Automatic Transmission Solenoids. Classification. Problems and Diagnostics

Before Solenoids

Before appearance of solenoids automatic transmission manufacturers used the governor, a primitive mechanical-hydraulic valve that shifted gears in hydraulically-controlled transmissions. In old (700R4) automatic gearboxes speeds were changed with the help of hydraulics, not electronics. For this purpose the gearbox would generate three types of pressure: mainline, throttle and governor pressure.

• Mainline pressure is the source of pressure for the torque converter, valve body, and the clutches and bands. As pressure comes from the gearbox pump, the regulator exhausts excessive pressure to avoid any malfunction that may occur to internal components.
• Governor pressure grows with the car speed. Older gearboxes had mechanical governors made up of springs, flyweights, and a spool valve to control this pressure. Governor pressure forces a gearbox to gear up and throttle pressure forces it to gear down.
• Throttle pressure shows the engine load. Some gearboxes have a vacuum modulator or throttle linkage to control the throttle valve. Modern cars are equipped with electric solenoids to do this job.

In the following videos you can get acquainted with the governor and its purposes in the automatic transmission 700R4

A solenoid operates on the principle of electromagnetism. To put it simply, a solenoid is a cylinder-shaped coil of wire, which operates as an electromagnet when electricity passes through it. According to more extensive definition, solenoid is an electro-hydraulic valve in the gearbox, which opens and closes the channel in the valve body to hold control over the ATF flow in response to an electrical impulse from the transmission control module (TCM) or engine speed sensors. The structure of solenoids includes a spring-driven plunger muffled with a coil of wire. When this coil of wire gets an electric charge from the TCM, it forces the plunger to open, enabling ATF to flow into the valve body and raise pressure in required clutches and bands. Engine sensors specify the particular time for changing gears and solenoids, in their turn, are directly responsible for changing of speeds.

Location

Solenoids are placed in the valve body of the gearbox. The solenoid is inserted in the valve body channel, it is mounted with help of the bolt (or clamping plate), and on the other side solenoids are connected to the TCM via the wiring plug. In many cases plug and wiring-tail turn out to be the main reason of solenoid failures. In some transmissions the valve body and the pan cover are located not at the bottom part of the transmission, but on its side. The solenoid connects the transmission hydraulic system to its electrical system. In many cases the transmission computer detects errors in solenoids.

Solenoid Classification Based on Design Differences

On-Off Solenoid

Auto transmission manufacturers started using solenoids on a massive scale for American automatic transmissions since the 80-s. These solenoids consisted of the coil with a copper winding. Their main function was to push the plunger rod in the valve body and open or close channels used for oil transfer to the system. The spring brings the plunger rod to initial position when the current is not provided to the coil winding. This type of solenoids has two options: “Closed” and “Open”, therefore it is called as “On-Off solenoid”.Typically, when dealing with On-Off solenoids transmission service centers often encounter such problems as short-circuit failure, winding failure, breakdown of the back-moving spring. The repair of old solenoids usually consisted of rewinding of ragged or burned wires, soldering, cleaning, and replacement of weak springs.

Solenoid-electric valves (solenoid-operated valves)

The next generation of solenoid-electric valves (solenoid-operated valves) was used before 2006 in European Volvo S80, ХС90 and still may be installed in many American high-priced cars such as Buick, Oldsmobile, Pontiac, and Chevrolet. In terms of design this type of solenoids is more complicated. It is not just a simple winding with the plunger, but this type of solenoid also has the oil channel (made of white plastic) with two outputs and a metal ball valve, which opens and closes the channel. This solenoid is a hydraulic valve in itself. This solenoid is called a “solenoid-controlled valve” because it combines hydraulics and electrics in one unit. It is much easier to replace this solenoid by disconnecting it from the hydraulic system in which solenoid keeps the pressure with the help of rubber gasket rings and from the power supply by disconnecting the plug.

The solenoid-controlled valve can be “normally open” or “normally closed”. The spring operates in no-voltage condition. When the voltage is applied, the winding magnetic field gets activated and resists (counteracts) the spring. With time, car transmission specialists started installing the filter-grid preventing magnetized iron dust from penetrating the channel with ATF.

3-WAY Solenoid

The next type of solenoids was designed in the 90’s. It was named as a 3-way solenoid. The first solenoids were just on-off switchers, but the 3-WAY solenoid in its turn operates as “shifters”. 3-WAY solenoids connect 3 channels: in one position (ON) the ball opens the way from the 1-st to the 2-nd channel, in (OFF) position the 3-WAY solenoid opens the way from the 2-nd to the 3-rd channel. Usually Off position is used for the clutch package depressurization. Thus, one unit is used for activation of the friction clutch package as well as for switch-off operation. Previously, this function was performed with help of an extra mechanical valve located in the clutch.

