'82 911 VSS (Speed Sensor) Sine or Square Wave?

[jaustinmd]

I'm programming a Rostra cruise module for my '82 911SC and need to designate the wave form of the VSS. Does the speed sensor on the transmission generate a sine or a square wave?

Also, I "think" I saw somewhere there are 8 magnets actuating the sensor so I assume there are 8 pulses per revolution of the wheel? I wanted to verify that so I can calculate the PPM (pulses per mile).

[scarceller]

I have lots of details on the speed signal in the 84-89 cars, I think the 82 should be the same. PM me I'll send you what I have, I even created a circuit to clean the signal up to 0-5vdc very clean square wave. The signal from the sensor is a sine wave and not appropriate ttl signal.

Here's the basics:
First, how many times does the rear tire turn per mile?
Inches per mile / Wheel Circumference = Tire turns in 1 mile.
63360 / 78.54 = 807 turns per mile

You can double check the math above by marking the rear tire and the pavement then push the car forward 1 complete tire rotation and measure the distance traveled in 1 tire rotation. I recommend calculating both ways. The 2 results MUST match! You don’t need to be 100% perfect here, the software will later let you fine tune the calibration of the Digital Gauge on the computer.

On the Carrera the sensor gets it’s signal from a large disk that has 8 magnets and the disk turns at the same speed as the wheels. So, we can calculate the number of pulses per mile like this:
Tire revs per mile * 8 magnets = number of pulses per mile
807 revs * 8 = 6,456 pulses per mile

Originally Posted by jaustinmd

I'm programming a Rostra cruise module for my '82 911SC and need to designate the wave form of the VSS. Does the speed sensor on the transmission generate a sine or a square wave?

Also, I "think" I saw somewhere there are 8 magnets actuating the sensor so I assume there are 8 pulses per revolution of the wheel? I wanted to verify that so I can calculate the PPM (pulses per mile).

[jaustinmd]

Originally Posted by scarceller

I have lots of details on the speed signal in the 84-89 cars, I think the 82 should be the same. PM me I'll send you what I have, I even created a circuit to clean the signal up to 0-5vdc very clean square wave. The signal from the sensor is a sine wave and not appropriate ttl signal.

Here's the basics:
First, how many times does the rear tire turn per mile?
Inches per mile / Wheel Circumference = Tire turns in 1 mile.
63360 / 78.54 = 807 turns per mile

You can double check the math above by marking the rear tire and the pavement then push the car forward 1 complete tire rotation and measure the distance traveled in 1 tire rotation. I recommend calculating both ways. The 2 results MUST match! You don’t need to be 100% perfect here, the software will later let you fine tune the calibration of the Digital Gauge on the computer.

On the Carrera the sensor gets it’s signal from a large disk that has 8 magnets and the disk turns at the same speed as the wheels. So, we can calculate the number of pulses per mile like this:
Tire revs per mile * 8 magnets = number of pulses per mile
807 revs * 8 = 6,456 pulses per mile

Thanks for the detailed information! The Rostra cruise module can use a sine or square wave but you have to designate which one. I've got 255/40 VR17 tires and had looked up the revs per mile for a tire of that diameter and multiplied by 8 and came up with 6446 PPM - close to your number! The Rostra module has a "6400" setting and I'm sure that will work - I think it just needs to be in the correct range as all the settings were rounded off to hundreds or even thousands.

Sent you a PM.

[jaustinmd]

Just an update - I finished up the frunk wiring and temporarily plugged-in the Rostra cc module/servo so that I could set it on the hood and see the diagnostic LED. I have a 964 speedo and I "thought" the VSS was available on the red/white output wire. However, the module did not read a signal. I moved the input wire over to the "raw" signal coming to the speedo from the sensor on the transmission (brown/red wire) and set the Rostra to read a sine wave and it appears to be working perfectly!

I have LED brake lights and Rostra suggests to install a relay in the brake wiring so the module can read a ground. I wired the relay through the normally closed terminal and then ran the ground in series to the clutch switch relay and then to ground. I was able to shunt the two wires to the original servo (no longer needed for the new servo) and access the wires to the original cruise module in the frunk so as to be able to use my OEM clutch switch in the circuit - according to the diagnostic LED's, it's working OK!

I've mounted the module/servo in the engine compartment and now need to extend the wiring harness between the frunk and the engine compartment and also connect the throttle cable and I'll be ready for a test drive!

[jaustinmd]

Today, I finished the extended harness and went for a test drive and the cruise would not engage! I repeated all the diagnostics, and on a hunch, reset the dip switch for a square wave VSS rather than a sine wave. I took another test drive and it worked perfectly! Very smooth engagement without surge or delay and very tight speed control - about +/- 1 mph! It instantly disengages with clutch or brake application. I especially like the "tap up" and "tap down" buttons so I can increase or decrease the set speed in small, 1.5 mph increments.
If you need a cruise, it's not very expensive (a little over $200), but fairly labor-intensive and you sometimes get a headache from following all the wiring diagrams! Ha! I also had to devise a few circuits of my own so I could utilize the car's original clutch switch and relay and also had to add a brake relay to provide a reliable ground since I've got LED brake lights.

[ischmitz]

I was under the impression the round pickup in the transmission is a simple REED contact that get closed/triggered by the magnet disk? That would make the output a square wave with the appropriate pull-up resistor.

Ingo

[jaustinmd]

Originally Posted by ischmitz

I was under the impression the round pickup in the transmission is a simple REED contact that get closed/triggered by the magnet disk? That would make the output a square wave with the appropriate pull-up resistor.

Ingo

Sal sent me a PM and said the reed contact outputs a "dirty" square wave - ramp times like a sine wave but with the peaks clipped off. So, I tried the sine wave input first, but it didn't work, but the square wave input did. Guess it is "square enough" (how's that for a technical term? Haha!) for the Rostra computer as it works perfectly!