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The ALFA Basic Sending Unit

Only $49.00

(Also see the EZ-Pulse sending unit)

The function of a sending unit (also known as an impulse unit, sensor, or probe) is to transmit wheel rotation to the ALFA computer so that it can accurately measure the distance traveled. The Hall Effect sending units supplied with your ALFA-tsd are mounted at the wheel with magnets in the rims. Alternatively, the magnets may be attached to the back of the wheel hub, or inside a brake drum. The advantages of the Hall Effect sending unit are higher speed capability and insensitivity to vibration. These units are usually mounted on a non-driven wheel for greater accuracy

Typical sending unit mounting
Mounting the sending unit is different for every car, and requires some imagination. The ALFA sending unit works best when mounted on a non-driven wheel (rear wheel for front drive cars, front wheel for rear drive cars). If your car is equipped with disk brakes, the back side of the dust shield makes an excellent mount. If your car is equipped with drum brakes, you may need to build a small bracket to hold the sending unit close enough to the wheel to sense the magnets. In either case the sending unit should be mounted at the highest possible location to minimize it's chances of being hit by a rock kicked up from the wheel (see illustration). The sending unit may also be attached to a strut or any other part of the car that maintains it's distance from the rotating wheel. For maximum protection of the sending unit, the magnets may be mounted inside the drum, or on the back of the wheel mount studs. The sending unit would be mounted directly to the brake assembly, where is is shielded from rocks and other road debris. A possible alternative to wheel mounting would be mounting the magnets to the cars drive shaft and the sending unit to either the transmission or axle casing. (Do not mount the sending unit or magnets where the temperature will exceed 250 degrees for any extended length of time.)

The magnets (usually two) are mounted to the wheel or hub with the marked side facing toward the sending unit. You may use magnets other than the ones provided with your ALFA, but they must be oriented with the south pole of the magnet facing the sending unit. Typically leave 1/8" to 1/4" of gap between the sending unit and the magnets. Powerful "rare earth" magnets are supplied with your sending unit and in some cases allow gaps up to 1/2 inch. It is recommended that you mount the magnets using a Silicone adhesive which remains flexible when cured. Epoxy, super-glues, and other bonding agents which become hard or brittle will not stand up well to rally use.


Be careful to protect the wires attached to the sending unit from the weather by wrapping the splices fully with vinyl tape. All splices in the wiring should be soldered. The best way to route the wires from the wheel is to attach them to the brake hydraulic line with nylon cable ties. The brake line is usually routed such that it will not be stressed as the suspension does it's job, and also is least prone to breakage from road hazards. The wire should enter the inside of the car at the earliest possible point, and care should be taken to be sure that it is not crimped at the point of entry. The length of all wires associated with the sending units should be kept as short as possible and should not be routed near ANY ignition components.

Various color coded wiring schemes have been used for the ALFA sending units. Your sending unit will have a tag attached to it indicating the function of each wire. ALFA rally computers and odometers supply an operating voltage of 5 volts DC to the sending unit. Be very careful not to attach the power supply wire to the cars 12 volt electrical system or permanent damage may result to the ALFA computer. Some ALFA odometers use a 12 volt supply to power the sending units. ALFA sending units can withstand up to 20 volts and operate properly. See the instructions packed with your computer for details.

Most ALFA sending units use these standard color codes:

RED: +5 volt power input
BLACK: Ground
GREEN: Odometer impulse

(Older Alfa sending units used different color schemes. The most popular was Orange (+5), Green (Ground), and Yellow (Impulse). If there is ever any question of the correct wiring of a sending unit or computer, it is always best to call for exact information rather than guessing.)
Mounting inside hub

Photographs of some typical installations:
Photo of wheel installation
Shows typical installation with magnets mounted on the wheels, and the sensor attached to the strut tube.

Photo of hub installation Photo of hub installation
Shows typical installation with the magnets mounted to the hub, and the sensor attached to the disk brake shield. This is the preferred method because changing the wheel will not disable the sensor.

Shows typical installation on a driveshaft or axel. Make sure that the magnets are mounted to a point which will not move relative to the sensor.

ALFA Rally products are built in the USA by the rallyists at:

Small Systems Specialists
201 N. Lobb Ave.
Pen Argyl, PA 18072
Phone 609-301-0541, Fax No Longer Available!
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