AW55-50SN Archives - Transmission Digest
Testing the Speed Sensors in the AW55-50SN Transmission

A 2002 Volvo S60 arrived at our shop with two codes in the TCM: 0023 “Transmission input shaft speed sensor signal missing” and 0089 Transmission input speed sensor, signal too low.” The vehicle was in “limp-home mode,” which allows the transmission to be operated in 3rd and 4th gears only and with high line pressure. The customer had bought the vehicle with these conditions and did not know its history. Visual observation showed that someone had spliced into the speed sensors (Figure 1) and had replaced the transmission with a used one.

The Importance of Proper Testing

We can all agree that testing today’s transmissions can be more complicated then it was 20 years ago. We have to deal with more electronics and hydraulic design, not to mention programming of the transmission control module by the design engineers. Proper testing procedure and testing equipment become a necessity.

Included in the Quote

Let’s look at an example of what I’ve discussed. Let’s say that you were working on a 2001 Volvo V70 XC AWD, with an AW55-50SN transmission, and the vehicle had been towed in because it suddenly quit moving. You discovered that the transmission had a problem in the final-drive area but you found no other issues. All the clutches were like new, and there was no obvious wear in the valve body etc. That being said, the transmission was probably operating normally just before the final-drive component failed.

AW55-50SN Initial Engagements

I will try to explain one of the most-common questions being asked, concerning Drive and Reverse engagement problems. Some of this information has been covered at the ATSG seminars, so I will be using some amperage and pressure readings taken from a 2005 Nissan Altima as a visual aid so we can see how this transmission accomplishes smooth, almost-unfelt initial engagements, or garage feel as it is known to some.

An Interesting ‘Spin-off’ of a Hall-Effect Speed Sensor

Since the time of writing that article, it seems the Hall-effect style of sensor is favored among the manufacturers, most likely because of its reliability and the cost effectiveness of producing it. This sensor can be either a two- or three-wire design, with the frequency signal being generated by a magnetic wheel (ring) or by the teeth of a gear. The three-wire design consists of a voltage-supply wire, a ground and a signal wire. The two-wire design has a voltage-supply wire and a signal wire. For this two-wire sensor to work a resistor inside the TCM/PCM is wired to ground, allowing the voltage on the signal wire to drop when the rotating magnet is used to “chop” the sensor circuit and producing the signal pulse.


By the time they arrive, they are well-seasoned units, although not necessarily what you would consider high-mileage. Although these automakers use the same transmission, there will be variations in the shafts, drums, converters, valve bodies and TCM calibration. The 2-3/3-2 shift issues are common, regardless of brand, and may be accompanied by codes P0780/785 or P0745.

Adapting to the AW55-50SN Valve Body

Some things are less intimidating if you’re not the first to do it. That statement applies to both crossing a frozen lake and working on a problematic AW55-50SN. On the lake, heavy snow cover can prevent you from telling whether the ice is sufficient for support, and what you can’t see may end in a sink-or-swim experience. On the 55-50SN valve body, a clean core does not mean the valve body is functioning correctly.

Sometimes It’s Not So Bad When It’s Bad

The input-shaft and output-shaft speed sensors used in the AF23/33-5 (AW55-50SN) are two-wire Hall-effect sensors, which means they are supplied with a voltage source on one wire and send a signal back to the computer on the remaining wire.