VDC and speedo experiment

the head · 10-22-2004, 06:43 AM

In my quest to gather information about the speedo operation and effects of tires on it I did a simple experiment last night involving jacking up the rear and letting it "drive" while in the air.

Two things of note happened:
1) The speedo does indeed function off the rear wheels. It might be reading just one wheel but I don't know.
2) The VDC detected slipping (the light came on) once I got past 8 mph on the speedo. It basically wanted to idle at 10 mph. I took the VDC off and had no other effects.

So I may try to see what kind of signal the wheels are putting out since that is actually a hard wire that can be tapped into. The speedo and ECM rely on the CAN wiring to get speed inputs from the brake controller so I can't tap them. I'm hoping this is as simple as buying two of the little boxes that correct speedo readings and putting them inline with the rear wheels.

For those of you wondering why I'm doing this, it's because I want to run a shorter tire at the track for better overall gearing. As it stands now I can only run a 29" tall tire at the least since anything shorter risks the speedo error causing the speed limiter to kick on.

KL99SEA · 10-22-2004, 07:27 AM

I would think the speedo signal comes from the transmission, it has on the other 2 Nissans I have owned. I think it reads the shaft speed before output. Doesn't VDC use a gyro (accelerometer, like a gtech) and compare speeds to introduce VDC control? Thats the way I picture it anyway...........and i could be WAY OFF.

JerseyDirt · 10-22-2004, 07:50 AM

Did you hear the brakes on either wheel kicking in as the ABLS tried to eqaulize rear wheel speeds?
And I agree with KL99SEA, I think the speedo is in the tranny.

the head · 10-22-2004, 08:24 AM

The Nissan service manual says that it uses the wheel speed sensors that are transmitted to the brake controller and then to everything else. It is not done via the transmission, according to the manual.

frogui · 10-26-2004, 03:28 PM

I don't know if it pertains to your quest but I know VDC does not kick in when both tires are slipping at nearly the same rate. My drive way is tiled and I've florred it there to check when wet. No VDC light, full engine power and no forward movement. Which sucks since I plan to buy a boat and i don't know if this will embarass me at the ramp. Anyone have any experience wioth 4x2 titans at the launch ramp?

Anyway, I know some sportbikes have different rev limits at diff rpms and maybe ours work the same. In bikes you just add a resistor to the gear position wires and speed limit does not kick in. You just let the ECU think it's in fifth instead of sixth(Suzuki Hayabusa). But I don't think this works on an automatic, like we have. might be worth a try if you can find the right wire.

I haven't been able to get to the track becasue of work but I will ASAP. So I have one more question do all five gears engage at the track?

the head · 10-26-2004, 04:07 PM

I'm going through at the top of 3rd gear.

The transmission is also hooked up to that dad-gum CAN line so there is no wire to tap in for that particular reading, I think. It is what connects to the BRC that then goes to the speedo and ECU.

Interesting on the dual wheel thing. Every bit of info helps!