Archived Acceleration shudder

[HHhuntitall]

If it's shifting normally while driving, I don't think you have a problem with the electronics. I think the problem is mechanical, and the only way to keep it from doing it is to back off the throttle when you're under 2000 rpm, and downshift it when you're under that. Those trannies do fine until you start to up the HP, then they don't hold up very well. The only way I know to fix it is to rebuild the transmission... . upgrading the pump, clutches, bands, sprockets, and converter when you do it. Personally, I'd call ATS or Suncoast, and get away from the BD stuff. I have a '98 4 door conversion that would do exactly as you describe, but it only had 35k miles on it. I had a lock up switch on it, too, but it didn't matter. If engine rpms were too low, the transmission pump just didn't have enough pressure to keep the clutches engaged... ... That truck now has a NV4500 and Southbend in it.

 

[Kirwin]

Torque convertor slippiage

The torque converter lockup pressure is at a minimum at low speeds. As the transmission wears, read as accumulates miliage, line pressure falls due to leakage inside the transmission control circuits and accumulators. The most likely time for torque convertor slippage is when you step down on the throttle hard enough to generate quite a bit of torque but not enough to cause the PCM controlled torque converter to unlock. This prevents a manual down shift out of OD to drive or 2nd gear depending on the transmission control cable position which is directly connected to the throttle lever. So to clarify this; until the transmission is unlocked, the hydraulic transmission controls will not allow a down shift. Therefore the torque conveter clutch slips and since this in not a designed slip point it sounds like a rough, grating, sound that is very unnerving. If you drop the pan you can see the results if its severe enough. Small fine metal dust is OK. Larger pieces of clutch facing not good. Ken Irwin

 

[Kirwin]

TPS operation and its effects on torque convertor clutch

The Throttle Position Sensor (TPS) does not directly affect torque convertor condition. It is merely an input to the PCM. The PCM provides the ground path for a solenoid valve in the transmission valve body that directs transmission line pressure to apply hydraulic pressure to the "apply side" of the torque convertor clutch. This forces the internal clutch faces inside the torque converter to come in contact and after a short engagement period where slippage is allowed and the two pieces reach the same speed, the converter drive and driven hubs are effectively one piece IE "locked" together. It is important to understand that if the torque input on one end exceeds coefficient of the friction material under the existing applied line pressure condition the two clutch faces will rotate relative to one another just like a manual clutch will. This is not supposed to happen and later transmissions have an input and and output speed sensor that was added to detect this exact condition to prevent damage to the transmission. Several other upgrades were done over the years to further help maintain line pressure at a pressure high enough to prevent slippage as this can destroy a transmission in short order. Remember, just like a manual clutch is not designed to slip under full load conditions, neither is the one in an automatic transmission. Its designed to slip for a short period of time while adjusting the speed of one shaft to the speed of another shaft.

Now the TPS can cause the PCM to order the torque convertor to unlock if it detects a significant throttle opening or a sudden increase in throttle position. This is most noticeable when operating on cruise control as the wiper inside the TPS loses electrical contact with the fixed portion of the resistor. The PCM (via this input from the TPS) sees a momentary wide open throttle condition which causes the PCM to order the torque converter to unlock and RPM will rise about 150 RPM; but since the actual throttle position is not wide open, the truck will slow and this causes the cruise control unit to open the throttle slightly reestablishing contact inside the TPS and then the PCM realizes the truck is actually cruising close to cruise control set speed and will order the torque convertor to lock up and RPM will drop about 150 RPM. Then the process repeats itself as the wiper inside the TPS moves across the worn area of the TPS resistor.

 

[KRStone]

I understand.



Also, I very well may be getting into the pedal too much at lower RPMs in overdrive. Someone mentioned downshifting if RPMs drop below 2,000.



This is a bit of a surprise, I try and never go over 2,200 rpms, it just sounds like it's revving too high. I try and stay between 1,500 - 2,000. Have I been wrong?



A rebuild (and upgrade) will be in the future, but I'd like to put it off as long as possible.



KRS

 

[Kirwin]

RPM range

The 12 valve engines have a rather "peaked" torque curve. I did the 6 speed conversion on my 97 in an attempt to take advantage of this. The 97 also sounds like its making a lot of noise over 2100 RPM to me also. I know that when my stock clutch slipped it was right at peak torque, in high gear. There is a lot of torque available at peak tuque. A rebuild will restore your line pressure and there are several upgrades that can be done while your in there to help it last. Dusty Hawk at HTS transmissions built my last 46RH and its a very stout transmission. I highly recommend him. Ken Irwin

 

[HHhuntitall]

Very good break down, Mr. Irwin.



And as for the higher RPM, you may not like it, but for the sake of making the transmission last longer, it's just about the only way to keep it together. If you keep making it slip, it will, in short order, destroy the overdrive clutches, and you'll have to rebuild it. FWIW, I had a '96 I ran 4100 regularly. I took that pump(and twins, injectors, etc) off the truck, sold it, to a friend no less, and it's almost to 300k on the bottom end. Another fellow I once knew, who lived/s in Weatherford, Tx, has/had a Dodge Dakota, narrowed, tubed... . basically a chassis car... . that he used to race with. It's a stock 24v bottom end with a p-pump conversion, and he shifts the custom 727 at 5400rpm. Yes, I typed 5,400rpms. The last time I saw it, it had several hundred passes on it, and still no signs of bearing or rod fatigue. It used to run 9. 80s in the quarter on diesel, (around 70psi boost), and 8. 60s on propane, around 110psi boost. Fun would be a good description..... I wouldn't dare run mine to that extreme, but MY POINT being, rest easy on running your Cummins above 2000rpm, and an occasional 3000 won't hurt it, especially if you have the stock governor springs in the pump... Cummins wouldn't have designed it to go that far if it wouldn't have lasted. Remember, these engines operated several years in farm tractors and construction equipment prior to being installed in the Dodge, all governed around 25/2800 rpms. Talk about a harsh environment!! All dust, no air movement, surrounded by hot metal... . And they'll run 9/10,000 hours with proper maintanance. That's around 1 million in truck miles!!

 

[Missouri Mule]

KRSTonne, I run the Mule to 2750 rpm when entering a highway but I have 5 spd and I'm shifting into higher gears. Cruise between 2000 and 2250 rpm.



For upgrading trans also consider DTT and Goerends.



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