sergeant5

I installed the KD switch which made quite a difference. I notice however, that the shift point seems to be rather low, especially 2nd (I believe). Shifts out of second around 4000 rpm. I would like to change this. Is this controlled by the ECU or is there an adjustment somewhere?

TIA

Jay Wellwood

Sarge-

I seem to remember that the shift point is controlled by ECU via the Tach. The tach inputs to the ECU to control the shift point so as to not permit over-revving.

The kick down feature is controlled by relay in conjunction with the KD relay (number XV on my 87).

You may look at the KD cable for proper adjustment as a side note - but this wouldn't affect the upshift points.

A quick email to Wally P may also be in order - 'tech@928gt.com' should get him on board.

HTH-

2V4V

If your kickdown cable is properly adjusted, there really aren't any other adjustments.

Even the GTS a/t is pretty much an off-the-shelf MB unit, and has no electronic controls save for the kickown solenoid.
(The PSD diff is electronically controlled, but not the trans itself.)

If you haven't done a fluid and filter lately, that MAY change things a wee bit.

Or, you may have a leaky vacuum modulator/ vacuum modulator hose which can make shifts a bit off. Check the hose going to the modualtor for cracks/hardening, and if you have a vaccum pump, check if the modulator leaks.

HTH

Randy V

Sarge, I assume you don't read the responses to your previous posts. If you did, you would have seen the reply I submitted to your post " Wow, Installed A Kickdown Switch".

I'll try again - see this site for instructions on changing the shift points by adjusting the vacuum modulator:

LVT's Kickdown/Shift Point Mod

I have done this on 16 and 32 valve cars and the improvement is impressive.

Oh, and Jay, while we appreciate your enthusiasm, perhaps you will provide assistance on issues you have proven experience in. It's great that you want to be helpful, however providing inaccurate information can be counterproductive - just a suggestion.

Jay Wellwood

Randy-

First - please understand that I took no offense to your comments...my experience is undoubtedly not complete as yet....however - if you have been monitoring the main 928 message board - you should find that Wally did poost a response to this particular issue. As such, I passed on what I could remeber - and also suggested that the original poster contact Wally directly.

That being said, I may not understand your comment regarding the shifting modulator solenoid adjustment - since this (as I understand it) changes the shifting 'hardness' but not the speed (engine rpm) at which the shift point occurs.

Please advise - as I am definitely interested in this particular area.

Lastly - I am certain that most would agree with this position - any information gleaned from this list is 'free' and may or may not be the gospel. Mostly by experience and wisdom of others...and may not be the most correct in the eyes of some.

BTW - I searched the archives and found the message....here is is what Wally P at 928 Specialists had to say...
The automatic transmission is primarily controlled by internal hydraulic
pressures. This includes the speed at which upshifts and downshifts occur,
and the firmness of the shifts.

The kickdown solenoid is only one of several devices that adjust the
control pressure to change the shifts. When the kickdown switch is made
(closed), power flows from fuse #10 (on the '87) thru the switch, then into
the 30 terminal of the kickdown relay (XV on the '87), thru the normally
closed contacts of the relay, and out the 87a terminal to the kickdown
solenoid mounted inside the transmission. When the solenoid is energized,
it reduces control pressure, raising the shift speed close to the maximum
speed.

The kickdown relay doesn't do what most people think that it does. The
power flow from the switch to the solenoid is thru the NORMALLY CLOSED
contacts of the relay. This means that the power normally flows thru the
relay. In order to control the shift points more precisely (since the speed
is very close to engine redline), the kickdown relay receives a signal from
the tachometer when engine speed hits 5800 +/-60 RPM (USA and Japan - it's
100 RPM higher for the ROW). This signal open the contacts in the kickdown
relay, breaking the power to the kickdown solenoid. This has the same
effect as instantaneously lifting your foot from the throttle switch,
causing an instant shift. As soon as the engine speed goes back below 5800,
the relay closes the contacts, lowering the control pressure to hold the
next shift.

So, the kickdown switch (and the added parallel kickdown switch) supplies
power to the kickdown solenoid only when the engine speed is below 5800
RPM. Above that speed, the kickdown relay opens, breaking the circuit.

You can check by pulling relay XV (on the '87) and jumpering terminals 30
and 87a in the socket, then driving the car. If there is now a difference
in the shift speeds, relay XV or the contacts for it are bad.

I would suggest that you NOT try the full throttle shift points with the
relay jumpered.

Wally Plumley
928 Specialists


Please advise...as YMMV

Randy V

Jay and Snowball (do you have a real name Snowball - we're all friends here), see my latest response under the topic "I finally Installed a Kickdown Switch - Wow". I believe it answers your questions. I'd be happy to quote you chapter and verse from the workshop manuals, but they are at home and I'm at the office.

