Second Generation Steering
Wheel Controls.
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History of the second generation steering wheel controls
The above picture is of the second generation steering wheel control
(SWC) that GM made along with the part numbers for these buttons. It was
available in several 1989 GM vehicles (Pontiac Grand Prix, Bonneville and
6000 STE; several Buick models and several Oldsmobile models). It controlled
a special radio made to accept the SWC commands through a new bus called
the E&C bus (Entertainment & Comfort). This system fixed the major
drawback to the first generation system that could only control radio and
audio functions. Tape deck (not too sure of the tape deck -- I think Delco
might have made a few models that the E&C bus could control) and CD
player functions were able to be controlled by this new E&C system.
In fact, the E&C bus could also control the HVAC system on some of
the models listed above (example: the Pontiac Bonneville SSE).
In 1990 and 1991, the system did not really change much. It continued
to be offered in the models above and slight cosmetic changes were made
to several Oldsmobile models.
In 1992, airbag equipped vehicles from Oldsmobile and Pontiac required
and new type of steering wheel control. This is what I call the third generation
steering wheel control. Otherwise, the system stayed the same as previous
years.
1993 was the last year that Pontiac used these controls. I am not
sure of the other divisions usage of the E&C bus steering wheel controls
(SWC's).
In 1994, Pontiac redesigned the Grand Prix interior and it had
an airbag equipped steering wheel. This required a third generation steering
wheel control system. I believe that other divisions had also switched
over to airbag equipped steering wheels because of a federal law that required
all vehicled to be equipped with them by 1994 (any help here from any readers?).
So I believe that 1993 marked the end of the E&C bus steering wheel
control. The E&C bus lives today, just not inside the steering wheel
(although I think some Oldmobile models convert the third generation steering
wheel control signals into E&C bus commands to control the radio and
HVAC controls).
How does this thing work?
The second generation SWC's required several new and some updated
components from the first generation SWC's. First of course was the steering
wheel. The steering wheel needed to be designed to allow the SWC pad to
be secured to it. It was a different design than the first generation controls
simply because of added functions and more models available with SWC's.
Next was the SWC pad itself. GM did service this unit. New buttons and
case parts could be purchased from the dealer. Next in the chain was a
new cancel cam. The standard cancel cam had only one slip ring (for the
horn). The first generation cancel cam had four slip rings and six infared
LED transmitters. The second generation cancel cam still had four slip
rings, but now only had five LED's but these LED's were transmitters AND
receivers in one package. This allowed bi-directional communication unlike
the uni-directional communication of the first generation SWC's. This is
still an expensive replacement piece. The four slip rings handled the horn,
ignition power, ground and illumination power. The cancel cam had a standard
horn connector and a plug with FIVE (not four like the first generation
SWC's) pins in it (for the SWC pad). The infared (IR) light emitting diode
(LED) transmitters turned on and off with the digital signal sent from
the SWC pad when you pushed a button. There are two IR receivers AND transmitters
on the turn signal switch. The IR LED transmitters/receivers (five of them)
are placed on the cancel cam in a circular arrangement, evenly spaced.
This arrangement allowed at least one of the transmitters/receivers to
be in the line of sight with one on the receivers/transmitters on the turn
signal switch no matter where the steering wheel was. This formed an optical
slip ring which was necessary because using a standard slip ring would
cause too much digital noise and interfere with the correct decoding of
the transmitted signal. This optical slip ring solved that problem. As
I mentioned above, the turn signal switch had to be modified to accept
the four slip rings on the cancel cam as well as the two IR receiver/transmitters.
A new component in the chain is called a translation module. This module
converted the signals from the receiver/transmitter IR LED's in the turn
signal switch to a single output. This allows for the E&C bus connection
as this bus is a single wire bus just like the first generation system.
Remember that the E&C bus us a bi-directional system. Components (radio
and HVAC controls) are connected to this bus and all components can "talk"
(send commands) and "listen" (receive commands). So, the translation module
looks at the E&C bus and when the bus is transmitting data, the traslation
module will pass this information to the transmitters in the turn signal
switch, which gets received by the receivers in the cancel cam, which gets
input to the microcontroller inside the steering wheel pad. If the command
was directed to the SWC's, the microprocessor then performs the requested
task. If you press a button on the pad, this first thing that happens is
the microprocessor looks to see if the bus is transmitting a signal. If
it is, the processor waits till the bus is not sending data. If you let
go of the button before the bus becomes "free", the button is ignored.
If the bus is free when you push a button, the processor sends a signal
over the bus that tells either the radio or HVAC control it is about to
send a command to it. The radio or HVAC responds and tells the SWC processor
it is ready to receive the command. Then the SWC processor sends the command.
This turns on the transmitters in the cancel cam, which is received by
the IR LED receivers in the turn signal switch. This goes to the translation
module which converts the signals to go out over the E&C bus. So, the
translation module simply converts from a one wire system (the E&C
bus) to a two wire system (the SWC) and vice-versa. And lastly, the car
harness had to be made to accept the new turn signal switch and translation
module and supply it with the correct power, ground and illumination signals
and well as take the signal from the translation module and supply it to
the radio and/or the HVAC module. Below are some block diagrams to help
you understand the entire system.
Picture is coming...
This diagram shows a simplified block diagram of the entire system.
The infrared detectors in the turn signal switch number only two, not six
as shown. There are also six transmitting LEDs in the cancel cam.
Picture is coming...
In this diagram, a more detailed breakdown of the system is shown.
The switch package assembly is what I refer to as the SWC pad. The steering
column contains the cancel can and the turn signal switch. And you can
see the data path to the radio.
Here, we see how the internal radio circurity is configured. On
the back of a first generation SWC capable Delco radio, there is a five-pin
connector on the back (in addition to the normal connectors). Only two
pins are used as you can see. D6 and D7 are used as spike suppression devices.
The asynchronous serial data is fed through a 47K resistor to pin six of
the microprocessor. The program in the microprocessor decodes the serial
data and performs the desired function pressed on the SWC pad. Note here
that the tape deck is not connected in any way here. This is why the first
generation SWC's cannot control deck functions. The decks used in these
units had no data communication to the microprocessor head. Second generation
compact disc decks did have communication to the microprocessor head and
therefore could control deck functions. Some tape decks I believe received
communication capability and could also be controlled by the SWC's.
Can an interface be made to work with these controls?
The answer is unknown at this point. I have tried to decode the
serial data stream and have found that I need to have the whole system
working in the car to decode the proper serial stream. All I get trying
to decode the steering wheel pad alone is is a series of pulses which are
the same no matter what button you press. I suspect the pad is trying to
"talk" to the radio and tell the radio that it is going to send it some
data. But since I don't have the radio hooked up, it never gets a reply
from the radio and therefore continues to try to tell the radio that it
is going to send it data. I will keep trying as time permits. If you are
interested in an interface for this type of steering wheel control (second
generation), let me know! Since this is an older system, I haven't been
really working on it hard. Seems the third generation SWC's has a wider
market (it came on more cars) and is easier to interface with. The second
generation is the most difficult to interface with as it uses a bi-directional
serial data bus and requires much more decoding as several components share
this data bus.
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Copyright © 1998 by Carl Miles
Last modified: June 30, 1998
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