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  #51  
Old 12-22-2018, 07:59 PM
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Originally Posted by Overboost View Post
...The first hex byte of a line represents the number of data bytes to follow in that line and the last byte is a simple checksum. You aren't changing either of those.
Pardon my ignorance here. Just trying to learn, and am interested in this...

How can it be that you are not changing the checksum when you change other values. I thought maybe if the multiple changes were symmetric they might offset and the checksum would remain unchanged, but that does not appear to be the case with the new temperatures you chose. What obvious feature am I not understanding? Thanks.
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  #52  
Old 12-22-2018, 08:08 PM
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I was thinking the same thing. Once you know the Ω value at a few values you can tweak the apparent resistance with either series or parallel to offset the temp to have the light come on at any temp you want.
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  #53  
Old 12-22-2018, 08:50 PM
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Quote:
Originally Posted by oldskewel View Post
Pardon my ignorance here. Just trying to learn, and am interested in this...

How can it be that you are not changing the checksum when you change other values. I thought maybe if the multiple changes were symmetric they might offset and the checksum would remain unchanged, but that does not appear to be the case with the new temperatures you chose. What obvious feature am I not understanding? Thanks.
We aren't changing any of the arrangement of the bytes in the lines of code, just the temperature values the angles of the gauge uses. We are simply changing the hexadecimal values that relate to temperature according to the gauge angle. The first byte dictates how many bytes in the line. The second byte is the first angle of the gauge, the third byte is the temp value for the first angle, the 4th byte is the second angle, the 5th is that temp value displayed on the second angle, etc.

So for 15C and below, the gauge is at the bottom. From 15C - 50C, it moves from 0 to 31; from 50 to 75C, it moves towards 90. From 75-115C it stays at that angle. From 115 to 124C it gets to 148, and then 125C onward it jumps to 164.

15C - 0; 60C - 31; 85C - 90; 95C - 90; 110C - 148; 115C - 164

The 22 page thread I posted from E46F is a good read that explains everything in detail.
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  #54  
Old 12-22-2018, 10:13 PM
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Originally Posted by Overboost View Post
... We are simply changing the hexadecimal values ...
Yes, so that's my basic dumb question ... if you change one of the hex values, isn't that something that the checksum checks? And if so, wouldn't the checksum value for that line change? Handled automatically somehow?

No doubts about the needle angles and old vs. new temperatures you want programmed in there. Just the checksum thing does not make sense to me.

I had thought that maybe if the changed parameters were all symmetric changes, so some went up by the same amount that others went down, the required checksum changes might self-cancel. But it does not look like that is the case with the numbers here.
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  #55  
Old 12-23-2018, 12:53 AM
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In this case there is enough eeprom space to keep a duplicate line of the code so there is no checksum needed.
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  #56  
Old 12-23-2018, 03:45 AM
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Originally Posted by 80stech View Post
In this case there is enough eeprom space to keep a duplicate line of the code so there is no checksum needed.
I'm finding they do the same thing in the motorcycle dash EEPROMs - duplicate the code to remove the requirement for a checksum.

I guess storage is cheaper that CPU cycles.
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  #57  
Old 12-23-2018, 12:41 PM
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Originally Posted by andrewwynn View Post
I was thinking the same thing. Once you know the Ω value at a few values you can tweak the apparent resistance with either series or parallel to offset the temp to have the light come on at any temp you want.
No, first of all the 1/2 of the temp sender that works the light appears to be an old skewel normally closed switch, so can't really do much to change that other than maybe use a different sender. The wiring diagram for the temp sender shows both sides the same and NTC resistors internally but I think that is misleading and from what I can tell. I am going to double check that though. So what I am trying to do is read the temp from the DME side and turn the light on with that.
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  #58  
Old 12-23-2018, 02:29 PM
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Originally Posted by 80stech View Post
In this case there is enough eeprom space to keep a duplicate line of the code so there is no checksum needed.
Thanks, I figured it was something like that.
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  #59  
Old 12-23-2018, 09:54 PM
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OK, I actually got into the harness(and put a new battery in my multimeter) and did some more serious testing and I was wrong about the wiring diagram being wrong! Sorry guys, but might be good news. The light side of the sending unit appears to be indeed a NTC resistor although with a completely different resistance value than the DME side. So might be entirely possible to tweak it as Andrewwynn suggested. Will keep you guys posted.
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Last edited by 80stech; 12-23-2018 at 10:06 PM.
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  #60  
Old 12-24-2018, 01:56 AM
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I did a fair bit more investigating and there is definitely more going on here than I thought! Anybody dig into the temp light before, or am I in uncharted territory? Either way I am up for the challenge but if anybody can add some knowledge I would not complain! As it sits now I think there is some information sharing going on between DME, the dash, and both parts of the sending unit(or maybe I blew something in the dash) so planning on looking in that direction next.
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