How long an engine speed sensor lasts on heavy equipment, and the signs it is failing

An engine speed or position sensor on a machine has no service interval. Nobody changes one at a set number of hours, and none of the manufacturer material we checked from Hella, Woodward, MOTORTECH or Caterpillar gives one. The sensor itself has no moving parts, so it lasts until something outside it gets to it: heat, a lead that rubs through, a plug that lets water in, steel debris on a magnetic tip, a tip that touches the teeth, or damaged teeth on the wheel it reads. The signs it is failing are a no start with the engine cranking and no speed reading, stalling, a rough or misfiring engine, and speed or timing fault codes. Test it before you buy one, because the same symptoms often come from the harness.

Why there is no hours figure

Woodward describes its magnetic pickups as having no moving parts and being self powered. A Hall effect sensor has no moving parts either. There is nothing inside that wears with each revolution the way a bearing or a filter does, so the part is changed on condition, when a test shows it has failed.

That makes the question "how long should it last?" really a question about what is around it. A sensor in a cool, clean bore with a well clipped lead has nothing attacking it. The same part next to a turbo, with its lead resting on the exhaust, has heat working on that lead every hour the machine runs. Every cause below is something you can see, measure or fix.

The two types fail in different ways

Our store lists 87 heavy equipment sensors. Sixteen are speed sensors and eight are position sensors, five of them throttle position sensors. The speed and crankshaft or camshaft sensors come in two kinds:

• Inductive (magnetic) pickups. A coil around a magnet that makes its own AC voltage as the teeth pass. Usually two wires. The Caterpillar 196-7973 magnetic speed pickup for 315C and 320C excavators and the Cummins 4327233 dual output pickup for QSM11, M11, N14 and K19 engines are this kind. The coil can short or break, and the magnet tip attracts steel.
• Hall effect sensors. Powered by the controller, giving a square wave signal. The Caterpillar 216-3446 position sensor for 3054E, 3056E, C4.4 and C6.6 engines is a Hall effect sensor with a bracket and a two pin plug. These depend on the supply voltage reaching them, so a supply fault in the harness looks exactly like a dead sensor.

Knowing which one you have matters most when you test it, as the test section below explains.

What shortens a speed sensor's life

Hella's technical information on crankshaft sensors lists five causes of failure: an internal short circuit, a short or open circuit in the wire, mechanical damage to the ring gear, soiling through metal abrasion, and a short circuit to ground. Add heat and a wrong gap and you have most of what goes wrong on a machine.

• Heat. Woodward rates its magnetic pickups for an operating temperature of minus 54 to 107 °C. Flywheel housings and timing covers sit close to the exhaust manifold and turbo on many engines, and the lead often passes closer still. A lead that touches hot metal hardens and cracks.
• Lead damage. A lead out of its clips rubs on a bracket, a belt or the alternator until a wire breaks or shorts to the frame. The break is often hidden inside the insulation right where the lead leaves the sensor body, where it flexes with every vibration.
• The plug. Water, dirt and corrosion in the connector, or a latch that was never fully home, cut the signal on and off. Hella's first electrical check is that leads and plugs are fitted correctly and making contact. On a machine that is washed down regularly, keep the lance off the sensor plugs.
• Metal debris on the tip. A magnetic tip collects steel particles from the flywheel housing or gear case. Hella calls this soiling through metal abrasion. A thick coat of fuzz weakens the signal and points to wear somewhere else in that housing.
• A gap that is too wide or too tight. Too wide and the signal is weak. Professional Motor Mechanic lists an air gap that is too large among the causes of inductive sensor failure. Too tight and the tip strikes the teeth. MOTORTECH gives 0.5 to 1 mm for its pickup and recommends checking regularly that it is still tight, because a lock nut that works loose lets the gap change.
• A damaged wheel. Chipped ring gear teeth or a damaged timing wheel give a bad signal from a good sensor. Fitting another sensor will not cure it.
• A bent bracket. On bracket mounted sensors, Caterpillar's procedure says the flange must sit flush and the bracket cannot be replaced separately. A bracket bent during a repair holds the tip in the wrong place for the rest of its life.