PWM (power-width modulation), VBS (Variable Bleed Solenoid), VFS (Variable Force Solenoid)

In the mid-90s, engineers came up with more sophisticated instrument for hydraulics control, namely solenoid-regulators. In terms of design “electro-regulators” operate on the principle of “Valve/Faucet” in contrast to the principle of “Switcher” applied in On-Off solenoids which have fully opened or fully closed states. These solenoid-regulators half-open and half-close the section along the curve, depending on the nature of the impulse voltage received from the computer (current is applied intermittently).

Firstly, engineers designed PWM solenoids, which are quite simple and not too expensive in terms of manufacturing. The main advantage of PWM solenoid design is the possibility to use more durable, wear-resistant and anodized (as a result, it makes them more expensive) materials for narrow spaces of the channel-clutch through which the valve moves in dirty and hot oil.

Later transmission specialists developed rare VBS (Various Bleed) solenoids, in which the flat valve has opened and closed states. These solenoids can adapt to changes in oil pressure, but they are mainly used for a limited set of tasks related to the low oil pressure in the line.

VFS (Variable Force) solenoids are the most complicated, they deal with high oil pressure in the line and are almost insensitive to changes in applied pressure. VFS-solenoids may have the spool valve. ZF Company very often uses this type of solenoids in its products. The VFS solenoid has a complicated control system. VFS-solenoids have a lot of peculiarities related to their settings. Moreover, this type of solenoids has shorter service lifespan if compared to linear Aisin solenoids. In well-known ZF transmissions – namely, 6HP21 and 6HP28 – these VFS solenoids have practically become spare parts and they are usually scheduled for replacement after 3-5 years of intense service.

Linear (proportional) solenoids

This type of solenoids is used by Aisin, the supplier of automatic transmissions for Toyota-VAG-Volvo. Linear solenoids have a valve plunger moving on the coupling clutch with port-holes, which previously was the part of the valve body design. In other words, this most problematic part of the valve body plate – coupling clutch with port-holes – has been placed in the construction of linear solenoids.And now in many cases it is no longer necessary to rebuild or replace the valve body itself, but it will be enough to replace the worn-out solenoid with the built-in valve. Thanks to this innovation the recurring problem of all automatic transmissions, the wear of the valve body channels, has been solved and now valve bodies operate much longer.

For example, in the process of the valve body repair in the 6-speed automatic transmission Aisin A960E, car transmission repair specialists mainly replace 4 linear solenoids from the set of 9 solenoids. The remaining 5 solenoids (On-Off solenoids) practically do not fail until the end of service life of the transmission. In recent decades engineers have been using light and soft aluminum allow for manufacturing of valve bodies (and solenoids) instead of cast iron. Nowadays, the mechanical part of solenoids (manifold and spool/plunger) is made of an aluminum alloy anodized with robust and dirt-resistant materials.

In the following video transmission specialists provides some information on repair of Linear Solenoids

Solenoids Classification Based on Functional Differences

Solenoids can also be classified according to their purpose. Specialists distinguish the following types:

• EPC (Electronic Pressure Control) or LPC (Line Pressure Control) solenoids , which are used for monitoring of the line pressure. It is the first and the most essential solenoid-operated valve which appeared in the valve body. This solenoid is responsible for distribution of all oil to the remaining solenoids and channels. In 4-speed automatic transmissions the EPC-solenoid is the first to get out of order.

• TCC (Torque Converter Clutch) solenoid (or SLU- Solenoid Lock-Up). This solenoid controls the torque converter lock-up. This electric valve does “the dirtiest work”- forces the torque converter clutch to be connected-blocked in order to enhance the efficiency and ensure the “sport mode” acceleration. This solenoid in transmission is the first destination of dirty and hot oil from the torque converter. Therefore, TCC/SLU solenoid is considered to be the weakest link in many valve bodies. The torque converter is locked-unlocked every time the car brakes or accelerates. In addition, the torque converter friction clutch in modern automatic transmissions operates in the so-called “regulated slippage” mode, when the torque converter heats up the oil in the transmission more intensively and contaminates it by the friction lining. In recent times the design of these overloaded friction clutches includes graphite (or Kevlar) linking elements that have a negative impact on the endurance of solenoids and valve body.

• Shift-solenoid is a simple solenoid-shifter, responsible for shifting gears. The valve body usually includes a certain number of such pressure regulators, which are mainly responsible for shifting gears up and down.

