VOLVO 850
Transmission Service

When Volvo produced the 850 series in 1993, it was the first Volvo imported to the United States that had front-wheel drive. The majority of these vehicles featured automatic transmissions, and Volvo offered the model with only normally aspirated engines. With 80 percent of the passenger cars sold in North America the prior five years being automatics, and 90 percent of these are front-wheel-drive units, Volvo had to offer a durable front-wheel-drive automatic with a lock-up converter to compete with other manufacturers.

The AW 50-42, designed for Volvo by Aisin Warner, is an electronically controlled four-speed automatic with torque converter lock-up in the three highest gears. The transmission is controlled by an electronic control module, making it possible to control gear shifting so the correct gear is always selected to be consistent with driving conditions, engine load and speed. This yields good fuel economy, combined with top-class performance and increased comfort, by ensuring gentler gear changing and lower noise levels.

The control module receives information concerning the selected gear and driving mode. It then processes this information along with signals from a number of sensors describing the running conditions of the car, and calculates optimum shifting and lock-up engagement points, depending on the driving mode selected.

The control module then controls shifting and lock-up engagement functions by changing the direction of the fluid to the valve body. To facilitate fault tracing, the control module has a built-in, on-board diagnostic system that records faults in the control module. These faults subsequently can be read with the help of the diagnostic unit. The control module also affects the quality of gear changing by modifying the hydraulic line pressure the instant a shift is made.

The AW 50-42 electronic control system continuously receives information from a number of sensors as well as from the engine control system. Unlike a transmission with only a hydraulic control system, this information enables the electronic control system to accommodate extremely small changes in operating conditions and to control the various transmission functions so that the correct gear is always selected in accordance with the selected shifting mode.

The slippage that normally occurs in the torque converter and in the clutch and brakes during engagement is registered and stored in the control module’s memory. This information is used to calculate the engagement times for the various gears. By allowing the control module to modulate the system pressure in response to the engagement times, the shifting comfort level is increased and the wear on mechanical components is minimized, thereby significantly reducing the need for service.

Solenoid valves (SL, S1, S2, STH), see top photo, sit in the hydraulic system valve housing, which is located beneath the housing cover on the transmission. Solenoid SL regulates the torque converter lock-up function, and valves S1 and S2 control the shifting. These valves are activated by the transmission control module, as is solenoid valve STH, which controls the transmission line pressure.

If there is a code found for an open circuit in one of the shift solenoids and erratic shifting (starting off in second or third gear), it is possible to replace them with the transmission in the car. I recommend replacing both S1 and S2 solenoid valves even though the ohm readings are in specification (10-15 ohms), as I have seen the solenoid valves work intermittently.

The NTC temperature sensor measures the oil temperature. The control module receives this information and utilizes it to calculate shifting and lock-up engagement points.

The gear position sensor (see Fig. 1) has three separate functions. They are: inform the control module of the selected gear positions by electrical means; operate the backing lights when the gear selector is in position R; and prevent the engine from being started when the gear selector is not in positions P or N. This signal also is passed on to the engine control module and is used for drive compensation in connection with idling.

The gear selector is mechanically connected to the transmission by cable, which operates the gear selector valve in the control system. To avoid any chance of the gear selector moving from the P position, the electrically operated interlock function (shift lock) interlock pin is located in the selector lever.

Failure of the small micro-switch (see Fig. 2) located in the gear lever housing is common. It enables the customer to move the lever even when the brake pedal is down. The override lever could be used at this point.

If the transmission has major failure due to worn-out clutches and contamination, I recommend replacing it with a quality remanufactured transmission, or overhauling the transmission with a rebuild kit and replacing worn parts as needed. After the unit is replaced, the transmission oil cooler must be flushed out. Metal filings could be harmful to the valve body causing binding.

We use a fluid exchanger in our repair shop that I have modified to perform this task. I installed a large inline oil filter to the machine. It helps after installing a replacement or rebuild to filter the fluid that comes out of the oil cooler before it reaches the transmission.

The 850 and some other transmission coolers are equipped with a thermostat that controls the fluid at certain temperatures. Attempting to flush or clean the cooler when it is cold is impossible and, chances are, the contaminated fluid from the old transmission is trapped inside. I run the car on the lift with the idle raised a few rpms and wait for the thermostat to open. I feel the inline oil filter get warm to know that it is filtered and circulating back to the transmission. At this time, fluid can be added or removed to maintain the correct level.

In our repair shop, we fluid exchange the transmissions every 35,000 miles when the major routine services are performed.