Question of TC lock-up feature

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CPLTECH
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Question of TC lock-up feature

Postby CPLTECH » Sat Dec 04, 2010 12:18 pm

My question is for someone who is familiar about how the normal operations of the transmission’s torque converter lock-up functions.

It only locks up when in 5th gear when at 45-50MPH, (correct per page AT-60 of Service Manual) then after all components are warmed up it will lock up at 40MPH. However, 4th gear NEVER goes into lock-up for me and since when towing 4th is the mfg suggested gear, the feature of no lock-up causes heat buildup in the transmission and the resultant loss of MPG. Is this the way it is designed? Is the converter mechanism too weak to handle engine torque in 4th , thus the slush-a-matic feature to save itself from destruction? If not, can someone tweak the computer settings?

The reason I bring this up is that I came from a 98 Tahoe 4sp and it’s TC would go into lock up also in 3rd. When it did I saw a jump in instantaneous MPG (per aftermarket gauge) or anytime the trans went into TC lock-up. My guess is that the PF would outperform the Tahoe if it had lock-up in 4th , as is it currently gets similar MPG towing at similar speeds. Yet, many on this forum are pleased with towing trailers. I just think fueling up would cost less if it went into lock. I already have 2 gas cards that give 5% rebates.

Any thoughts appreciated,


amr40509
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Postby amr40509 » Mon Dec 06, 2010 7:24 am

Your logic makes sense to me.

I tow with mine also - in 4th gear if I'm going <65 the RPMs vary a lot at a constant speed. When I get to 70-72ish it seems to hold in there at a consistent RPM much better. I have confirmed that with our boat in tow I get much better MPG at the faster speed where RPM is more consistent.

CPLTECH
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Postby CPLTECH » Mon Dec 06, 2010 9:39 am

Thanks for your reply amr40509,
I will test that when on some rural road in the near future. However, I don’t want to pull a 4K# travel trailer that fast with this size vehicle and lose control.
On the instructions of the “Bully Dog GT” engine/trans tuning device, it can change some transmission settings in several mfg, but not the Nissan. Wonder why Nissan is so different in their transmission?
Last edited by CPLTECH on Mon Dec 06, 2010 7:13 pm, edited 1 time in total.

amr40509
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Postby amr40509 » Mon Dec 06, 2010 11:11 am

At 4,000lbs you are right in the ballpark of where we are with the boat, you might be a little lighter. Pulls just fine at a bit over 70MPH for us and jumps from about 11MPG to about 14-14.5 with the speed increase.

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disallow
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Postby disallow » Mon Dec 06, 2010 10:27 pm

I tow a TT that is near the max tow weight of 6000lbs. I also exceed the recommended front face area of the trailer, strictly speaking. I'm OK with this, however, as the trailer (see avatar) has an aerodynamic nose section, which I feel lowers the drag and decreases the front facing area enough to make me feel better... :) (Manual says 64sqft = 8'x8', I'm at 8' x 10' but not at a 90 degree angle) (and yes I understand that my setup pushes the envelope).

I usually tow in 4th. When the wind picks up, as it often does here in the prairies, I have been known to downshift to 3rd if I feel the trans is down/up shifting too much. Here is my speed/gear/RPM info:

100kph (62mph)/4th/2900
100kph (62mph)/3rd/3600

90kph (55mph)/4th/2600
90kph (55mph)/3rd/3100

What I do notice is that the transmission rarely "settles down" in 4th gear in the wind. What that means is that I do notice the RPM go up about 300-500 on flat/up grades, but on down grades, the RPM will drop by that much. It certainly never feels as though its slipping (ie the engine RPM never runs away, its stable), which makes me think that the TC is simply not locked up.

I do plan on installing a temp gauge (its in my toolbox in the garage), and have also heard that Superchips may try to add it to the diagnostic feature on the Cortex. But I definitely have a feeling that I push the trans too hard, and will also be adding another cooler next summer.

And I'm with the OP on the towing speed, I don't feel at all comfortable at speeds over 65mph. Part of that is the length of my rig. It can apply alot of leverage to my vehicle, making it a bit of a handful. Lower speeds make it much easier to handle.

t

CPLTECH
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Postby CPLTECH » Thu Dec 16, 2010 12:27 pm

Pardon me for asking more questions, but I have yet to find a satisfying answer to calm me down.
I saw elsewhere on an Xterra forum that only gear #3 & #5 go into TC lock-up. So I looked at the Serv Manual and sure enough that is what the chart below shows. But why did Nissan choose to ignore Gear #4 TC lock-up, the gear for towing?
I am also confused (perhaps due to a translation from one language to another) as to the statement that “Slip Lock-up Control” IS used in gear 4&5 to improve FE (how if half-clutched?) but “Lock-up Control” is not used in 4th. Confusing.
There must be a reason why Nissan and aftermarket programmers have not ventured into this area. Why? What do they know that is not common knowledge? Yet this product family is advertised to tow 5-6K#. So, is this transmission able to withstand the temps that occur in 4th? But I want better mileage? I recall that in the GM 4L60E, whenever towing and the TC unlocked in final gear you had best drop a gear or the heat build-up would damage the trans.