New types of solenoids

- Control solenoids (which control valves of the valve body). Functionally, these solenoids can control valves of the valve body as a transistor in the electrical circuit. Such solenoids only apply the control pressure to the valve of the valve body, and then this valve applies or reduces pressure on pistons and friction clutches. Control solenoids ensure smooth gear shifting.

-Solenoid of gear shifting quality (this solenoid operates only at the time of gear change for soft shifting with “slippage”)

-Solenoid of oil cooling control. This type of Solenoid is responsible for the oil cooling control (as a thermostat it opens the channel for oil cooling)

The specific nature and design of solenoids is constantly expanding and becoming more complicated. The diagnostics and repair of solenoids is limited to a simple replacement.

Typical solenoid problems

Typical symptoms of transmission solenoid malfunction:

1. Harsh shifting . Generally, the process of changing speeds should be performed smoothly and unnoticeably for a driver. If the driver experiences roughness while shifting speeds, it may be caused by excessive ATF pressure in the hydraulics (faulty solenoid(s)).
2. Shifting delays . Gear shifting happens in two stages: activation of clutch, followed by engagement of the gear. In a properly functioning transmission, this happens almost instantaneously. Hence, if you happen to notice that the gears in your car don't engage on time causing occurrence of some delay, during which your car behaves as if it were in neutral; in this case, it is most likely that your car has a bad transmission shift solenoid.
3. Transmission does not downshift . Properly operating gearboxes automatically downshift when the car slows down and get to the 1-st speed once the car reaches a complete stop. Problems with shift-solenoids may cause the gearbox to downshift uncontrollably or not at all.
4. Transmission does not shift into the right gear. A defective solenoid can also cause your transmission to shift to the wrong gear, skip a gear, or shift back and forth unpredictably. Take notice of these changes as you attempt to shift – as any of these reasons warrants a look at the solenoid.

When solenoids turn out to be the cause of trouble in the automatic transmission, it is indicated by specific “fault/false code”. Transmission repair shops often have to deal with the following solenoid issues:

Problem #1: Solenoids get clogged with oil varnish consisting of glued dust particles (including paper, aluminum, steel, bronze) from worn-out and damaged spare parts. This problem reveals itself when the spool valve of the solenoid operates well in cold oil and becomes wedged in hot oil (or vice versa). To eliminate varnish, solenoid-valves (and valve body components) are washed with the help of different removal fluids and cleaned by various methods using ultrasound and alternate current. In the process of the overhaul it is recommended to perform demagnetization of solenoid steel parts.

Problem #2: Leakage occurs as a result of the plunger or manifold wear-out. When one of PWM solenoids is weakened, TCM considers this fact and redirects the part of load to other solenoids. It slightly prolongs the service life of the worn solenoid. But hot oil and intensity of loads quickly wear out a weak solenoid, therefore it is necessary to replace this part. High intensity of operation, when redirecting pressure and some functions to other solenoids, wears channels and plungers of these solenoids (chain reaction).

Problem #3: Weakening of the back-moving spring, body cracks, structural failures, winding resistance drop (break or short-circuit failure). In this case, transmission repair specialists apply such methods as soldering of contacts, rewinding, replacement of bushings and other components.

Problem #4. The main reason for premature breakdown of modern solenoids is the wear of manifold channels, bushings, valve or plunger. The problem starts with clogging of the plunger with wear debris. Initially, the plunger becomes wedged leading to problems with gear shifting (depending on the function of the first clogged solenoid), then varnish damages surfaces of the plunger, bushings, and valves. After 2003-2004, valves and manifolds are usually made of anodized alloys which withstand significant wearing loads. In most cases, bronze bushings of solenoids turn out to be the parts that get worn off.

Sometimes transmission specialists repair worn linear solenoids by re-bushing the plunger. There are repair kits for replacement of solenoid bushings. These kits prolong the service life of solenoid bushings for another 30-60 thousand kilometers (depending on the condition of other electric regulator components). The resource of high-quality solenoids is measured by the number of open-close cycles. According to this parameter, Hyundai solenoids are usually not so good as corresponding American solenoids and are definitely worse than products of Aisin, Jatco or ZF.But even the most reliable solenoids have the operation resource which does not exceed 300.000-400.000 cycles. Solenoids may get out of order even after 400 thousand kilometers and maybe much earlier (depending on the driving style aggressiveness). Due to design peculiarities, in older versions of automatic transmissions (for example DP0, 01N), some solenoids (usually EPC solenoids) operate 2-3 times more intensely than other solenoids; therefore EPC solenoids are the first to exhaust their resource in these transmissions.

