Checksum correction using the math!!!

[El Berto]

Hello all, I am quite ignorant on the subject, maybe you can enlighten me....

I was looking at a file .the gold of a FIAT Coup? (1996) with HxD (hex editor).
By selecting the various blocks of code, I found that the calculated checksum is equal to the one contained in the byte immediately after the block.
From what I've read the ecu, ignition, controls in any way the checksum.
Now, if I go to edit the map, of course, change the checksum; if you wish to leave the checksum intact, I suppose you could change a few bytes.
How do I know which is which ? the part of the map that I can change without changing the behavior of the car?
Thanks in advance.

[sergiot]

first of all you need to know which unit it is.
however, you have found the point and the method.
the solutions are 2
1 change, we recalculate the checksum and write in the address that you have found
2 let the checksum inaterato and write something that makes up for it (to go to the accounts) in an area of 00 or FF just before the checksum

[El Berto]

first of all you need to know which unit it is.

Bosch Motronic (I can't remember the initials) of a FIAT Coup? 20v turbo

2 let the checksum inaterato and write something that makes up for it (to go to the accounts) in an area of 00 or FF just before the checksum

Then I confirm that the block of code all oxFF does nothing with regard to the map?

O. T.
It's an old ball that I try to do for years: a double map.
In practice, I install 2 memory chips in place of the EPROM original, with a switch select the EPROM or the other.
The maps both work, only that they have obviously, the checksum is different, in fact if I move from one map to the other with the switch, the machine does not turn on, do I have to staccara and re-attach the control unit (or cos? I suppose I am).

[sergiot]

a block of ff in the queue at the maps before the checksum, you can compensate for.
but considering that you can calculate and know where scrivrlo not waste time offset calculations, the new and the write

if you can even do the double eprom you can even have 2 checksum is different, each person counts for his map

who is doing the double map must not disconnect the ecu, just the switch should do it with the engine stopped. ( though not essential)

[El Berto]

But the ecu, the ignition does not check the checksum?
In the sense: the attack on the ecu (I select map 1), I put in motion, everything is ok.
I turn off the engine, I select map 2, I put in motion.... should not be.
Ex-control unit (is select it in the map 2), piecing unit, I put in motion, the engine part.
So I assumed you must have the 2 checksum equal....