Accessory Belts, Tensioners And Misfires: A Balancing Act -

Accessory Belts, Tensioners And Misfires: A Balancing Act

On an engineering level, the design of the accessory belt drive is a balancing act of efficiency, force and vibration. The more contact or wrap a belt has with a pulley, the more force or energy can be transmitted without slipping and the less tension the belt will need. But, this is not great for efficiency because it creates more friction and drag.

belts-tensioners-misfiresEver been amazed how complex some accessory belt routing diagrams can be when all they are driving is an alternator, A/C compressor and maybe a power steering pump? The belt might be 8-feet long and loop around one or two idler pulleys.

On an engineering level, the design of the accessory belt drive is a balancing act of efficiency, force and vibration. The more contact or wrap a belt has with a pulley, the more force or energy can be transmitted without slipping and the less tension the belt will need. But, this is not great for efficiency because it creates more friction and drag.

The design of the accessory belt also creates a balancing act for the accessories. A water pump pulley may require less contact or wrap with the belt, while the A/C compressor may have a lot more contact or wrap.

The other issue is keeping the tension on the accessory belt under different speeds, loads and temperatures. If the tensioner is worn, the belt will slip and then catch suddenly.

belts-tensioners-misfires-tensionerTensioners are more than just a spring. Most tensioners have a dampener mechanism inside or connecting to the arm. This dampener prevents wild movement of the tensioner’s arm and pulley. This helps to keep the belt tension within specifications as the speed of the crankshaft and loads change.

The tensioner dampener is a sacrificial part that will wear out over time. When it does, this is when rapid changes in belt tension occur. This can be seen through belt flutter and even excessive movement of the tensioner.

If the dampener is not able to control the tensioner arm, it can cause fluctuations in tension. The accessory belt may oscillate between too little tension and too much tension. The belt will slip and catch, causing noise and vibration.

This can cause a change in crankshaft speed that is severe enough to cause the engine management system to think that a misfire just occurred.

Modern crankshaft position sensors measure more than the position of the crankshaft, they measure changes in the speed of the crankshaft caused by combustion events. Over the past decade, crankshaft and camshaft position sensors have become more sensitive and accurate in detecting these changes. These sensors can even detect clutch engagement in an automatic transmission and small changes in traction.

Additionally, these sensors can detect changes in the loads on the drive belt and how the engagement of the A/C compressor can change crankshaft speed. Under normal conditions, the engagement of the compressor is below the threshold of a misfire.

One A/C clutch engagement incident will not set a P0300 code. But, if enough occur and it is above the threshold of the monitor, a code will be set.

Courtesy Underhood Service.

You May Also Like

Updating Your TPMS Tools Regularly Matters

You can ensure your read is accurate by updating your tool regularly.

Courtesy of Tire Review by Yanick Leduc

Picture this: Mark, an experienced tire service technician, is on the phone with a technical support representative from his TPMS supplier, inquiring why his new sensors don’t seem to work properly with the 2019 Silverado he’s servicing.

Mark: “The truck doesn’t seem to pick up the signal from the new sensor!”

Cabin Air Filters

Cabin air filters will protect the health and well-being of the driver and passengers.

Diagnostic Procedures For Stop/Start Systems

Stop/start technology is more than just a switch wired to the brake pedal and starter.

Brake Pad Wear Sensors

Brake pad wear sensors have been around for years and continue to evolve.

Topology and Your Scan Tool

Topology influences how you access the modules with your scan tool.

Other Posts

Ride Control Diagnostics From Tire Wear Clues

Learning how to read the tire and communicate your findings is the key to ride control service.

Understanding Engine Timing- Chains, Gears and Belts

Engine timing is critical for overall performance.

Air Filters and Fuel Trims

Modern engines rely on precise airflow and oxygen sensor data for optimal performance.

Mercedes Ignition Coil Pack Diagnostics

Replacing a single coil might not be the best approach to curing an engine of a chronic misfire.