What is the purpose of this flag? I am running an aftermarket VSS now which is driven by the speedometer cable. Works fine. But I am getting ready to install a 4l60e tranny which has a 40 tooth reluctor type VSS. So now I'm gonna have a much different frequency for the ecm. Is that what that flag is for?? C'mon you experts....

check the flag for mech vss. Uncheck it for an electronic vss.

I belive your wrong on this one, for f-bodys from 1989 and down they used a magnetic type speed sensor that produced 2000 pulses per mile, then in 1990 they changed to a different type that produced 4000 pulses per mile. Guys like me want to check this flag so we can run are original speed sensor in the newer 1990 computers (730ecm) and not have to buy a new sensor.

I have a 87 formula. All firebirds from 87-up has a electronic vss. I had to uncheck it so I'm not in cell 4 all the time and to make the ecm see the road speed. Unchecked would mean electonic vss. A check would mean that your car has the mech speedo. That is what my original post stated, what do I have wrong.

Thanks,
Tim

The options are Optical sensor (early), and electronic (late).
The optical are generally 2002 PPM, and the electronic can vary from a low of 4004 to I don't know what, on the high side.

The flag sets which input is used, the electronic needing 2 inputs since it's an A/C signal, and the optic just one since it's a pulsed D/C signal.

Which ECM terminal do you connect the optical sensor to? On my wiring diagram for the '727, it shows the original electronic VSS connecting to C2 and C8. I'm using the optical sensor so do I just hook it's output to one of the terminals and leave the other floating? Should I tie it to ground?

I don't know, on the 727.
My best guess, would be getting a 730 and comparing the guts to resolve which pin you'd have to use.

The whole VSS signal issue is a deeper quicksand pit than most people realize.
The more posts that pop up on this subject, the more confusing it gets. I've been searching for the answers that make me comfortable for the last year. Yeah, my projects do tend to stretch out.
Here are some answers that I've found and am comfortable with. Don't take my answers as the gospel - GM loved to change things around (probably based on what parts were left in the inventory bins and could be used on the next line run).
In other words, Don't Try This At Home without supervision, kiddies!

Example #1 - Bruce's answer was right on - early GM's (grin - like my '87 GN) used a speedometer cable and an optical VSS sensor behind the speedometer to feed the ecm a 2000 ppm (pulses per mile) signal.

Example #2 - Late '80's and early '90's (and aftermarket adaptors) - Speedometer cable drive in the transmission (with plastic gears to allow for rear end gear changes) and a VSS built in at the transmission end. The speedometer cable drove the speedometer and the VSS wire(s) fed a Signal Buffer (or DRAC - digital ratio adapter/controller). These are a small plastic box located somewhere behind the instrument panel. This Signal Buffer converted the raw VSS signal to the proper signal for the ecm (usually 4000 ppm for the newer ecms and 2000 ppm for cruise control).

Example #3 - Electronic speedometers - many used the same plastic gear drives in the transmission, but no speedometer cable, just VSS wires. With many of these, the Signal Buffer (or DRAC) converted the raw VSS signal, but also added terminals for speedometer output (usually 4000 ppm). Optional - See Example #5 - 7730 ECM with built-in signal buffer.

With these, you could still adjust for rear gear changes by changing the plastic gears on the transmission drive.

Example #4 - True Magnetic VSS - no more plastic gears in the transmission housing. The transmission output shaft use a 40 tooth reluctor ring and had a magnetic pickup in place of the older speedometer cable housing.
This also can send the raw VSS signal to the Signal Buffer for conversion for the speedometer, ecm, cruise control, and anti-lock brake system. As you can see, there are no plastic transmission gears to change to match the rear gear ratio. You must change the Signal Buffer (or DRAC) for one made for your new rear gear ratio. OR, you can find info on the internet (I did) on how to calculate and modify DRACS by changing resistor jumper settings.

Example #5 - AAH, NOW on to the 7730 ECM.
The 7730 ECM has a VSS Signal Buffer built into it. It can handle the VSS signal in two ways:

(A) It will accept a 4000 ppm signal from an external Signal Buffer (DRAC) on pin C6.

(B) Or, it will accept the raw VSS signal on pins B9 (purple) and B10 (yellow) and output on pin B11 (grey) for electronic speedometer and output on pin C1 (red) for cruise control.

Using option B, correction factors for rear gear changes are made in the ECM itself - one area for speedometer output correction and one factor for the ECM usage itself.

Now, I've offered all that I've chased down for the past year.
But I still need help on the ECM side. The speedometer correction factors are B1 thru B7 (including one titled Mag VSS).
The ecm correction factor is a 3 positon number.
HELP - can anyone give a detailed description of how these work?

David

Pin B21 on the '727 is the optical input pin. You shouldn't have any floating pins. The optical units (as far as I know) have three wires: B+, ground, output. Lime green box with an optical sensor on a short cable?

RBob.

Yep. I'm using the optical sensor with the lime green box. Has the 3 wires like you described.

The '727 diagram I have (from the diy-efi site) just shows B21 as "not used". So, if I understand right I should connect the output of the optical sensor box to B21 and leave C2 and C8 disconnected?

Yep'er, and the bit flip mentioned previously.

RBob.

Well, in the VSS calculation part of the code, which begins at BF9A in the ANHT it is apparent even to me that speed is calculated one way when it thinks a mag sensor is installed and another when it thinks it is an optical sensor. The mag sensor part of the code uses the pulse per mile speed constant at $8015 but I cant really figure what it is doing there and likewise I havent been able to understand what it does on the optical sensor calculation right above.

Would one of you code guru's mind going thru that little subroutine and explain to me? It's been so long since I did any assembly stuff I cant remember many of the basics.

Steve