Can A Bad Alternator Cause Limp Mode? Help & Advice

Today’s cars are so reliant on sensors that any voltage drop can cause an error message to be sent to the ECU.

The error dictates what action the ECU takes depending on what the sensor has sent.

Briefly, before we get into details.

A bad alternator can indirectly cause your car to go into limp mode. If the alternator is not producing the correct voltage, sensors can give wrong signals to the ECU, instigating limp mode. 

It’s quite unusual for the alternator to fail without giving you an early warning. 

Normally there will be classic signs such as dim lights, windshield wipers slowing, etc. Have you had these happen?

Limp mode is a state that the ECU puts the car in when it identifies a problem with a critical part. Limp mode limits the car to 2,400 RPM to save further damage being caused but allows the driver time to get the car to a safer place to stop and get the car checked.

What Sensors Can Cause Limp Mode?

Transmission Sensors and Limp Mode

Low voltage plays havoc with transmission sensors. They need consistent voltage to enable them to give accurate readings to the ECU. When the voltage is low or erratic, the ECU interprets this as a transmission problem. 

Sometimes, gear changes will be sluggish or not happen at all. This results in a car being stuck in low gear and revving excessively. Usually, limp mode kicks in at this point to prevent high RPMs and any potential damage that could cause.

Mass Airflow Sensor and Limp Mode

Commonly called the MAF sensor, this sensor sits between the air filter and the intake manifold. This sensor has two wires, and one is live and heated by the power sent to it from the alternator. 

This sensor’s job is to measure the airflow and adjust it accordingly. This ensures that the correct amount of air is sent into the engine to combust with the fuel.

If the current being sent to the sensor is low, it is unable to give a correct reading to the ECU. The ECU may then allow too much or too little air into the engine and cause a rough idle.

The ECU will detect an issue inside the engine and put the car in limp mode to prevent any damage it thinks may occur.

Manifold Absolute Pressure (MAP) Sensor and Limp Mode

Usually, a car will only have a MAP if it doesn’t have a MAF, as they serve the same purpose. 

The MAF, however often sits behind the throttle body. This, too, uses a current and the changes within it to send signals back to the ECU. Any change in the voltage can hinder the flex within the chip, which divides the sensor vacuum and the manifold intake.

These different pressures can cause the chip to seal the vacuum when it should be open, resulting in the wrong amount of oxygen entering the engine. 

Crankshaft Sensor and Limp Mode

The crankshaft sensor synchronizes the position of the valves within the engine and the speed of the revolutions within it. It’s one of the most vital sensors on your car. Without it, your engine would risk becoming badly out of time and the pistons being damaged.

Like most sensors, they rely on receiving the correct voltage. Without it, they can give false readings to the ECU. When a false reading is received, the ECU may recognize the reading as being outside of its normal operating range and put the car into limp mode.

O2 Sensor and Limp Mode

This sensor doesn’t rely on receiving voltage from the alternator. It creates a small voltage itself. If you get an o2 sensor error code, you can be sure it’s not related to the alternator at all.

Cause of Erratic Voltage In An Alternator 

Alternator Ground Straps Causing Limp Mode

If your alternator grounding strap is loose, it may cause the voltage to jump and spike. This will play havoc with all the sensors that require a constant voltage to work correctly. 

The camshaft sensor is the most susceptible to alternator grounding problems. The ECU is extremely sensitive to any bad readings from this sensor and will often place a car in limp mode if it identifies any potential problems. 

Bad Voltage Regulator

The voltage regulator, when working correctly, ensures that the alternator doesn’t produce too much voltage that could damage electrical parts and circuits.

A bad one will allow excess voltage to leave the regulator or restrict the flow to low voltage. Either of these two scenarios will cause many sensors to stop taking accurate readings. When working well, the regulator should limit the voltage to 14.2 v. Anything more than this will blow bulbs and fuses.

As we’ve discussed, a bad reading at the ECU risks the car going into limp mode.

Here’s a great video on How To Test For A Bad Voltage Regulator 

Can A Bad Battery Cause Limp Mode?

Your car battery is only used to start your car. After that, the alternator takes over. Sometimes the battery warning light illuminates before a car goes into limp mode. This light, however, is a warning that the battery isn’t being charged by the alternator.

Only on rare occasions when the alternator is going bad will the battery be forced to provide any power when the car is running.

When the alternator is good the battery won’t be called upon.

Popular related article often read by other visitors: How Long Can You Drive A Car With A Bad Alternator?

Will Limp Mode Always Cause a Check Engine Light?

The first question to ask yourself is does your car have a limp mode setting. Many cars don’t. 

Check your handbook to make sure. The older the car the less likely it will have it.

If your car does have limp mode it should cause a check engine light to come on when it happens. 

The two go together, the warning light lets you know there is a problem and limp mode ensures you can’t do too much damage.

Can I Continue To Drive In Limp Mode?

Limp mode is designed to allow you to carry on driving. However, its aim is to allow you to get to a safe place before pulling over and getting the car towed. 

Limp mode isn’t intended to allow a car to be driven around for extended periods. This may cause the check engine light to flash and the car to stop.

A flashing check engine light is a sign of a serious problem. Your car won’t restart and significant damage may have been caused to your engine.

Cost to Fix Limp Mode

  • Bad ECU – $1000 – $2000
  • Faulty Sensor – $100 – $250
  • Low Fluids  – $40 – $80
  • Worn wiring  – $50 – $250
  • Transmission $500 – $2000

In Summary

Limp mode can indirectly be caused by the alternator. If it goes bad it could send erratic voltage to the sensors. If this happens they’ll send mixed messages to your car’s ECU. The ECU will read these signals as potentially harmful to the engine and place the car in Limp mode.

Related content:

What Causes An Alternator To Fail

Do You Have To Replace A Battery When Replacing An Alternator?

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