The oil sending unit, also known as the oil pressure sender, is an important component in a vehicle’s engine. It monitors oil pressure and sends signals to the oil pressure gauge on the dashboard. A faulty oil sending unit can cause erratic oil pressure gauge readings or prevent the gauge from working at all. Recognizing the symptoms of a bad oil sending unit can help identify the problem and prompt repair before major engine damage occurs.
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What is the oil sending unit?
The oil sending unit, typically located near the oil filter, is screwed into the engine block where oil galleries are drilled. A pressure sensor inside the unit monitors oil pressure through a diaphragm or sensor probe. As oil pressure fluctuates, the diaphragm or probe moves, altering resistance. This resistance value is transmitted to the oil pressure gauge, which converts it to a pressure reading displayed on the dash.
On newer vehicles, the oil pressure sender may be electronic rather than mechanical. An electronic sender generates a varying voltage signal instead of changing resistance. This voltage signal is sent to the car computer, known as the ECM or ECU. The computer then converts the voltage to a digital signal before sending it to be displayed on the oil pressure gauge.
Low or Fluctuating Oil Pressure Readings
One of the most common symptoms of a faulty oil sending unit is unusual oil pressure readings. Here are some things that may point to a problem with the oil sender:
- Oil pressure reads very low when the engine is running, even when it is warmed up.
- Oil pressure surges up and down erratically at idle.
- Gauge reading doesn’t change much between idle and higher engine speeds.
- Gauge seems accurate at times but then shows incorrect readings.
A bad oil sender can cause oil pressure readings that are too high, too low, or simply erroneous. Strange readings that don’t seem to align with your vehicle’s normal oil pressure range often indicate the sender is malfunctioning.
Causes of Inaccurate Readings
There are a few reasons an oil sending unit may give false pressure readings:
- Damaged diaphragm or sensor probe – The sensing mechanism inside the unit can wear out or break over time. This prevents accurate monitoring of oil pressure.
- Sticking sender contacts – Mechanical senders can get stuck open or closed so the resistance doesn’t change properly with pressure.
- Faulty wiring – Frayed or broken wiring can cause sporadic connectivity problems.
- Ground issues – Senders need a good ground to reference oil pressure changes. Poor grounding prevents accurate signal transmission.
- Sensor contamination – Dirt, sludge or oil varnish buildup on the sender contacts interferes with pressure monitoring.
No Oil Pressure Reading
In some cases, the oil pressure gauge may stop working entirely. Some signs of this issue include:
- Oil pressure gauge is pegged at the lowest reading.
- Gauge stays exactly in the middle regardless of engine speed.
- Gauge needle doesn’t move at all when engine is started.
A reading of zero oil pressure is a serious issue that requires immediate attention. Continued driving can quickly lead to catastrophic engine damage when oil isn’t circulating properly.
Causes of No Oil Pressure Reading
Here are some typical reasons for the oil pressure gauge completely failing:
- Broken sender wire – The sender wire detaches from the unit or gauge, opening the circuit.
- Faulty ground wire – An improper ground connection prevents the sender signal from completing the circuit.
- Blown fuse – The fuse providing power to the gauge or sender burns out.
- Failed sender – The sender contacts stop working or its sensing element breaks entirely.
- Bad instrument cluster – The gauge itself is damaged internally.
It’s important to note that a blank oil pressure gauge doesn’t always mean the sending unit is bad. Low engine oil, oil pump issues or broken oil galley plugs can also cause zero oil pressure. The engine should be inspected immediately if the gauge reads nothing to identify the underlying cause.
Oil Light Comes On
In addition to oil pressure readings, drivers should watch for illumination of the oil pressure warning light. This light comes on when oil pressure drops below a safe level. Reasons the light may activate include:
- Low engine oil level
- Damaged or worn oil pump
- Faulty oil pressure relief valve
- Broken oil galley plugs
- Blocked oil pickup screen
- Bad oil pressure sending unit
Don’t ignore the oil light – it should prompt an immediate shutdown of the engine to prevent damage. Determine the cause and resolve it before continuing to drive the vehicle.
High Oil Consumption
Excessive oil burning can sometimes result from a leaking oil sending unit. Here’s why:
- Oil drips from the compromised seal around a damaged sender.
- Improper re-installation can distort the oil sending unit, causing leaks.
- Removing the sender loosens sludge that clogs oil passages, increasing oil burning.
While not definitive, oil loss from a degraded sending unit may contribute to high consumption. The source of any oil leaks should be determined when addressing high oil usage.
Dashboard Indicator Lights
Along with oil pressure readings, warning lights related to engine performance can indicate issues with the sending unit or oil circulation. Potential indicator lights include:
- Check Engine light
- Low oil level or change oil notification
- Engine or temperature warning light
- Charging system/battery light
- ABS/stability control lights
These various warning lights can come on if engine computer codes detect abnormal oil pressure readings from a faulty sender. Diagnostic trouble codes (DTCs) stored for the sender circuit can help identify the specific problem.
No Symptoms
It’s possible for an oil sending unit to fail without any noticeable symptoms. Oil pressure readings may remain in the normal range, no warning lights turn on, and no obvious oil leaks develop. There are a few reasons a “silent failure” can occur:
- Intermittent electronic sender failure may not produce symptoms all the time.
- Short circuits in the wiring only cause problems under certain conditions.
- Compensating oil pressure relief valves maintain safe oil flow despite issues.
- Sudden complete failure of the sending unit doesn’t produce warning signs before quiting.
To detect problems before the sender completely fails, periodic inspection and troubleshooting are recommended. Otherwise, the first sign of a bad oil sender may be a dead engine from lack of oil flow.
When to Replace an Oil Sending Unit
Here are some general guidelines for when to replace an oil sending unit:
- Inaccurate, erratic or zero oil pressure readings
- Oil pressure warning light comes on
- Sender is obviously damaged or leaking
- High mileage, typically over 100,000 miles
- External oil leaks from the sender mounting area
- Engine performance indicator lights related to oil pressure issues
- Routine maintenance alongside timing belt, water pump, etc.
Replacing the oil sending unit as preventive maintenance provides peace of mind about oil pressure accuracy and gauge function. Consult your vehicle repair manual for the specific recommended replacement interval.
Conclusion
Catching oil sending unit problems quickly can help avoid unnecessary engine wear and damage. Be alert for common symptoms like fluctuating gauge readings, illumination of warning lights, oil leaks, and high consumption. Schedule periodic inspection and consider preventive replacement of older sending units. With proper attention to this critical engine component, major oil circulation and engine failures can be avoided.