Signs the sensor is failing

Hella lists four effects of a failed crankshaft sensor: the engine will not start, the engine misfires, the engine stalls, and a fault code is stored. On heavy equipment that usually looks like this:

• Cranks but will not start, with no engine speed showing on the monitor or diagnostic tool while the starter turns.
• Stalls or cuts out while running.
• Rough running or misfire when the controller loses the timing reference.
• Erratic tachometer or speed reading, a needle that jumps or drops to zero for a moment.
• Speed or timing fault codes that come back after being cleared.

A fault that comes and goes with vibration, rain or washing points at the lead or the plug before the sensor.

How to test it before you replace it

Read the fault codes first, then check the plug and lead, then look at the tip for debris and damage. Those three checks find a lot of faults without any meter. After that, Hella's procedure depends on the type, and it starts with a warning: if you do not know whether the sensor is inductive or Hall effect, do not put an ohmmeter on it, because the meter can destroy a Hall sensor. You cannot always tell by looking. Hella says a two pin plug usually means an inductive pickup and a three pin plug could be either, so check the parts catalogue.

The 216-3446 is the exception that proves the warning. It has a two pin plug and our listing describes it as a Hall effect sensor, so do not test it as an inductive pickup on the strength of the plug alone.

• Inductive pickup, resistance. Unplug it and measure across the two pins. Hella gives a general range of 200 to 1000 ohms, to be checked against the value for that sensor. Woodward's own pickups range from 40 to 85 ohms on one model to 360 to 540 ohms on another, so use the figure for your part, not a general one. Zero ohms means a short. A reading in the megohms means the coil or lead is open.
• Inductive pickup, short to ground. Measure from each pin to the engine or frame. Hella gives more than 30 megohms as good, which is effectively no connection on an ordinary meter. Any real reading means the insulation has failed.
• Inductive pickup, output. With the meter on AC volts across the pins, crank the engine. A reading that rises with speed shows the coil and magnet are working. An oscilloscope should show a strong, even sine wave.
• Hall effect sensor. Hella says you can only check the supply voltage and the signal, which is a square wave. With the plug connected and the key on, check that the supply reaches the sensor. If the supply is missing, the fault is in the harness or controller, not the sensor.

If the sensor passes, look at the harness back to the controller, the plug pins on both halves, and the teeth on the wheel.

How to make the new sensor last

• Fix the cause first. If the old lead was cooked, chafed or out of its clips, sort that out before the new one goes in. Our guide to fitting a heavy equipment speed or position sensor covers seating, gap setting and lead routing step by step.
• Find the source of debris. If the old magnetic tip was coated in steel, something in that housing is wearing. Wiping the new sensor at every service does not cure that.
• Check it at each service. Look at the lock nut or bracket bolt, the plug latch and the lead clips. MOTORTECH recommends checking regularly that a pickup is still tight.
• Protect the plug. Clean, dry pins and a latched connector stop most intermittent faults before they start.
• Keep the old part number. Some sensors are sold under more than one number. Our SPM-HEP-4062 replaces both Caterpillar 196-7973 and 125-2966, for example, and the number on the old sensor is the surest way to match it in our heavy equipment sensors.

Crankshaft sensors on larger excavator engines follow the same rules. The Isuzu 8-97606943-0 crankshaft position sensor for 6WG1 and 6UZ1 engines has a flange, an O-ring and a three pin plug, so check its type in the parts catalogue before you put a meter on it. For a sensor number you cannot find in our heavy equipment engine parts, send us the number from the old part through our quote request page.

Frequently asked questions

How many hours does an engine speed sensor last?
There is no hours figure. It has no moving parts and is replaced on condition. Heat, lead damage, plug corrosion, debris and a wrong gap decide how long it lasts.

Can I test a speed sensor with a multimeter?
An inductive pickup, yes: resistance across the pins against the figure for that part, each pin to ground, and AC volts while cranking. A Hall effect sensor, only the supply voltage and the signal. Do not use an ohmmeter on a sensor whose type you do not know.

Should I replace the speed sensor when I change the flywheel or ring gear?
Only if it fails a test or the tip is damaged. Clean the tip, check the gap or the seating, and refit it with an undamaged O-ring.

The engine cranks but will not start and shows no speed. Is it the sensor?
It is one of the first things to check, along with its plug and lead. Test the sensor before you replace it, because a broken lead or a missing supply voltage gives the same symptom.

Why does my new sensor show the same fault?
The fault is probably in the harness, the plug, the controller supply or the wheel teeth. Check those before ordering another sensor.