Izzy

Figured I'd post this here since there are more 928 automatics than the rest of the Porsches, that and a 928 will most likely be the application. How is upshifting on a 928 controlled? is there a computer to shift the transmission up and down at certain rpms, and coordinate the torque converter with the shifts, or is it strictly mechanical based on the rpms to the transmission? To continue with the barrage of questions, is there such thing as a way to override the prompts to shift? Maybe even set up a system with a paddle shifter or custom upshift points?

Michaelathome almost convinced me to buy a 928, though I don't like how the automatics almost take away control from the driver.

Izzy

I see Jager Engineering has almost what I'm thinking of: http://www.jageng.com/shift_tip.htm

perrys4

The auto can be manually shifted as I am sure you know and really is the best way to drive one. I don't know the technical answers to your questions.

Bill Ball

There is an electrical kickdown switch and a bowden cable that affect shift points, and an adjustable vacuum modulator that affects firmness but everything is mechancial. A paddle shifter would be quite a feat. There are no electronic controls.

That being said, you can use the shifter like a manual tranny and hold it in gear or downshift when you want. It's not a tip, but it's a nice, strong tranny. You can look at the performance figures for AT vs. manual and see you don't give up much. I shift mine at will and as you can see from my sig, the car pulls just fine off the line, and even when stock (before the SC) I did a documented 165 MPH at 6000 feet above see level.

DANdeMAN

I've got one, and I shift all the time.
If you go for a test drive, lever in 2nd, it will start in 1st and stay there til you hit ~5,500rpm; but once in 2nd it is hard to get back to 1st without smashing the pedal or use a kikdown bypass switch. Most of the time, I'm in 3rd (or 2nd), that gives firmer shift and responce than D. D is good for highway cruze only. I very rarely use the kikdown, I prefer to downshift manualy than slaming the pedal down.

Izzy

Whats this kickdown switch? And of course, how does the device on Jager's site work, is this the kickdown switch?

DANdeMAN

Here is a good example : http://members.rennlist.com/v1uhoh/kickdown.htm

G Man

The Jager unit apperas to do the same thing as the kick-down mod mentioned before. Looks like they mount the switch in the shifter handle instead of elsewhere.

whitefox

When you "floor" the pedal you press down a button that is behind the pedal which generally tells the transmission to downshift, its my understanding that it isn't that basic, other conditions must be met such as speed and RPMs.

The Jager system works just like a parallel kickdown switch. IMHO its not worth the money for the Jager system.

Izzy

So would I be jumping the gun to say its primarily the vacuum pressure thats controlling the shift points, and the relative pressure on the throttle pedal is just acting to shift the car UP sooner?

Steve Cattaneo

This sould help, one my post from Mercedes shop.



04-17-2004, 08:15 PM
C32AMG
Registered User Join Date: Mar 2004
Location: new york
Posts: 183

The primary pump, supplies pressurized fluid to the hydraulic circuit. This pressure applies bands, clutches and lubrication for all moving parts. The most hydraulic circuits are found in the valve body. As the fluid moves into the valve body it has the potential to actuate every single valve. When the valves move they redirect the pumps pressure to control the application of bands and clutches thus providing up and down shifts and different ratios through the planetary system. The pumps working pressure is always the highest pressure in the hydraulic system.

All other pressures are derived from this maximum pressure and is reduced and controlled by regulating valves. The primary control valve is the pressure regulator valve. Mercedes calls this valve the control valve working pressure. It controls and regulates the amount of working pressure in the transmission. This pressure varies depending on which way the valve moves. If the valve moves towards the spring, which sometimes breaks, we have low working pressure. If it does not move, or moves a little, we have high working pressure. There are other pressures that balance and regulate the pressure regulator but when we have 210 pounds of working pressure in a vehicle in drive, the problem is usually the pressure regulator.

The governor is the road speed input from the transmission. It has no regulating pressure from a stop and does not control a first gear start in any valve body. In a Mercedes first gear start valve body, the 1-2 command valve is stroked in the first gear start position; by design once the car is started, commanding the B2 band on and the F, one way clutch locked. If the valve body were a second gear start, then the one two command valve would be stroked in the 2nd gear start position commanding the B1 and B2 bands on.

