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Old 06-20-2008, 09:30 AM
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b3lha b3lha is offline
Phil & Belha
 
Join Date: Aug 2001
Location: Alcyone Limited, Buckinghamshire UK
Posts: 2,671
Solenoid A

Having read about Harvey's new gadget I started looking at how the solenoid A duty cycle gets generated by the TCU. The QC shift kit, as I understand it, generates a duty cycle that is proportional to the throttle opening.

I haven't managed to figure it all out yet, because there are still lots of unknown variables in the code. But it looks like the TCU also bases the sol A duty cycle on the throttle opening.

For a USDM TCU, the function in question starts at location EBAA in the TCU code.

At location EC49, it selects a map, based on the gear that the transmission is currently in. A different map is used depending on whether the car is in 1st, 2nd, 3rd or 4th. The maps describe a graph using the straight line formula y=mx+c and each map has up to 6 segments.

For example: The map for 2nd gear is stored at 0xCABE and it looks like this:

1F 3E 2580
3F 4B 23F0
5F 0C 3390
7F 0C 3390
BF 03 3840
FF 19 27D8

The first byte is X, the throttle signal as a proportion. 00 is closed, FF is full. The second byte is M, the gradient of the line. The third and 4th bytes are a 16 bit word representing C, the Y offset of the line. Converted from hex to decimal it looks like this:

012% 62 9600
025% 75 9200
037% 12 13200
075% 03 14400
100% 25 10200

This means from 0 to 12% throttle, the duty cycle is ( 62 x throttle ) + 9600. From 12% to 25% it is ( 75 x throttle ) + 9200. To convert the result Y to a percentage, you divide it by 200. I have plotted the duty cycle versus throttle opening graphs for each gear.



But there is more to it that I haven't analysed yet. For vehicles fitted with an atmospheric pressure sensor (USDM and any others with the 2nd byte of the ROM ID less than 55), the TCU subtracts a correction value based on the atmospheric pressure and throttle opening. Then later it adds a correction based on the current gear and vehicle speed.

Then there is the question of how the TCU calculates the duty cycle to reduce the line pressure during gear changes. I don't know yet.

It seems to me that a similar effect to Harvey's QC could be achieved in software by simply modifying the data in these maps to change the shape of the graphs. Obviously a lot more analysis is required before any kind of TCU mod could be developed. When I'm a little less busy, I'm going to use my TCUSCAN program to log the duty cycle and throttle opening values on my car and try to reconcile them with the graph to make sure I'm on the right track.
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1992 Alcyone SVX Version L
1992 Alcyone SVX Version L
1994 Alcyone SVX S40-II
2004 Subaru Legacy 2.5 SE Sports Tourer
1996 Subaru Legacy 2.2 GX Wagon
1988 Subaru Justy J12 SL-II

Last edited by b3lha; 06-20-2008 at 09:33 AM.
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