overdrive???

Postby detroit671 [OP] » May 20 2012, 10:01pm

Hi everyone, I am new to this forum but have already learned alot from searching. I have seen alot of posts on this subject but not exactly my situation. So here goes.... I have a 1999 Silverado, 4x4 ext. cab with 170k miles. I have only had it for a year and a half. The previous owner had the transmission rebuilt less than 10k miles ago. The truck is a daily driver not worked very hard. Until recently it was only driven around town by me. I have been traveling once a week about 2 hours on the highway. The truck shifts fine but seems to fluctuate a couple hundred RPMs. Like its coming out of overdrive? But it doesn't seem like the RPM change is enough to be coming out of overdrive? Some of the posts talk about the torque converter not locking up? I have read so much about it that I am totally confused. Any help would be much appreciated. Thanks Steve
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Re: Truck going in and out of overdrive???

Postby ASTE25 » May 21 2012, 2:03am

It does sound like the converter is locking and unlocking. It will do this if you have a slight load applied to the truck (like going up a small hill on the interstate). It sounds like it is working correctly to me.
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Re: Truck going in and out of overdrive???

Postby lynx440 » Nov 27 2012, 6:42pm

detroit671 wrote:Hi everyone, I am new to this forum but have already learned alot from searching. I have seen alot of posts on this subject but not exactly my situation. So here goes.... I have a 1999 Silverado, 4x4 ext. cab with 170k miles. I have only had it for a year and a half. The previous owner had the transmission rebuilt less than 10k miles ago. The truck is a daily driver not worked very hard. Until recently it was only driven around town by me. I have been traveling once a week about 2 hours on the highway. The truck shifts fine but seems to fluctuate a couple hundred RPMs. Like its coming out of overdrive? But it doesn't seem like the RPM change is enough to be coming out of overdrive? Some of the posts talk about the torque converter not locking up? I have read so much about it that I am totally confused. Any help would be much appreciated. Thanks Steve

This is the exact scenario I am experiencing with my truck. Especially around 40 mph. It seems to float in and out of full TC lock. Goes up and down a couple hundred RPM. At highway speeds, it doesnt seem as pronounced. This is under light to no load conditions. My truck has about 170k miles on it with a recently replaced transmission. I purchased it after all of this work was done. Is it common to replace the TC along with a transmission? Or could it be that in both cases the original TC was used on a rebuilt transmission, and is worn out to the point where it cannot fully lock up? My truck has the 4.8L with 4:10 rear gears, and while I don't expect terrific gas mileage, I can't seem to do much better then 16 MPG on the highway at 60 MPH. Seems too low to not be a problem.
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Re: Truck going in and out of overdrive???

Postby ASTE25 » Nov 28 2012, 6:53pm

Typically they replace the converter with a remanufactured unit when they rebuild a transmission. FYI a misfire condition is often mistaken for a transmission problem. I'd have it scanned and check the misfire section. I have seen engines misfire but not set a code. If it is a light misfire it may not set a code, but cause what you describe.
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Re: Truck going in and out of overdrive???

Postby lynx440 » Nov 28 2012, 7:58pm

ASTE25 wrote:Typically they replace the converter with a remanufactured unit when they rebuild a transmission. FYI a misfire condition is often mistaken for a transmission problem. I'd have it scanned and check the misfire section. I have seen engines misfire but not set a code. If it is a light misfire it may not set a code, but cause what you describe.

Thanks for your response. Perhaps I do have a misfire. I just replaced all the plugs and wires with NGK iridium's and AC Delco OEM Wires, so it can't be those I would think. Would an Ignition Coil start to go intermittently like that?
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Re: Truck going in and out of overdrive???

Postby Dean508 » Nov 28 2012, 8:02pm

X2.. Check for any CEL codes stored, pending and current.
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Re: Truck going in and out of overdrive???

Postby lynx440 » Nov 28 2012, 8:04pm

Dean508 wrote:X2.. Check for any CEL codes stored, pending and current.

I may have hindered that lately. I got a "Left Bank Lean" code once about 3 weeks ago. I had a scanner and I cleared it. I hope I didn't take out any other codes with it. I'll check to see if there any more rightnow. Thanks for the help!
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Re: Truck going in and out of overdrive???

Postby ASTE25 » Nov 28 2012, 8:24pm

A coiuld could go bad after jostling it around during service. I would check all the codes as stated above.
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