Is it nessassary that the ecm knows the exact speed the car is going for it to work properly. The reason I ask is if I slowed the vss signal down 50% so the speedo was only showing 30mph when the car is doing 60mph, would the ecm care as long as all other sensors and inputs stayed the same.

Steve

 

The VSS does have effect on some of the logic of the computer. The idle routine needs the VSS to determine when the vehicle has come to a near stop so the engine can go into closed loop idle. It also effects the operation of the shiftlight/TCC. Incorrect VSS values will mean that the light wont come on at the right time or the TCC lockup will occur at the incorrect speed. Those are the only two I can think of now but there may be other stuff as well, depending on what ecm is used. It is possible to correct for it if you want to modify the code.

 

dimented, I have a 91rs and as you know the speedo only goes to 110mph. I was thinking about removing every other tooth on the relucter gear in the transmission so the vss would only pick up half the signal at any given speed. Then have a new face plate made for the speedo to read up to 220mph. I just wanted to know how this might effect the ecm. I thought about modding the drac but it's not possible to slow the speedo down to half no matter what gears I wanted to use in the rear axle. It would be nice to be able to know how fast you are going above 110mph as well as the wow factor of having a 220mph speedo. The only thing I can think of that would be effected on a down note would be the odometer which would only register half the miles driven. Maybe that wouldn't be so bad after all as I already have 190,000mi's on the clock already.

Steve

 

That sounds like an idea that would work. I think there is the ability to change how the ecm reads in the vss but I can't remember if it's in the eprom or rom. I thought some of the code was in the eprom but I don't have the hack right in front of me and the hack doesn't tell you what does what. Only that the values stored are for the vss calculations (needs a test bench).

 

The actual value thats used in the div. routine to determine speed from the period from the VSS pulses is in the rom. Cant touch it unless your romless. If you wanted to be ghetto about it you could jsut change the parameters around in the prom to take the change in registered speed into account.

 

If you want to be even more 'ghetto' you could wire up a handful of parts from Radio Quack with a divider IC and a couple of BCD thumbwheels that woulf allow you to set the division value.

Run the VSS into it and take the output to your speedo.
USe the VSS by itself to the ECM so it knows what's really happening. There are some calcs. it does based on speed particulary for an A4... TCC control and stuff like that.

 

Steve, there is an even easier method. Find the 4-out buffer box, under the dash probably on the passenger side.

There are four outputs, 2 are 4000 PPM, 2 are 2000 PPM. Currently the 4000 PPM goes to the speedo. Move that wire to one of the 2000 PPM outputs. Done.

If you have cruise one of the 2000 PPM outputs are already in use.

RBob.

 

I like Rbob's solution much better than mine.

Rbob, "That's a lot better than the one I had".

 

RBob, did Doc tell you that? I know Grumpy has learned alot from Doc. I hear he's the real master electron guru. Thanks for the info, I'll try it out after I get back from riding the wooden rails at Kings Island. Thanks again.

Now to find someone to make a custom overlay for the speedo. Oh one more question RBob. This will only effect the speedometer not the ecm correct. I was kind of hoping it would do both but I guess I'll just have to raise the speed limiter in the chip to 220
.

Steve

 

No, not from Doc I actually figure some of this stuff out on my own (surprises me!).

There is an easy bin change to convert the speed limiter into a rev limiter. Rev limiter's are always a good idea.

As for the ECM it already gets the 2000PPM signal. So no dice unless the 4-out buffer is modified. I looked at the wiring last night and the 2000PPM signals are used for the cruise control and the ECM. SO if you have cruise you'll need to double up on one.

Foreseeing our next question. . . red (pin E) is cruise, brown (pin C) is speedo, and brown (pin F) is ECM.

RBob.

 

Thanks RBob, I knew you learned the smarts on your own. I was just kidden around. No really, thanks for the info.

Steve