Kickdown relay (two versions, this is the pre-1989, starting 1989 the relay has extra inputs)
What is the sole purpose of kickdown switch relay and how will I know that it is malfunctioning?
Hello Bora, The kick-down switch closes a circuit to the automatic transmission and forces a downshift to the lowest possible gear for the speed/RPM of the car. A relay is a switch, with the "open/close" controlled by a relatively small current compared to the amount of current flowing through the actual switch. A relay is used when it is desirable to control a large amount of current with a small current. The kick-down switch (and wires) at the throttle are small compared to what would be necessary to manage the solenoids in the transmission. If the kick-down relay isn't working, you will see two possible scenarios; - if it is stuck in the open position, you get no kick-down when stomping the throttle petal to the floor - if it is stuck closed, you will always be in kick-down mode and will have a much too aggressive shift program for normal driving (always revving to near the redline prior to upshift) My guess is being stuck in the closed position would be rare.
2 928's with kick-down switches
For all those auto. trans. out there. I installed a temporary switch to bypass the kickdown process for my '85. I've read a few messages on the list that extoll the new found gitty-up that there cars seem to have at lower speeds without having to put the pedal through the floor. Well, after I screwed up a $10 job, (pinched wire = meltdown), I had my mechanic go through it, (now $200 job w/wire harness, accordion conduit around the wires etc.), and it is functional. However, the only real benefit I have found is at the higher cruising speeds where if you need to pass, with a simple press of this button, (mine is on the left footpad), it shifts down into 3rd for that added response. But that is it! From what I read on the list, I was expecting it to send it to 1st or 2nd for that true "kickdown" effect. It absolutely had no benefit for freeway onramp acceleration. Am I missing something? Thanks,
Rory Hart '85S A/T
Dear Roy: On my previous 86 928S AT I installed a parallel kick down switch. When pressed once it was on. To turn it off, it had to be pressed again. During autocrosses I would "set it, and forget it." I was always in the lowest possible gear that did not exceed redline. During DE, I prefer to determine the gear I want on my own, so it rarely was used. For street use, it was good for a few low speed thrills, but not often. Besides my mileage was not great to begin with, and using it made it much worse. The fact that your AT is not shifting down to the lowest gear possible may indicate there are other problems. Try the following experiment. First, do not use the kick-down switch at all. Second, from a standing start, place the gear selector in "2". Accelerate slowly to 25 MPH (40 KPH) and take notice of the Tach. (You should be in 1st gear). When you get to 4,000 RPM move the selector to "3" and back to "2" quickly. (You should now be in 2nd gear.) Continue to accelerate and at 3,000 RPM move the selector to "3" and leave it there. (You should be in 3rd gear.) Finally, move the selector to "D". You should now be in 4th. Merry motoring. ~Ed~
At 08:46 PM 7/24/01, Dennis Wilson wrote:
>I recently tried doing the "kickdown switch modification" as documented on many sites. Result was no discernable change in shifting behavior. After checking my connections and ensuring the toggle switch is really switching the circuit (probing continuity across the foot switch connector), I'm fairly sure there is nothing wrong with my wiring job. That leaves me to suspect faults in how this circuit is sending control signals to the AT. I think I'm in for some under-car exploration... Has anyone BTDT? I'm guessing there may be some kind of relay involved.
>Anyone know how/where the kickdown circuit connects to the AT?
As usual, it would have been nice to know the specific year model, but ....
The automatic transmission is primarily controlled by internal hydraulic pressures. This includes the speed at which up shifts 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.
I recently learned more about the kick down relay operation (even though I
don't have one). It is not intuitive.
The relay is not activated by the switch (or a bypass installed instead). Instead it is activated variously over the years by engine RPM, throttle position, speed, dynamic throttle inputs).
The relay terminals are normally closed and the solenoid is activated directly by the kick down switch (or bypass) though these normally closed contacts. When the solenoid is activated the AT control pressure is lowered & the shift point speeds are raised.
The kick down relay is activated primarily by an engine RPM signal (at high RPM points - from dash) which causes the relay to turn off the solenoid - this is what causes the shift to happen - which itself causes the RPM to drop and reengages the solenoid to hold the next gear until it gets to high RPM again...
So bottom line is you will engage the solenoid almost full-time if your kick down is always activated/bypassed. It does not seem to be a major reliability factor since I haven't seen reports of solenoid failure and as far as I can see its just operating a pressure relief valve so is probably quite low power...
So in summary:
The kick down switch causes the trans to hold in gear to higher RPM/speed.
The relay momentarily breaking the connection causes the high RPM shifts to happen.
I do also think a (left) foot operated momentary switch is probably the best option to control this.
1994 928 GTS Black/Black Manual "AZ Desert Gang"
I didn't realize that the relay has normally closed contacts. What I said earlier should have more correctly stated "Looking at the circuit, the KD switch applies battery volts to the 12v side of the CONTACTS of the kick down relay".
So as I understand it, the KD relay may be opening its contacts at some predetermined rpm but until the KD switch is bypassed or normally activated then the KD relay can't activate the gearbox solenoid because there are no volts available to energize it (and then allow the relay to break those supply volts)
Do you agree ?
John '86 Euro S2
Yes - the KD relay is actually more of an "anti K-D" relay in most versions. Its job is only to disable the kick-down when the trans should shift or before a min speed has been reached. This is what is not intuitive at all about it - it works quite opposite to the switch.
Only in the GTS dynamic kick-down can the relay actually activate the solenoid itself. In that case it still retains the disable function - its just as if there is now also a parallel kick-down switch inside the relay. The schematics for the later relay do not show this extra contact set - but they must be there.
In the Book from Austin " Buch zur Lektüre" , page 137, top left ....
Kickdown- up- and down switching are not triggered hydraulically anymore, but more precise via a kickdown relay, which triggers gear change at 4950 RPM from 1st gear into second. 2nd gear and 3rd gear are made up shift at 5950 RPM.
End of quote
I'm not 100% sure this is accurate but it supports that the gear change is triggered electrically as well as mechanically/hydraulically. It says also that up/down shift is not (!) triggered hydraulically anymore and that is not true imho.
new TPS & New KD relay and a bit of additional wiring - 1 new wire between the new TPS & the relay should get you exact GTS 94+ behavior. Actually not that hard to do electrically (but you have to pull the intake to swap the TPS. Its a little more complex on 88 and older...
I made a schematic annotation to help understanding the kickdown relay power flow:
1992 Porsche 928 GTS midnight blue
In the manual the kickdown is mechanically preset. Here is an extract from the manual: