MX-5 ND power windows are controlled by a dedicated controller. When the soft top latch is unlocked, the windows go down automatically a few inches and stay there until manual interaction.
This is a little bit annoying when you just locked the top and are leaving the car. I decided to implement an automatic closing windows feature after you lock the top.
Bellow is the schematic diagram of the power window system for MX-5 ND
As you can see, there is no communication with CMU. Everything is controlled inside the main switch, which will not be easy to modify without altering hardware.
updated on 5/16/17
Finally, I got the actual switch from MX-5 2017, part number NA1J66350B. The main processor is marked as “109GCK”, which is R5F109GCKFB from Renesas. It has 6 accessible test points (GND, Reset, Debug, SCK, SI, SO) for programming/testing.
I did a quick scan on LIN bus wire; it seems to be very simple communication with passenger-side door switch. It would be very easy to control the passenger side window, but there’s no similar interface for the driver side. The driver’s side is controlled by a button or internal software.
LIN master on the driver’s side is sending 2 bytes of data every 10ms. The first byte provides information about switches (door locked, panel “Lock window” switch, top latch and soft top release). The second byte has commands to the passenger side motor control (up/down and auto). The passenger side controller responds to the master, probably with door switch status.
My current plan is to first add a small microprocessor, to implement and test features I want. Later I will attempt to hack an existing processor. I’m not sure if hacking will be possible at all. Available debug tools only support flash programming; no reading function supported. Also, this chip has a security feature. If enabled, it will not be possible to communicate without a known security ID.
My external microprocessor will check status on the LIN bus and soft top position, and based on this input will activate “Close” signals. I think the 8pin PIC12F1572 microprocessor will be good enough for this project.
One possible scenario:
If windows where all the way up, unlocking Top latch will force windows down for a few inches. Once the top latch is locked again, in the current implementation, the windows will stay in the same position. I will check this status. If there is no manual interaction with switches, windows will be forced to go all the way up automatically.
On the picture above is the implementation with PIC12F1572 microprocessor for auto-closing function.
The new version with PIC16F18323 has automatic windows down feature when the convertible top is open.
In this implementation:
Top locked up, windows up: opening the top will force the windows to go all the way down.
Top locked down, windows up: closing the top will force the windows to go down a few inches; after the top latch is locked, the windows will go up all the way.
Top-down, windows down: closing top, windows will stay down.
Replacing the switch in the car is a relatively simple process, takes about 20 minutes of work.
Feedback on Miata forum:
I made one more option by request, windows will go up after lowering the top. Here is a customer’s video.
Step by step instruction for modifying switch: Switch mod.pdf
See the next post about key fob control.