The description of operation in that PDF is for a 1996 C22, which Is a 722.4 transmission that starts in second gear.

What they are referring to by the engagement dampening circuit, is the accumulator cutting in oil circuit wish is a dampening system designed to soften shift engagements from a dead stop (not at road speed):

The dampening circuit just cushions the selected gear engagement.

N to D = B1 and B2 = 2nd Gear
N to 3 = B1 and B2 = 2nd gear
N to 2 = K1 and b2 and f is locked = 1st gear
N to R = K2 and B3 = Reverse




In the neutral position there is no power flow between the engine and transmission, no bands or clutches are applied. This is also the towing and push start feature of the car.

By moving the transmission lever from neutral to drive at road speeds, you are going from a pressure less system (neutral) to a high pressure system (drive). At that high pressure you are probably freeing up whatever is stuck in the valve body.

I think the problem is in the valve body. If that car was at my shop I would change it.


In any 722.3-4-5 MB or Porsche 928 if you move the selector lever manually to second, the transmission will start in first gear. This is engine breaking. Try that as a test.






How a transmission shifts is based/ derived from a balance /unbalance of pressures


The main plays are

Governor, road speed input oil.
TV Bowden cable / road load input
Modulator pressure, load and speed dependent.
Working pressure, pump

Shifts happen when governor pressure moves the shift / command valves in the valve body (control center) in opposition of TV, Bowden cable/ rod, control pressure, making the connection between main pressure and the appropriate band and clutch drums

m21sniper

A paddle shifter would be doable, but be quite involved. You'd have to hook the outputs of the paddle to some sort of a progressive two way solenoid, and then mount the solenoid so that it could manually move the gear change lever on the transmission.

Bill Ball

Well considering the shift pattern is not exactly linear (2 for 1st gear, up to 3 and back to 2 for second gear, then 3, then D), it would be quite a feat.

deranged

I have a 1986 928 with 60k miles on it. I have done all the necessary repairs and replacement of
parts to get the vehicle running again; vacuum lines, tune up, water pump,
timing belt, trans. service, etc.

When I was doing the top end refresh, I removed the throttle, bowden, cruise
and pedal cables from the quadrant. I think when I reconnected them I lost
some adjustments. I reviewed some of the forums to find some solutions to my
problem of hesitation in upshifting at high rpm.

The vehicle starts fine, and idles at about 750-800. The trans. fluid level is
normal and recently replaced. There is vacuum at the brake booster. If I
accelerate normally, the vehicle shifts perfectly. However, when I push the
pedal to the floor, it starts hesitating and does not want to upshift. If I
back off the throttle, it will shift. If I do not aggressively accelerate
the vehicle shifts perfectly. If I am at 60 mph, and press the pedal to the
floor, it does not kickdown, but sputters, hesitates as if it wants to shift
to a lower gear. I will only shift if I back off the throttle. I have
adjusted and readjusted the bowden and throttle and pedal cables dozens of
times with no correction. Interestingly, it seems to run find, even at full
throttle, when it cold. I may be mistaken, but I think these symptoms occur
only when the vehicle is at operating temperature.

On a 30 mile trip recently it does not want to upshift on hard acceleration, but bucks and hesitates. I can drive in the morning for 30 minutes with no problems with acceleration, kickdown at 55 and so forth.

Then I park it for 5-10 minutes. It always starts right up.

Then take off again and the symptoms of hesitation in upshift, bucking occur, and will not upshift unless I get off the accelerator. This hesitation/bucking occurs on hard
acceleration.

It seems to me heat is the culprit. I have checked the
vacuum line at brake booster which checks with good vacuum. I have not checked the line going to the modulator on the tranny.

Would a vacuum leak occur only after the vehicle is hot?

I thought I would try manually shifting to see what
happens.

So I put it in 2nd and when I get up to 20mph it stars bucking,
like it does not want to go faster than 20. I did not see what would happen
if I manually shifted to 30. But when the shift lever is put in 2, and I
drive, it will not go above 20 and then starts this bucking that I
experience when it automatically shifts.

So is driving in 2 and not going above 20 normal?

86 928 automatic