TC Unlock RPM Dip - Ford Powerstroke Diesel Forum
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  #1  
Old 07-15-2012, 10:36 AM
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TC Unlock RPM Dip

Anyone ever heard of the RPM's dipping slightly (to around 500-600) when the TC unlocks coasting to a stop? Mine has been doing this now for a couple of months and is starting to bother me. The fluid is good as I just drained the pan and changed the filter less than 500 miles ago. Not knowing what type of fluid was in the transmission from the previous owner I put in DexIII/Mercon which is what I was told was the preferred. The TC locks up perfectly at 45'ish MPH and the truck pulls well. Any ideas. Searching for others with this issue lead me nowhere. Thanks!
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Old 07-15-2012, 07:19 PM
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not to bomb your thread but how do you drain your torque converter?
im going to put a mag hytech pan in next week and was reading the instructions and it said to do that but didnt go any further in depth....
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Old 07-17-2012, 06:26 AM
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I didn't do anything with the converter. I simply drained the pan, dropped it, changed the filter and refilled it. Anyone have any ideas? Rude had a brand new ICP (old one was oily) and the IPR nut looks good, but I'll check the tightness tonight.

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Old 07-17-2012, 09:40 AM
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I'm no expert. But here are my observations from my pickup, which I put a manual TC lockup in.

If you are in OD (light off), and your speed drops down to around 40mph, the transmission will downshift to 3rd without engaging the (crap, can't remember exactly what it's called) brake clutch? Meaning, regardless of whether the TC is locked or not, the engine will drop to an idle unless you hit the OD off switch. In my pickup, with the TC lock on, if I'm in OD and the tranny shifts down to third, I only get the benefit of the lockup on the acceleration side, not the coast side. Dunno if this is the phenomenon you are experiencing or not, but in a stock transmission, the TC unlocking and downshifting to third are simultaneous. With OD on, there also is no compression braking action in the low gears, meaning it basically coasts to a stop with the engine at idle.
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Old 07-17-2012, 12:54 PM
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I understand that the TC unlock when releasing 4th gear. I expect my RPMs to drop to idle when my coasting speed drops below 40ish MPH. Idle (at least on my rig) is right around 800 RPMs. The issue i am experiencing is my RPMs drop BELOW idle (down to roughly 600 our so) when coasting and then bounce back up.

I have read issues like this could be anything from an IPR, to a solenoid in the transmission, to a bad TC, to a dead cylinder. My truck does not stall out and die but does stumble and then seems fine on take off. Anyone? Thanks for all the help so far.

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Old 08-24-2012, 02:32 PM
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I started a thread on this not too long ago. My truck is having the EXACT same symptoms. I got the same responses as far as what to check and what it could be also. I haven't really had a chance to mess with it. Sucks we're having this problem but glad to see someone with the exact same problem as mine. Hopefully one of us will get this figured out. Also, does yours seem more prone to doing this once its warmed up? And does it do it more so when you quickly slow down as opposed to cruising to the point where the converter unlocks?
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Old 09-21-2012, 01:05 PM
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Cool Potential Update??

Got this as a reply on the PHP forum from Cody. This sounds like its gonna suck. Time for a BTS I guess.

"I will, however, tell you that your RPM drop "problem" is mechanical. It's transmission mechanical. Sorry for the bad news. It's not going to hurt anything, but as you've found out, it's really annoying not having any engine braking. I've found it in two transmissions now (one was my own, the other was one I rebuilt for a guy). The coast clutch snap ring pops out of its groove and you lose much of the clamping force on the frictions and steels.....and they eventually burn up. The only way to fix it is to take the transmission apart."
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Old 09-21-2012, 01:28 PM
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Crap! That's not good to hear. You have a link to that thread by chance?

Last edited by STIgone73; 09-21-2012 at 01:41 PM.
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Old 09-21-2012, 01:49 PM
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I've noticed something else mine does now. If I'm cruising along and the converter is unlocked and I throw the truck into neutral, it will rev up to about 1100 RPM and drop back down to idle. It only does this while moving. If I'm already stopped and try this it will hold idle fine. Does yours do this as well?
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Old 10-07-2012, 06:05 AM
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Just curious if you've had anymore developments with this? I haven't been driving mine much because of the issue but know it's something I need to ultimately fix. Just trying to figure out which way to go with it.
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