American automotive repair specialists prefer to perform scheduled repair of solenoids. Repair specialists replace bushings and clean all the insides of solenoids and the valve body from varnish in the process of each transmission overhaul. Timely cleaning and “re-bushing” of linear solenoids increases the resource of solenoids and the valve body by 40-70%. But at the same time it is necessary to replace all worn seals, rings and bushings (through which the oil pressure is lost), otherwise solenoids start operating at full cross-section. Torque converter repair with replacement of worn clutch lining is also considered to be a part of actions on prolonging the service life of solenoids.

There are only a few automatic transmissions in which solenoid issues can be solved only by replacement of worn solenoids. For example, in automatic transmission DP0 the service life of EPC and TCC solenoids is short in comparison with other spare parts of a car transmission. In certain cases of 4-speed transmissions repair replacement of both solenoids may bring the car back to life and help to forget (for a short time) about the reason of transmission failure (replacement of Teflon rings and bushings). But unfortunately replacement of solenoids is a temporary solution that often turns out to be unnecessary waste of time and money. In addition to solenoids it is recommended to pay attention to the valve body, torque converter and the transmission itself.

In the following videos transmission specialists share their experience in repair of solenoids

Solenoid Testing

Even if trouble codes specify a particular solenoid, it must be tested using solenoid diagnostic equipment. The so-called “resistance fork” is a parameter which determines the operational capacity of solenoids. Therefore, the first test to be performed when testing solenoids is the resistance check with the help of an ohmmeter.Reason: with time and due to severe operation metal in wires gets worn, resistance of winding increases and when the ohmmeter shows that the resistance exceeds the maximum point the ECU detects such solenoid and requires its replacement by means of an error code.

If the solenoid-operated valve shows normal resistance level and clicks when voltage is applied, then repair specialists limit themselves to cleaning and washing activities. Besides solenoids and their terminals, problems may be caused by powered wiring-tail. But nowadays modern solenoid-operated valves can not be properly operated just by ohmmeter. PWM solenoids require some sophisticated tools for complete solenoid checking.

Solenoid testing equipment

For proper solenoid testing professional repair specialists have to use special equipment. We will review 3 solenoid testers designed by different companies, namely, Hydra-Test (Cottingham Engineering), Axiline ( SuperFlow), and Raymond Technology (RayTech).

Hydra-Test SOL (HT-SOL) is a sophisticated solenoid testing unit designed by the English company Cottingham Engineering LTD for testing solenoids of different automatic transmissions. Engineers of the company developed a wide range of solenoid testing blocks ensuring the possibility for checking an operational condition of solenoids installed in most popular automatic transmissions. At the current stage, HT-SOL is activated by the HTC-K Controller, which has been adjusted for solenoid testing.

Key features:

• Single tank with enhanced for testing in cold/hot conditions
• Capacity to perform tests at max required temperature
• Solenoids are checked exactly as they operate with the valve body
• Integrated flow-rate meter and reliable data collection system
• Developed for application with Hydra-Test supported controllers. Can be used manually with possibility to add dual trace oscilloscope.

The Axiline is the latest solenoid tester, which ensures effective and time-efficient testing of transmission solenoids with the help of not complicated laptop interface. SolX PRO is equipped with the same electronics and software as the Axiline VBT 8000 valve body tester and SuperFlow’s Axiline and Hicklin brands of transmission dynamometers.

Key features:

• Advanced testing of On-Off solenoids, pulse width modulated (PWM) solenoids, linear solenoids, and complete solenoid packages
• Full adaptability of frequency range, operation cycle, and ramp time
• Manual control of up to 5 solenoids

Another automatic transmission solenoid tester is provided by the Chinese company Raymond Technology.

Key features:

• This tool ensures testing of all linear and ON-OFF solenoids
• User-friendly interface allows the user to analyze the information on tested solenoids and compare it with standard data, moreover, user can customize standard specifications
• Possibility of performing durability testing of On-Off solenoids (max time of repetitions 9999)
• Wide coverage expressed in high control frequency (0-4000 Hz), 100% operation cycle control, which can cover all types of solenoids
• Time-efficient diagnostics of solenoids operational characteristics
• Quick transmission fluid heating and maintenance of required temperature

Modern designs of solenoids are much simpler than structural solutions of their predecessors. Previously, valve bodies were made of cast-iron steel, and now they are made of Aluminum. Advanced solenoids are more sensitive to ATF quality. Upgrades of solenoids helped to decrease fuel consumption, enhanced dynamics and comfort of the car driving, all the transmission mechanics started operating more smoothly and accurately. But such changes, in turn, led to the rapid wear of transmission components and oil contamination. In many cases solenoids turn out to be a major source of automatic transmission problems. Therefore, well-timed diagnostics and professional repair of transmission solenoids is a key to fail-safe operation of automatic transmissions.