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Old 11-18-2011, 01:07 PM
TransEngineer TransEngineer is offline
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Originally Posted by snrusnak View Post

I know this is slightly off topic, but I went to the dealer this morning to do my "ride along" to have my 3rd to 4th slipping issue that I've been having looked at. I am frustrated. I made an appointment then had to wait like 30 minutes, then they wouldn't let me drive, which I knew they wouldn't "be able to replicate the customer's issue", and that's exactly what happened. If I drove it I could have made it happen. The guy also did not know what I was talking about with the tool to plug in like transengineer suggested. I started to question his knowledge (and he was their "trans guy") as we started talking more and more about the truck and about tow/haul and 2nd prime and he said he didn't have a clue what I was talking about. He told me "can't fix it if it ain't broken", and told me I must be imagining things. Then told me if I want it fixed to abuse it and break it then he can fix it....great advice. He said the only thing he could think of was the TCC engaging/disengaging between gears, but I thought it didn't engage in between gears and under load. I thought it only engaged in overdrive gears at cruise. I'm just frustrated as he blew me off and I took a day off work to go do this. Transengineer, do you think it could simply just be the TCC? He also said to me "if there's no engine light there's no problem", but I'm sure what is happening in my truck is not normal, or intentional.
Yes, I think there's a good possibility you're just getting a 3-4 upshift followed by TCC engagement, but the only real way to confirm that is with a scan tool. There are aftermarket scan tools that can also read our transmission data (I've had another owner send me recordings made using a SnapOn scanner). So if you have a buddy who works at a shop and has access to a scan tool, maybe that would work for you.

The TCC doesn't just engage in overdrive gears. We try to engage it as much as possible (even in lower gears) since it benefits fuel economy. But there is a whole separate "shift pattern" (so to speak) that controls TCC engagement, and it depends on turbine speed, throttle opening, temperature, whether you're in Tow/Haul mode, etc. So there's no simple answer as to "when" your TCC should engage.

In any event, when the TCC engages, it's a 2-step process. First, it goes from unlocked (open converter) to partial lock. This reduces the converter slip from, say, 400 rpm down to about 60 rpm. The TCC will remain in partial lock for a brief period (1-2 seconds), and then transistion to full lockup (where the slip drops to zero).

I suspect you are feeling a 3-4 upshift, followed by TCC partial lock engagement, followed by TCC full lockup, over a period of perhaps 4-5 seconds. That would be normal. If this is the case, you will see engine speed drop (in steps) as you go through these events.

If you actually had a slipping clutch, you would probably notice some engine flare during the events (engine speed goes up and then comes back down). So if you are just seeing an extended time for the engine speed to finish its descent (with no slips or flares) then you are probably OK. I should note, however, that in some cases you WILL get "flare" during an upshift, if the TCC is locked before the shift, then unlocks just before the shift (giving you the "flare"), and then re-locks after the shift. This is why it's really helpful to have a scan tool so you can see what is really happening during the events. I can't predict if or when you might experience an "unlock, shift, re-lock" event - again, it depends on the vehicle package, throttle, speed, etc.

The PCM is very good at detecting clutch slippage within the transmission. It will usually detect slippage before you (as the driver) ever notice it. It will then set a ratio error DTC, although the CEL (aka MIL) may not light up right away. It will also put you into "logical limp-in" mode, in which the trans will start in 1st gear from a dead stop, and then upshift directly from 1st to 4th. It will remain in 4th until you stop (or slow way down) again. It will operate (in Drive) in 1st and 4th gears only, until you restart the engine. So if your trans is making normal upshifts, then it has NOT detected any significant clutch slippage. I think that's what your tech meant by "if the light's not lit, it's OK," although as I've noted that's not always technically accurate.

Now I can already hear some folks starting to yell, "Baloney! My trans wouldn't move at all, and it never set a fault!" So before anyone starts a post like that, let me explain:

The ratio error faults (such as P0731, Gear Ratio Error in 1st), are technically "in-gear" ratio faults. This means that, in order to set a ratio fault, the trans must first actually achieve the desired ratio, and THEN slip. So if you shift into Drive and get nothing (neutral), where the vehicle won't move, in some cases you will NOT set a P0731 fault because you never made it into 1st gear.

OK, I know that's probably more info than you needed, but I figure too much info is better than too little.....
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