Murphy MP3298 Magnetic Pickup for Engine Speed Sensing

Magnetic pickup that turns flywheel gear teeth into a speed signal

The Murphy MP3298 is a magnetic pickup from the Murphy range of Enovation Controls, covered in one bulletin with the MP7905 and MP7906. A magnetic pickup is an AC generator. It is normally installed in the flywheel housing of an internal combustion engine, where the starter ring gear acts on it and produces a voltage pulse each time a gear tooth passes the end of the sensor.

The pulse train gives engine speed to a governor, tachometer, overspeed switch or controller. The internal portion of the pickup is filled with epoxy resin, which makes it oil- and moisture-resistant, and the two models differ in housing material, length and thread, as the table shows.

Technical specifications

Values are from bulletin 1511775 dated 2016/02/25.

Setting the gap and understanding the output

Drill and tap a hole in the flywheel housing to the thread size for the model. The bulletin warns that drilling too deep may damage ring gear teeth and advises blowing the chips out with an air hose. Insert the pickup and turn it until it stops at the face of the gear, then back it off by one quarter, one half or three quarters of a turn; the gap chart translates the turn into a gap distance. Check clearance by rotating the gear completely around.

Adjust the gap so the minimum voltage required is reached at the engine lowest speed, because voltage rises with speed. If readings are erratic, remove the pickup and check the magnetic tip for metal chips. Output also depends on gear pitch, and the bulletin includes a gear pitch versus output chart that shows coarser gears giving a higher percentage of the standard output and fine pitches not recommended.

Selecting the right pickup

Pick the model by thread and reach: 5/8-18 UNF for the stainless MP3298, 3/4-16 UNF for the aluminum MP7906 and a longer 3/4-16 UNF body for the MP7905. Check the flywheel housing wall, the distance to the ring gear and the gear pitch before ordering, and confirm the input of the instrument that will read the signal.

The pickup is a passive generator and needs no supply. It is a speed sensor rather than a switching proximity sensor and sits here because it is the nearest sensor category on the site. Enovation Controls lists this product family on its Legacy Products page, which describes those items as less popular or outdated but still available for purchase. Confirm current availability and any replacement part before ordering.

Key features of the MP3298

• Type 300 stainless steel housing and locknut, the only stainless model of the three in the bulletin.
• 5/8-18 UNF thread over a 3 in (76 mm) length, part number 20700162.
• Typical output of 200 V peak-to-peak under the bulletin test conditions, with a typical coil resistance of 975 Ohm.
• Epoxy-filled interior for resistance to oil and moisture.
• Two 20 AWG insulated leads, white and black, with insulation to MIL-W-16878D Type E.
• Wide temperature range of -54 to 107 °C, suitable for mounting on a running engine housing.

Comparing the three models

When replacing an existing pickup, the thread is the first check, because a 5/8 in and a 3/4 in hole are not interchangeable.

Wiring and signal good practice

The output is a low-energy AC signal whose amplitude and frequency both rise with engine speed. As a general rule for magnetic pickups, use twisted, shielded cable between the pickup and the receiving instrument, earth the shield at one end only, and keep the run away from ignition leads, alternator cables and starter cables, which can induce noise. Frequency equals the number of ring gear teeth multiplied by the engine speed in revolutions per second, so the instrument must be configured with the correct tooth count to display speed correctly.

Troubleshooting a weak or missing signal

• Measure the coil resistance across the two leads and compare it with the typical 975 Ohm value; an open circuit indicates a failed coil.
• Check the gap; too large a gap gives low voltage at cranking speed, so the controller may not see the engine start.
• Remove the pickup and clean metal chips from the magnetic tip, as the bulletin advises for erratic readings.
• Inspect the leads for chafing where they pass near hot or moving parts.

Applications in Oman and the GCC

Magnetic pickups of this type feed speed to governors, controllers and overspeed protection on standby and prime power generator sets, fire pump engines, compressors and pumping skids, which are widespread across oil and gas sites, utilities and buildings in the region. The stainless housing is a sensible choice where engines run in humid coastal air or are washed down regularly.

What to include in an RFQ

State the model MP3298 or part number 20700162, the quantity, and if possible the engine make and the instrument that reads the signal. If you are unsure of the model, send the thread size, length and housing material of the existing pickup.

Documents and quotation

The attached bulletin 1511775 is the source for the values above. Seven Star LLC supplies Murphy products as an independent supplier in Muscat. Send the thread, length, housing material, gear pitch and quantity with your enquiry. Other sensors are listed under inductive proximity sensors.