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Auto Transmission changing in and out of Overdrive

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Old 02-10-2009, 01:56 PM   #1
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Auto Transmission changing in and out of Overdrive

I have an 84 s10 blazer 2wd with a 3.4l swap. Just picked the truck up a few months ago, so I'm not sure what transmission is in it...I'm pretty sure it's a 4 speed w/ overdrive though. This happens about once or twice a day usually:I'll be driving on the highway (usually happens around 60-65 mpg) and it will start shifting back in forth from (what I'm guessing is) 4th to overdrive like every second. Like it can't make it's mind up. To get rid of it I have to either let up on the gas or give it more gas....if you keep a constant pedal it will continue to do it forever.Tranny fluid looks brand new, right on the level. What would this problem be, transmission or something else? It's getting really annoying, and is probably losing mpg over it. I've always owned manuals, so this is my first auto. Let me know how to get rid of this problem, thanks!
Old 02-11-2009, 09:05 AM   #2
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Re: Auto Transmission changing in and out of Overdrive

4th is overdrive. If it's jumping back and forth from drive to overdrive you would have a big change in engine RPM.
If the converter is locking and unlocking you would have a small change of about 200 RPM. Try taking out the fuse to the torque converter and drive it that way. If the "shifting" problem stops, you know there is a problem with the torque converter.
Old 02-11-2009, 11:26 AM   #3
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Re: Auto Transmission changing in and out of Overdrive

Oh okay, so it's just 1st, 2nd, 3rd, and overdrive (4th) then? So it must be shifting back and forth from 3rd to overdrive. I actually don't have a tach yet, but ones on the way...so I'll need to get it hooked up asap. I'm just going off of ear for now.

It isn't a huge jump or shift when it changes over constantly, just a little jump, but I can clearly hear it. I'll try pulling the fuse to see what happens. What if the problem still happens when I pull the fuse? Is that a bad tranny issue? Thanks.
Old 02-11-2009, 02:43 PM   #4
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Re: Auto Transmission changing in and out of Overdrive

A small jump in rpm I think would be the converter locking and unlocking. The reason why and the fix for it I can't help you with. If it's just the converter going bad it shouldn't be all that expensive to replace, but like I said, IDK why it would do that.
If it stops after pulling the fuse, then at least you have a clue as to what's going on.
It's 1st, 2nd, Drive (1 to 1), Overdrive .
Old 02-23-2009, 04:05 PM   #5
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Re: Auto Transmission changing in and out of Overdrive

Where is the torque convertor at? Sorry, never owned an auto before! Also, is the fuse in with the rest of them under the dash? I couldn't find it, maybe I didn't look hard enough though...
Old 02-23-2009, 04:55 PM   #6
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Re: Auto Transmission changing in and out of Overdrive

It's the coupling between the engine and transmission, instead of a clutch. It looks like a big friggin donut bolted to the flexplate. Sorry I can't tell you where the fuse is. Is anything labeled TCC? If you can't find the fuse you can unplug the electical connector at the tranny. I think it's on the drivers side, it's a sqaure 4 pin plug.
What I left out of the other post was: 1st, 2nd, Drive, Overdrive, and then you feel the converter lock up, you can easily see it if you have a tach.

Last edited by SH-60B; 02-23-2009 at 04:57 PM.
Old 02-24-2009, 07:18 PM   #7
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Re: Auto Transmission changing in and out of Overdrive

Check the brake pedal sensor.


When you press down on the brake pedal, it unlocks the torque converter. When that sensor is going bad it will do exactly what you're experiencing. Pretty common problem.
Old 02-24-2009, 10:07 PM   #8
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Re: Auto Transmission changing in and out of Overdrive

Oh, now that's something I wouldn't have thought of. How do you check to see if it's working? Just check to see it's got 12v or something? Good idea, thanks.
Old 02-24-2009, 11:13 PM   #9
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Re: Auto Transmission changing in and out of Overdrive

Well, yes but when it first starts to go it's pretty intermittent, as you've found out, so it may be kind of hard to diagnose without having a new switch to put in and just see if that fixes it. I'm not sure how expensive they are in your area, but autozone has them for like 8 bucks for the most expensive one here.

On our trucks I'm not sure if they get power when the pedal hits them, or if they become grounded when the pedal comes down. Maybe someone else will know for sure.

Last edited by Mattpyle; 02-24-2009 at 11:22 PM.
Old 02-24-2009, 11:34 PM   #10
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Re: Auto Transmission changing in and out of Overdrive

normally closed, then it goes open when you hit brakes. the tcc circuit and the brake lights are 2 seperate circuits on the brake switches

on your old truck, you have a cruise brake switch, which is normally closed, and the brake light switch, which runs from the fuse block, to this switch, then thru the turn signal switch to the brake lights.
the cruise/brake switch is the one you are targetting here.
it has a vaccuum hose barb on it.
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