Auto D40 revs higher when under load

[D_d_001]

Hey Guys,

have just found this forum and seems great. some great info here.

bit about me ...I live in perth West aust. have a 07 D40. tow the boat occasionally and (missus tows the horse float !)

have a question which I hope someone can shed some light.
auto gearbox slipping???

not sure if this is normal or not, did some tests today.
Traveled on straight level road not towing and did 80kmph OD = off (ie in 4th gear) and the rpm sat on about 2100.
put a 1500kg trailer on and travelled the same piece of road in same gear at 80kmph and the navara sat on 2500rpm.

this seems like some sort of slippage to me.

Cheers

 

[happy john]

It's not slipping mate the torque convertor is either locked or not ,pulling a load has an effect on this. I think there is a few threads here have a search.

 

[D_d_001]

I was under the impression that for the torque converter to lock up
my sort of vehicle had to be in "D" with OD Not disabled.

so the TC should be not locked cos I had the vehicle in 4th gear with the OD disabled both with and without a trailer.

 

[happy john]

Mate check later tonight Old Tony lugs 2.5tonne with an auto D40 he will give much better info as to how it performs.

 

[joe2006]

The stall speed of the tc is around 2800rpm. Certainly not promoting them but Wholesale Automatic Transmissions has some information on their site about the D40 torque converter. Yours seems to be acting like it should under load.

 

[Old.Tony]

^ Agreed. Stall speed on the TC is somewhere between 2700 and 3100rpm (that's the target range Nissan aim for). It's too high (my opinion), the TC supplied by Wholesale Automatics in Victoria has a stall speed of 2400rpm (much more sensible because peak torque is at 2,000rpm).

Your TCC (clutch) is working perfectly, as you note it's not supposed to lock in 4th. The exact conditions for TCC lockup in my car are:

* Vehicle road speed above 68km/h
* Engine load below 65%
* Throttle position sensor < 50%
* Gear selector in 'D' position
* Overdrive switch in the 'ON' position (indicator on dash is OFF
* Gear displayed on dash is 'D'

Some slippage is to be expected. The torque converter is a fluid drive, it relies on the oil's shear characteristics to provide the necessary connection between the input shaft and the gear shafts. This means RPM is lost.

I did a test once looking at the difference in fuel flow rates at different acceleration rates from 0-50km/h. Turns out that if you accelerate at 2500rpm you'll use less fuel than accelerating at 2000rpm. The reason is simple: because your TC is closer to stall speed, it slips less and pushes more drive through to the gearbox. That directly translates to higher acceleration for a (much) shorter duration = less fuel used.

 

[D_d_001]

Cheers everyone for the replys. have done some searches etc and ofcourse you guys are right. (just didn't think it would be 4-500 rpm diff but I guess load is the big factor)

which leads me to my next question. have tried searches but wondering if anyone has figures for transmission temps for
1. towing in 4th TC unlocked which ofcourse = lower EGT
against
2. tow in OD TC locked which = maybe EGT higher but less power loss and less heat generated by fluid only cxn in the TC.

 

[Old.Tony]

Unfortunately transmission temp signals are sent from the sensors to the TCM in the gearbox and only a "normal or not" signal is sent to the ECU. There's no way to tell what the gearbox temp is at any stage by querying the ECU - you'd have to put your own sensor in.

One of my next jobs with my car is to install a new, separate transmission oil cooler and remove the oil lines from the radiator. Last thing I want is cross-contamination in either direction. This may lend to installing a transmission oil temp monitor in the line from the gearbox.

I usually tow in top at all times unless I'm on a steep hill (in either direction). I rarely push the car hard, even with 2.5T behind her. I've tried towing in lower gears, but if I just stay in top as much as possible, my fuel consumption (with this van) is around 18LPHK and I'm pretty happy with that. I ought to be able to get that further down in the outback where there are fewer hills!

 

[joe2006]

Like Tony, I tow in drive with the TC locked whenever possible. If the TC won't lock, my fuel consumption is terrible.

 

[Foxbat]

which leads me to my next question. have tried searches but wondering if anyone has figures for transmission temps for
1. towing in 4th TC unlocked which ofcourse = lower EGT
against
2. tow in OD TC locked which = maybe EGT higher but less power loss and less heat generated by fluid only cxn in the TC.

Hi D_d_001, my last vehicle was a 3Litre Auto Patrol and I fitted a trasmission temp gauge as well as a manual lock up controller. I towed a 20' full sized caravan and found that towing in 3rd gear at 100kph with the torque converter locked the transmission temp sat around 90 degC at 2700rpm. At the same speed in 4th with the auto unlocked the transmission temp sat around 150 degC and the engine was doing about 2250rpm. Since the egt's dropped while I was in 3rd gear with the engine at 2700rpm and torque converter locked, I actually used a lot less fuel. I could lock the auto in 4th at 100kph but the revs dropped to around 2000 (from memory) and that made the car sluggish and the egt's went through the roof. So towing in 3rd with the converter locked saved fuel, had lower transmission temps, lower egt's and I didn't have to change down for small hills.
The torque converter will generate a heap of heat as it 'slips' due to the fluid coupling and this is the biggest killer in an auto.
Hope this helps.
Foxy.

 

[D_d_001]

I could lock the auto in 4th at 100kph but the revs dropped to around 2000 (from memory) and that made the car sluggish and the egt's went through the roof. So towing in 3rd with the converter locked saved fuel, had lower transmission temps, lower egt's and I didn't have to change down for small hills.
The torque converter will generate a heap of heat as it 'slips' due to the fluid coupling and this is the biggest killer in an auto.
Hope this helps.
Foxy.

thanks for the detailed reply foxbat...some good info here.
unfortunately I'm unable to lock the TC in 4th (5 speed auto) in the navara (only locks in 5th...as far as I know no aftermarket mods address this either)
in my perfect navara the TC would lock in 4th and I could cruise at 95kmph on 2300ish rpm. Problem solved !
so my dilemma is run in 4th "slipping" the TC by about 4-500rpm or locking the TC in 5th with increased EGT.
guess the only way to find out for sure is fit a tranny temp guage, scanguage and get the info myself.

Just thought that I couldn't possibly be the first to try and work this out someone has to have done this before me and had some data.

 

[joe2006]

It would be interesting to know what the manufacturer has determined to be the maximum safe EGT?

When towing I know my Engine Coolant Temperature increases with increased RPM.