Why no engine braking with AT?

jaberoo · Aug 4, 2003

I'm mystified why the my 2002 AT TDI wagon doesn't allow engine braking. On steep inclines, as soon as I lift off in D or in 3rd, rpm drops to 1K and the vehicle essentially free wheels down hill, with me riding the brakes. I've had a few ATs in my time but never one that acts like this.
Is this correct behavior for this transmission?

As I'm just about to exceed the 50K warranty, I'd really appreciate any info.

will360655 · Aug 4, 2003

My auto will hold the gear, and slow the car down. If going over 45mph, when I ger down to 38, it will shift into 3rd and slow the car down more. I don't know what the problem could be. Only really steep hills, or just for the heck of it, I will manualy shift the gears down.
Will

MOGolf · Aug 4, 2003

FWIW, here's a small bit of info that VW claims for the AT control module's fuzzy logic:

Control unit - J217 - with driving situation gear changes (fuzzy logic)

Determining the driving situation and driving resistance dependent gear change point happens automatically.

Advantages:

- Gear changes will be consumption oriented
- Maximum engine output is always available
- Individual adaption of gear change points in all driving situations
- Gear change points are infinitely variable

Gear change point variations for gradients

An additional gear change map automatically selects gear changes for gradients upon accelerator pedal position and driving speed.

- Gear change map for extreme uphill stretches is matched to engine output
- Gear change map for extreme downhill stretches is matched to the braking effect of the engine

Maybe yours doesn't know how to read a map?

Paul_ · Aug 4, 2003

I have only 300 miles on my auto TDI and I've noticed that releasing the pedal abruptly tends to make it stay in gear and even downshift as speed drops.

weedeater · Aug 5, 2003

Sounds like the torque convertor isn't locked.

cage · Aug 5, 2003

Don't release the pedal abruptly. It might think you will be braking quickly and is disengaging the converter lock so that you don't stall the engine with a quick brake situation.

bradbms · Aug 6, 2003

I may be way off base here, not knowing the exact layout of the TDI engine, but isn't it common for diesel engines to not have engine braking? Since a diesel engine does not have a typical throttle body with a butterfly valve and is controlled by the amount of fuel that is added there is nothing to provide a restriction on the engine, besides internal bearing friction. When no fuel is being applied (i.e. going down hill) it's really just an unrestricted air pump. This is why big diesel trucks have an exhaust brake, which is just a big butterfly valve on the exhaust side which retards the flow of the exhaust, to aid in braking for towing. Just my $0.02

Mike

OneBadBug · Aug 6, 2003

Mike,

There is also 19-1 compression to help engine brake.

weedeater · Aug 6, 2003

True enough. But his symptom is that when he lets off the throttle the tach goes to idle which indicates the torque convertor is not locked.

People reported non-locking torque convertors as a problem in '03s IIRC. The fix was a computer changeout.

GotDiesel? · Aug 7, 2003

OneBadBug said:
Mike,

There is also 19-1 compression to help engine brake.

What gets compressed in a diesel also decompresses on the powerstroke. It's like a big spring. Not 100% efficient so not 100% rebound but close.

The major force in gas engine braking is the closed intake butterfly. No butterfly in a diesel so not much retarding force when you let off the go pedal.

Sinner · Aug 7, 2003

I always understood that diesel engines are not built for engine-braking, mostly due to the (much) higher pressures in the fuel chamber. Alas, the Turbocharger + direct injection does not like to get rev up, like in engine-braking situations.

So far, I have not engine-braked my car. I don't want to damage my engine at all.

Am I right? Am I wrong?

Peace,
Sinner

MRIBOB · Aug 7, 2003

Not having a A/T in my TDI I cannot compare.

But if you still got 1 mile of warranty left get in and report it to VW.

From reading the post you have a small problem with something. ECU or locking TC.

Have VW check it out.
It is clear that you dont have a normal acting trans.

japedtdi · Aug 7, 2003

hey jaberoo, i'm curious as to your mpg. mines between 35-38 driving around dfw. not many hills but seems to do the
same as yours

98 exploder 02 glf tdi gls
the needs of the many are more important than the needs of the few

Why no engine braking with AT?

jaberoo

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will360655

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MOGolf

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Paul_

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weedeater

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cage

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bradbms

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OneBadBug

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weedeater

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GotDiesel?

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Sinner

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MRIBOB

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japedtdi

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