Rb25De Stalls When Put Into Reverse Or Drive

[GTSBoy]

Hey UCD

Now I have had Eff all to do with auto,s

Sorry to rain on your parade a bit ..... but stall speed and stall behaviour of torque converters, and the action of lock up clutches on torque converters are two completely separate things.

Any torque converter will have a stall speed. That is the speed at which if you stand on the brakes and floor the throttle, the car will either creep forward (more power than braking) or stall the engine (more braking than power). What happens at that speed is that the two halves of the torque converter effectively do lock up - it is the hydrodynamic stall point of the fluid coupling. That's where the "stall" comes from. It's similar to stalling a wing on a plane.

Modern torque converters may also include a clutch that literally forces the two halves of the converter to lock together. This is a mechanical clutch, effectively exactly the same as a clutch in a manual gearbox. The lock up is generally controlled by the TCU - ie, it is electrically activated by solenoid. The TCU can and will cause that clutch to lock up at just about any speed that it wants to. And if it is locked at any time other than when you are driving it should cause the engine to stall.

[FJ RB25]

Not raining on my parade at all GTS boy, like I said I am no expert on Auto's thanks for the info , always willing and open to people who know more than I, after all that's how we learn from folk that know more than us.

I have a question though, if stall is operated or initiated by the TCU ( transmission control unit) why do people go and purchase a higher stall converter for their car when they can just dial in a higher stall through the TCU ?

In saying that it would seem silly for all those people that go and buy a high stall converter for no reason.

The way it was explained to me was that if you have a 2500 rpm stall then the two halves and the stator would not be locked up and give you positive drive till 2500 rpm.

Is this correct?

Always willing to learn from someone with more knowledge.

Edited October 17, 2014 by FJ RB25

[GTSBoy]

Nah, you've still got it backwards. Lock up clutch is not stall.

If you put in a high stall converter it simply raises the rpm at which the converter will stall. So we'll go right back to basics.

Autos (historically) need a soggy connection to the motor. The torque converter allows the engine and the gearbox to run at different speeds. When idling in gear with the brakes on, the converter slips all the drive from the engine and turns it into waste heat. The fluid basically just gets thrashed around in the converter. If you let off the brakes then the converter may be able to transmit enough drive from the impellor to the stator to cause enough torque to turn up in the gearbox shaft to make the car creep forward. It certainly will if you give it a tiny bit more throttle. The engine is doing a fairly high speed (say 1500-2000 rpm), but the car is creeping forward at something equivalent to maybe 500rpm input shaft speed. Now that it's rolling, if you back off on the throttle the car will continue to roll on at that sort of speed and the engine revs will fall back to idle. Now if you floor the throttle, the revs will very rapidly flare up to a higher speed. On a standard setup that might only be about 2000 or 2200 rpm. That's the stall speed of the converter. At that speed the converter starts to behave as if it is a solid connection. So what it does is allow the engine to flare up to revs where some good power is made, and as it closes in on the lock up speed the car will get more and more drive through the tranny, so it will accelerate decently.

If you work a car's engine, like big porting and big cams, it might not come on song until maybe 3000 or 4000 rpm. In that case a normal stall speed converter will start to stall well below the point where the engine is happy and you will get very poor accel. In that case you put in a converter with a stall speed that suits the characteristics of the engine. If it needs 3000 rpm to come on song, then you put in one that stalls somewhere around there (maybe a bit higher to allow the engine to convincingly make power).

The lock up clutch is what is controlled by the ECU. That is a purely mechanical means of locking the converter solid. The reason they exist is that the slippy nature of converters wastes fuel when you are cruising below the stall speed of the converter. So when the TCU finds itself in cruise mode, it can lock the converter clutch and now the engine and box run at the same speed like a manual. When you demand a bit of accel, the TCU will unlock it to allow the converter to slip again (to retain the benefit of having a converter in the first place). Modern cars with very torquey engines will often leave the converter locked even under moderate accel from cruise conditions because the engines don't need the torque mulitplying effect of the converter the way that asthmatic old 60s and 70s engines did.

[FJ RB25]

Ah ok cool

Thank you for the post reply, while all my mates were playing around with v8's and Auto's in the 90's I was stuck playing with manuals.

Cheers once again