What is Transmission Solenoid? – A Quick Guide

What is Transmission Solenoid?

A transmission solenoid or cylinoid is an electro-hydraulic valve that controls fluid flow into and throughout an automatic transmission. Solenoids can be normally open or normally closed. They operate via a voltage or current supplied by the transmission computer or controller.

They control the flow of transmission fluid throughout the transmission, and they open and close as per the electrical signals they receive from your vehicle’s engine or transmission control unit. Which pulls its data from a series of speed sensors in the engine.

Modern automatic transmissions use pressurized hydraulic fluid for shifting. Whenever a gear change is required, the vehicle computer activates a transmission solenoid that directs transmission fluid into the valve body to engage the correct gear.

If one of these electromechanical valves fails, all sorts of transmission problems can arise. So let’s take a closer look at the solenoid and the general issues that come with it.

How Does a Transmission Solenoid Work?

As you drive down the street, the car’s computer analyzes data sent from vehicle speed sensors and engine speed sensors. Based on this information, the engine control module (ECU) or the transmission control module (TCM) performs the appropriate upshift / downshift by sending a signal to one of several shift solenoids.

These transmission solenoids have a spring-loaded plunger inside that is wrapped with wire. When this coil of wire receives an electrical charge from the TCM / ECU, the plunger opens allowing transmission oil to flow into the valve body and pressurize the desired clutches and bands. In this case, the transmission changes gears and you will drive down the road.

The car’s computer can control the transmission solenoid in a number of ways. If the vehicle is equipped with a special transmission control unit, it can open or close the hydraulic circuit with a direct 12 V signal.

Or the engine control unit can control the solenoid’s plunger by turning the ground circuit on and off. A solenoid can be used to control a single gear or multiple gears, depending on the complexity of the design.

Transmission Solenoid

Symptoms of a Transmission Solenoid Problem

A transmission solenoid can fail due to electrical issues, or dirty fluid that’s caused the shift solenoid to become stuck open/closed. Any change in the transmission fluid pressure can cause numerous problems, including:

  • Erratic Shifting – If you’re dealing with a failed transmission solenoid, the gearbox can skip a gear up or down, shift back and forth between gears repeatedly, or get stuck in a gear and refuse to shift.
  • Transmission Won’t Downshift – If the transmission will not downshift, one of the shift solenoids may have become stuck open/closed, preventing fluid from entering the transmission valve body to pressurize the correct gear.
  • Severe Shifting Delay/Stuck in Neutral – In order for an electronically controlled automatic transmission to shift gears, the solenoid must be able to regulate the fluid pressure to activate the appropriate gear.

If the shift solenoid is receiving too much or too little electrical power, or dirty transmission fluid has caused it to stay open/closed, it can become difficult or slow to shift into gear, which can cause the transmission to function as if it were momentary locked is neutral.

Since the solenoids are connected to a vehicle’s electrical system, the ECU usually registers a fault code and triggers the engine test lamp if something goes wrong. In this case, the transmission can switch to limp-home / failure mode, in which only second / third gear is engaged in order to limit the vehicle speed without locking it.

The first thing your mechanic should look out for are the fault codes. A scan tool can help the technician determine the cause of the solenoid’s problem. It can be as simple as a bad floor or as complex as a failed solenoid pack (a grouping of individual shift solenoids).

What to Do When Transmission Solenoid Troubles Arise

Like any mechanical device or component, transmission solenoids do wear out with time. Standard preventative maintenance for your transmission can help offset their deterioration to a degree.

If you’re experiencing transmission solenoid troubles, it will become evident in one of four ways:

  • Delayed gear shifting
  • You can’t downshift, and your engine continues to rev even when applying the brakes
  • Your transmission gets stuck in neutral
  • Shifting gears become rough and choppy

Depending on the make, model, and year of your vehicle, transmission solenoid replacement costs can vary.

Transmission Solenoid Replacement Cost – Parts & Labor

In most cases there are solenoids in the oil pan that are connected to the valve body. Depending on what you’re driving, the technician might just be able to replace the failed solenoid.

However, in some cases, the solenoids are shipped in these multi-unit packs. If a problem occurs with a package, the entire package must be replaced. This job usually takes 2 to 4 hours. Loading time is typically billed at $ 60 to $ 100 an hour. The average total cost of diagnosis and replacement is between $ 150 and $ 400.

Expect $ 15 to $ 100 for a single solenoid, depending on the make and model of your vehicle. A pack can cost $ 50 to $ 300.

TypeCost Range
Single$15 to $100
Pack$50 to $300
Labor$120 to $400
Total (Pack)$250 to $600

While it is not uncommon for the transmission shift solenoids to wear out over time, you can extend their life by changing the transmission fluid at the factory recommended intervals.

This will remove all of the dirt and sludge that has built up and the fresh liquid will prevent the plungers from sticking to the inside of the solenoids. If you don’t know the recommended transmission intervals for your vehicle, see the back of your owner’s manual or just ask Google.

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