7730 Vss signal processing

[terryk]

0x8018 Bit 6 (xXxx xxxx) is the mag/optical flag.
0x8014 is the IP divisor to set the speedo output.
0x8015 is the Road Speed Constant.

If it works anything like the Buick ECM's. the IP divisor sets the output to
the speed but doesn't affect the ECM's view of the MPH. The RSC sets the
ECM's view of the MPH, but not the output to the speedo.

It appears that the IPD is set for tire sizes meaning you can't fine tune
the speedo output for none standard sizes. You only get divide by 1, 6, 7,
8, 9, 10, 11.

Try
1  = 0x00,
6  = 0x02,
7  = 0x04,
8  = 0x06,
9  = 0x08,
10 = 0x0A,
11 = 0x0C

Or

1 = 0x00,
6 = 0x80,
7, = 0x40,
8 = 0xC0,
9 = 0x20,
10 = 0xA0
11 = 0x60:

I don't know for sure what divisors they use in the 7730.

Once the speedo is close to correct, adjust the RSC to make the ECM match
the speedo.

If any has seen it work differently, chime in. Again, I've only played with
this on the Buick ECM's.

Terry

----- Original Message -----
From: "K.E.T." <karljiml at compuserve.com>
To: <gmecm at diy-efi.org>
Sent: Monday, May 07, 2001 8:18 PM
Subject: 7730 Vss signal processing


> I think I've learned that the 7730 ECM, depending on the car in which it
is
> installed, handles the Vss signal two ways:
>
> A) Raw Vss signal from transmission sensor input on pins B9 & B10, and
> outputs signals for 4000p/mi on pin B11and a 2000 p/mi  on pin C1
>    OR
> B) Accepts a 4000 p/mi signal from an external signal buffer (yellow box)
> on pin C6
>
>  From experience, if the MEMCAL is expecting option B, but the car is
wired
> for option A, a digital speedometer reads much higher than actual speed.
>
> Question: Has anyone found the locations in the MEMCAL that control how
the
> ECM handles the Vss signal. I want to change transmission Vss sensors, but
> it looks like I will need to hack the MEMCAL to make it all work. I need
to
> decide if I should even attempt it, or leave it alone.  Changing the
MEMCAL
> to one from a different vehicle is not an option; too many other vehicle
> specific parameters would be affected. I have the capability to read, edit
> and burn a MEMCAL, but have no experience disassembling the code. I don't
> expect anyone to "do the mod for me," but was sure hoping not to have to
> reinvent the wheel.
>
> Thanks for any help!
> Karl
>
> --------------------------------------------------------------------------
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