If you like, do a search of the Rennlist 928 email list from about a year ago and you'll find my comments when last this issue was debated - the answer is still the same. Wally's description you posted eloquently supports the fact that kickdown and vacuum modulation are linked functions.

Steve Cattaneo

The tv cable (boawden cable) controls shift TIMING, EARLY OR LATE SHIFTS, the vacuum modulator controls shift FEEL HARD OR SOFT SHIFTS. It does this by controlling the line pressure (working pressure) in the transmission by way of a load input, vacuum. To make the trans shift later, make the tv cable tighter, no slack in the cable. A loose cable will yield a soft and early shift. This cable is very sensitive to adjustments, so you will have to adjust, drive, adjust, drive and so on till you get the right shift timing.

All 828 transmissions are non-computer controlled, non-overdrive, non-lock up torque converter. They are hydraulically controlled. As a transmission specialist the first thing that I would do with any transmission that has a driveability problem is to verify that it has the right line pressure (working pressure) with gauge. If the line pressure is not right any adjustments will not solve the problem.


Good Luck,

Steve Cattaneo
Master Tech Transmissions

Jay Wellwood

'Wally's description you posted eloquently supports the fact that kickdown and vacuum modulation are linked functions'

While they may be related, I was under the impression that the original post/question dealt more with upshifts vs. kickdown.

I may be wrong in my assumptions here - but isn't kickdown a downshift (ie 3rd to 2nd) vs. an upshift (ie 2nd to 3rd)?

As I understand the process, it would appear that the lower rpm upshift (from 2nd to 3rd) may be more likely controlled by the line pressure vs. modulation and indeed governed by the tachs input (amoungst several other unnamed ghosts in the machine).

I too have read the Shop Manuals regarding setting the pressures & etc. Still, they leave alot to be desired in this area. I would guess that troubleshooting this is like many other things on our cars - bottom line - make certain the basics are covered first, then move on to the more advanced (ie is the kickdown cable properly adjusted - how does the transmission shift - and what is the fluid level?).

I have to admit that I am a novice in the area of the transmission (who isn't - really?). However, I also must confess that after completing several other maintenance items with sucess - well, I feel as though I am gaining confidence with this beast. To insinuate otherwise would be insulting to both of us. After all - what are we here for anyway - obviously - to learn more about our cars!



Proud owner of the TT (and rebuild)/MM/Tbelt/CV Boots/Parking Brake/Cam Shaft/Interior replacement Tshirts - and many more (and some to come).

sergeant5

Randy - I did check the LVT's Kickdown site. As I understood his article the adjustments he was talking about was shifting frimness, which seem fine on my 928. I don't believe that I have a problem with the trans. It just seems to go into 3rd gear early, but only when the kickdown switch engaged, and under partial acceleration (about half throttle or a little more). This might be quite normal, but it seems that the car would accelerate faster if 2nd gear would hold out to the same rpm as first gear (under the same throttle position). I might try the kickdown cable as mentioned above. Or just experiment with the throttle positions and shift points to see what the exact pattern is. My seat of the pants feeling is 2nd should hold out longer, but I'm new to 928's so maybe this is normal.

Andy C

As the others have said the amount of slack in the transmission cable adjusts the shift points. The pressure regulator adjusts the firmness. That said, I took all the slack out of the cable and under full throttle it still shifts out of 1st a little past 4,000. I believe the shift you are feeling is 1st to 2nd because the other gears should hold to a little under 6,000 before the next shift occurs. I thought that was a little odd but I have read some articles that confirmed those shift points were designed into the system.

Steve Cattaneo

The kick down solenoid controls up shifts and downshifts. In the full throttle kick down mode, theA28 transmission changes it’s up and down shift points, the up shifts are slightly higher than full throttle shifts for maximum acceleration and performance. The other function of the kick down solenoid and tv cable is to force a 2nd to first gear downshift from a dead stop take off. At light and full throttle take offs the USA 928 starts off in 2nd gear.

Governor pressure pushing on the shift valve induces up shifts and downshifts, in a pressure less hydraulic system (the car not moving). The shift valves (in the valve body) are held in the lower gear by a spring. When the car is moving, governor pressure increases pushing the shift valve in opposition of the spring force, causing the transmission to up shift or downshift, depending on throttle position and road speed. The A28 trans has a kick down, downshift safety feature, called the speed relay. Lets say we are cruising at a 100 mph and we want to pass a slower car, while pushing down on the throttle, the kick down solenoid is energized. Thereby causing a 4th to 3rd gear downshift. The engine revs to 5901 rpm. The speed relay (rev limiter) cuts power to the kick down solenoid causing the transmission to up shift back into 4th, lowering the rpm’s below the cut off point. Think of the speed relay as a speed limiter, preventing the engine from redlining, this is the only function of the tach input.

steve c

Jay Wellwood

Thanks for the info Steve...definitely interesting stuff.