Front ABS sensor for 911/930

[dannobee]

A recent thread was started asking about retrofitting 911/930's for ABS sensors. There are a few applications where some sort of wheel speed sensor is needed, especially with new, modern engine management systems. Cruise control, traction control, launch control, ignition timing changes in certain gears, and the elusive ABS retrofit (BMW MK60 seems to be the most common) all need wheel speed inputs. And of course, simply wanting to data log the vehicle speed, especially on earlier cars that were equipped with a mechanical speedometer.

There are currently two types of wheel speed sensors, active and passive. The passive type is the old magnetic reluctance type of two wire sensor that have been used for decades and decades, for various things like ignition timing, cam timing, etc. It produces an AC voltage sine wave and begins producing a usable signal after a few rotations of the wheel. The active type uses a hall effect switch that produces a DC digital square wave, and while "more accurate," really only shines at very low speeds but it can also detect directional changes (in case you *need* a zero speed input or traction control in reverse). Since I doubt that torque management will help much when spinning out backward on the track, nor do I anticipate needing traction control while driving on icy roads here in California, I opted for the passive style of sensors (although one could easily "upgrade" to active sensors, if needed).

I pulled apart the front hubs on my 930 and started measuring and scheming. I perused some online catalogs, looking for wheel speed sensors and tone rings that might fit our applications.

The front hub has an outside diameter of 2.910" inches near the seal seating area with a slight taper, increasing in diameter further out.



The three bolts that hold the backing plate on seem to be perfect candidates for adding a mounting bracket. The struts with 3.5" spacing are flat, while the struts with 3.0" spacing have a step. I wanted a solution that would fit either strut, if possible.






A quick reassembly and sanity check confirmed that there's enough room for a sensor and bracket, along with a tone ring.



Next I fabricated an aluminum disk that would center on the step used for the backing plate (0.125" thick 6061 aluminum, 2.120" inside diameter, 4.75" outside diameter, to clear the step on the 3.0" strut). One disk is needed and will be cut in half. Holes drilled in each "half" to match the spacing on the backing plate.



I found a 47 tooth tone ring for a Saturn SC1/SC2 with an advertised inside diameter of 2.91". It was actually 2.90", and should fit the hub with an interference fit. After looking at a mind numbing number of wheel speed sensors, I settled on one for a 2002 Crown Victoria. It appeared to have the right tip, and a long enough harness that could be easily spliced for Deutsch DTC or Autosport connectors.



I chucked up the hub in the lathe, and a quick pass yielded a 2.91" diameter and a slight step to keep the ring in place.



And it fits like buttah!



I then cut a couple pieces from some 5/8" thick 6061 plate, to hold the sensor to the disk. I drilled the 0.700" hole for the sensor and tapped for M6x1.0 mounting bolts. One hole secures the sensor on one side and the disk on the other. Air gap is set to 0.5mm (0.020").

[dannobee]

Disk cut in half, sensor mount installed, edge prettified.





And the final results.








Parts used, Dorman 917-541 tone ring, Dorman 970-273 ABS sensor.

[911obgyn]

Inquiring minds want to know what you are using this for.
Very nice work there, and skillful documentation.

[Bill Verburg]

sensors and controllers are only part of the battle.

For ABS to actually work beneficially, i.e. self-correcting, The wheels need to have zero to negative scrub radius. unfortunately, all pre 964 911 variants have very large positive scrub radius.


The large positive s/r imparts the opposite of self-correction.

[dannobee]

Some kind of wheel speed input is needed for many of the features in modern aftermarket ECUs to work properly, such as traction control and cruise control. The aftermarket ECU's also use the information to determine gear selection (for speed referenced ignition timing) and launch control. Pre '76 911's still had a mechanical speedo and lack the requisite VSS.

Adding actual ABS to a vehicle not originally equipped sounds like a lesson in futility, for a number of reasons. Scrub radius being just one more consideration.

[jonesb930]

Are those sensors Hall or Magnetic?

[dannobee]

Those are magnetic. If you need Hall Effect, you'd have to peruse some catalogs and find what you need. What I listed as "passive" is magnetic. "Active" would be Hall effect. The only real difference is the zero speed and reverse capabilities of the Hall effect sensors. Most modern ECU's will accept either type as an input.

[jonesb930]

Thanks for the reply, my Motec M600 would need a hall sensor, or I would need to use a MOTEC DMCD to convert the mag signal to a digital one I suppose. Not the end of the word. Since I can fit whatever I want in the rear, I would only need one DMCD as it will convert two independent sensors.

[Cloggie]

I wonder if this could be used to drive the stock speedometer.

The speedo has 8 magnets with a pickup sensor which just generates a voltage proportional to the speed of the car.

Does this do the same thing? Obvious question is how many volts does it generate per RPM....if similar to the stock, then we might have a winner, as I am sure that the stock speedo has a voltage adjustment to calibrate it.

Reason why I am interested is my G50 (not stock) did not come with the speedometer magnets, so I put 4 small magnets on the front hub with a suitable mount for the sensor.
I'd love to replace it with this more elegant solution if possible


D.

[jonesb930]

Dnnobee,

I found a catalog of sensors and located one like what you used but it's active. Can you tell me the "A" dimension of your sensor using the below reference dwg? The one I'm looking at is used on a VW.

[brighton911]

Nice job!

[wazzz]

Originally Posted by Cloggie

I wonder if this could be used to drive the stock speedometer.

The speedo has 8 magnets with a pickup sensor which just generates a voltage proportional to the speed of the car.

Does this do the same thing? Obvious question is how many volts does it generate per RPM....if similar to the stock, then we might have a winner, as I am sure that the stock speedo has a voltage adjustment to calibrate it.

Reason why I am interested is my G50 (not stock) did not come with the speedometer magnets, so I put 4 small magnets on the front hub with a suitable mount for the sensor.
I'd love to replace it with this more elegant solution if possible
D.

The pickup sensor is a passive device that produces no voltage by itself. It is merely a switch that gets closed 8 times per wheel turn.
Therefore the signal that results from it has a frequency (not voltage) that is proportional to the speed of the car.
The voltage itself is 0V when the pickup sensor switch is closed and 8V when it opens.
The 8-volt voltage that is used to produce the square signal is the result of a current source that is inside the speedometer.
All you need to drive the stock speedo is a switch that closes eight times per wheel turn.

[dannobee]