Following info from Service Manual AT pages 33-34:
Lock-up Control
The torque converter clutch piston in the torque converter is engaged to eliminate torque converter slip to increase power transmission efficiency.
The torque converter clutch control valve operation is controlled by the torque converter clutch solenoid valve, which is controlled by a signal from TCM, and the torque converter clutch control valve engages or releases the torque converter clutch piston.
Lock-up Operation Condition Table

(Unable to paste chart. Look in Service Manual in "Sticky" posting.
Lock-up OK in 2nd, 3rd & 5th, not 4th
Slip Lock-up OK in 4th & 5th only)

Slip Lock-up Control
In the slip region, the torque converter clutch solenoid current is controlled with the TCM to put it into the half-clutched state. This absorbs the engine torque fluctuation and lock-up operates from low speed.
This raises the fuel efficiency for 4th and 5th gears at both low speed and when the accelerator has a low degree of opening.

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disallow
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Postby disallow » Thu Dec 16, 2010 2:00 pm

CPLTECH wrote:Pardon me for asking more questions, but I have yet to find a satisfying answer to calm me down.
I saw elsewhere on an Xterra forum that only gear #3 & #5 go into TC lock-up. So I looked at the Serv Manual and sure enough that is what the chart below shows. But why did Nissan choose to ignore Gear #4 TC lock-up, the gear for towing?
I am also confused (perhaps due to a translation from one language to another) as to the statement that “Slip Lock-up Control” IS used in gear 4&5 to improve FE (how if half-clutched?) but “Lock-up Control” is not used in 4th. Confusing.
There must be a reason why Nissan and aftermarket programmers have not ventured into this area. Why? What do they know that is not common knowledge? Yet this product family is advertised to tow 5-6K#. So, is this transmission able to withstand the temps that occur in 4th? But I want better mileage? I recall that in the GM 4L60E, whenever towing and the TC unlocked in final gear you had best drop a gear or the heat build-up would damage the trans.
All valid questions. Even more confusing when you consider that the RER505a transmission is also used on the Armada and Titan platforms with the same final reduction.

Would love someone knowledgeable to pipe in. Anyone? Anyone? :)

t

CPLTECH
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Postby CPLTECH » Thu Dec 16, 2010 5:14 pm

Terry,
Thought I’d see what experiences the folks at X might have had. A 2009 Xterra user says his has lock-up in both 4 & 5. Waiting for more input.
http://www.thenewx.org/forum/showthread.php?t=37305

CPLTECH
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Postby CPLTECH » Fri Dec 17, 2010 8:25 pm

One poster said a possible reason the tow gear (4th) stays unlocked is due to the "shock & awe" that a trailer puts on a drive train. Sounds reasonable enough.

Nuff said. Now let's get to the next subject at hand...

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disallow
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Postby disallow » Fri Dec 17, 2010 8:44 pm

CPLTECH wrote:One poster said a possible reason the tow gear (4th) stays unlocked is due to the "shock & awe" that a trailer puts on a drive train. Sounds reasonable enough.

Nuff said. Now let's get to the next subject at hand...
I dunno, CPL... I don't have all the answers I want...

Next time I can corner my Allison Transmission application engineer, I will run some of the stuff in the FSM past him and see what he thinks. Should be up in January to do some shift calibrations on our buses.

I was wondering too, whether I could take the TCM from an Armada or Titan and use it on the pathy. Then we'd get the benefit of a tow/haul mode. I guess to really evaluate it, a dyno would be required, or some better engine monitoring/management software than I have, such as Consult-II or better...

t

CPLTECH
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Postby CPLTECH » Sat Dec 18, 2010 10:38 am

Terry-
Willing to listen. Almost wishing for a 6sp manual Xterra or ???
It’s so hard to find one vehicle to do several functions: errands for 1 person, family car, something to haul a trailer (utility + camper). So to be politically correct and use less of the earth’s resources, I felt it prudent to downsize from the Tahoe. And for most occasions, the PF gets 10-15% better MPG. But when it comes to towing this camper, the mileage is similar to the 5.7L Tahoe. I had an S10 Blazer that got 13MPG towing this same camper, so why can’t this PF do the same? Yes I feel it is the slush-a-matic feature of 4th gear. But yet do not want to cause the premature death of a trans, not on retirement pay!
Then why not downsize the camper to a fold down style? Well it is nice to pull over and eat lunch while traveling, emergency pull off the road type pit stops for young & old alike, pull into a rest area and nap briefly, stay overnite in the RV area of a Flying J Truck Plaza. Plus this one is paid for.
So, just drive slower, plan trips to avoid those long grades, pay the extra gas it takes, and understand the limitations of your vehicle. Even with the high fuel costs, we find it less expensive than motels and restaurants.

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08Datsun
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Postby 08Datsun » Sat Dec 18, 2010 12:46 pm

I doubt your going to toast the transmission. The Titan and Armada have GCVWR about 4,000lbs heavier than ours. I haven't actually heard of anyone toasting one of these tranny from hard use. If it helps, this is only the 2nd tow vehicle I've even had that even has a lock-up converter. All the others just slipped on down the road tugging on their load. I still have a '79 Ford with a C6 and no lock up that's never been touched. It's done more towing than you can imagine and with the tiniest auxiliary cooler you've ever seen.

I have a gauge on my PF, but have don't tow as much as you. So far, I'm shocked by how cold it runs. Optimal tranny temps are typically around 175-195. It only gets up to that when it's out of lock up and slipping.


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