Auto Transmissions

[sean sherry]

Having had a couple of queries on the subject
.........To set up any make of Auto Transmissions it is imperative that, firstly the Engine is in a good state of tune and in good condition, not with low compression. An Engine and an Automatic Box should be matching bed fellows ! With vacuum modulator pressure control units the vacuum reading at the modulator end must be the same as manifold reading. Say 16 to 18 inches and the pressure reading should flick up and down with a blipping throttle. This tells that the internal valves are not sticking. Most low vacuum reading are down to retarded ignitions, for what ever reason. Fix it ! A slow modulator reaction will never fix a problem. A weak modulator spring can be checked using a kitchen scale to read Lbs pressure before breaking. But when in doubt .just fit a new one. They work hard, in and out like a fiddlers elbow !
For instance in G M Cars, with non adjustable modulators and a hollow rod, just put a brake rivet down the inside to raise the pressure, works wonders at times.
Mercedes Modulators are like Hinz's Beans.. they come in 52 varieties. Fortunately they are easily adjustable and respond well to a full turn on the adjuster to fix mild flares., if you are lucky. But the vacuum must be right !
Any questions I will be happy to address, if I can.

 

[c107]

Thanks Sean.
Interesting post.

Where you would you look if you had a car that will not exceed ~3500rpm unless you manually move through the gears with the shift lever.

Even if I floor the 450SLC, it will not move to redline (unless I use the shift lever). I had a mechanic look at it and he made some adjustment but it is still not revving out.

Kick down switch does work, but it is unwilling to kick down to higher RPMs.

Incidentally, I think my UK Spec 560 has an AU spec modulator on it. Not sure if this is a big deal?

Bryce

 

[sean sherry]

I would look at the linkage connecting from the Carb end running down to the Transmission. The modulator controls the Line pressure via the throttle Valve .
The linkage controlling the up shifts has it's built in limitations. Even in low gear it will up shift preventing the engine from over revving when using full throttle. It is not an uncommon problem to find that the accelerator pedal hits the floor before the carburettor gets full throttle. Wear and tear. Mostly this can be easily fixed by pulling the pedal up and bending it .

 

[c107]

Thanks - checking throttle linkage is a good place to start. Its an FI engine not a carby but the same would apply I assume.

 

[sean sherry]

Yes, the principle is the same There would still be a mechanical linkage connection to the Transmission, with the non Electronic controller transmissions.
Helmet would know how the later Transmissions are controlled.

 

[Styria]

Sorry boys, this goes beyond my feeble brain - but, keep it up as it makes good reading. Nothing like a brain teaser.........Regards Styria

 

[Oversize]

A lack of revs often points to a blocked exhaust but if it does rev out under some conditions tha would negate that theory.

 

[sean sherry]

Yes Mark, Engine Tune is always the first place to start, before blaming the Transmission. Another essential is a Stall Test to eliminate the Torque Converter. A slipping Sprag Clutch is not uncommon in older Cars. A rough guide is that the Stall should be around 1800 Revs with a strong Engine.
Large Cars have big Converters and small cars have small Converters, so it is an even playing field.

 

[c107]

Yes Mark, Engine Tune is always the first place to start, before blaming the Transmission. Another essential is a Stall Test to eliminate the Torque Converter. A slipping Sprag Clutch is not uncommon in older Cars. A rough guide is that the Stall should be around 1800 Revs with a strong Engine.
Large Cars have big Converters and small cars have small Converters, so it is an even playing field.

Hi Sean
What is a stall test and how do you perform one?

Bryce

 

[motec 6.9]

Hi Bryce I'm sure Sean will confirm but stall speed test to my understanding is foot firmly on brake pedal . And raise revs until it feels as though engine is about to stall or with more powerful cars it is about to launch

 

[sean sherry]

A common test Bryce, and a simple check to see that the converter is multiply Torque
A Stator sits between the Impeller and the Turbine and its job is to redirect the unspent fluid back to multiply Torque. If the Sprag ( one way ) Clutch is slipping then it has the opposite effect robbing Engine Power to the transmission.

A stall test is Transmission in D ,, left foot on Brake and floor the Throttle for one or two seconds. There is a very rapid build up of heat if held any longer. Note the Tacho reading. A bad converter will only show around 1000 RPM. But as before ensure that the Engine is up to the task. Many converters have been condemned due to an Engine issue.