The Allison 1000 and 2000 series transmissions were introduced in 2001 for GM vehicles and 1999 for non-GM applications. The 1000/2000 series units quickly developed a reputation as very reliable transmissions, which drove the sale of GM vehicles as well as those of other manufacturers.
Several different valve body designs and components have been used in the 1000/2000 applications. Applications from 1999 to 2003 did not have an electronic way to control line pressure. To control shifts and shift feel, they used a pair of solenoids known as “Trim Solenoids” or, as they are called in later applications, pressure control solenoids. Trim A and B solenoids are PWM operated to control the aggressiveness of the shift. Trim “A” is normally closed, while Trim “B” is normally open. The trim solenoids control the pressure to the shift solenoids (1/2/3, C/D/E). Shift solenoid C controls the C shift valve, D the D shift valve, and E the E shift valve. Shift solenoids C/D and E are on/off design solenoids. A TCC PWM-controlled solenoid is used to control the operation of the torque converter clutch.
The early Allison’s were unique when it came to GM transmissions. Typically, a GM transmission makes a shift by cycling a solenoid, which then moves the shift valve responsible for sending oil to the desired clutch. 1000/2000 applications use the shift solenoids to pre-position the shift valves in anticipation of the shift (staging) then the Trim/PCS solenoid is used to control the pressure applied to the clutch.
Starting with the 2004 model year, a new solenoid was added, solenoid “G,” which is an on/off line pressure solenoid. This solenoid was a pressure-reducing valve that was used to lower line pressure, thereby reducing pump noise and increasing lubrication flow. 2006 saw some changes in the solenoids again as the 5-speed was converted into a 6-speed application.
In 2010-11, the valve body was redesigned with the Trim solenoids changing to PCS designs and solenoid G becoming a fully variable modulated (PWM) mainline pressure solenoid, which is now located as part of the main valve body. Solenoids C, D, and E became shift solenoids 1, 2, and 3.
In 2013, the valve body received another update on some 2000 series applications. These applications added a solenoid, “PC3,” known as the RELS (Reduced Engine Load at Stop). This solenoid is in charge of releasing the C1 clutch when the vehicle is stopped but the transmission is still in a forward range, providing a neutral idle feature. Releasing the C1 clutch places the transmission into neutral, eliminating engine load at a stop, which improves fuel economy.
The 1999-2005 applications are experiencing shift quality-related issues after someone has worked on the valve body. During an overhaul or when doing valve body repairs it is common for the technician to replace the solenoids, including the trim solenoids. You may have discovered that it is very difficult to purchase the trim solenoids from GM or from Allison, and if you are able to purchase them, it may be cost-prohibitive to purchase the OEM parts for your application. Allison has captured the parts for a quality issue that they are trying to address, so the number of parts available for aftermarket consumption has become extremely limited. This has led most shops to purchase the trim solenoids as aftermarket parts.
A few weeks ago, I received a call from a remanufacturer regarding hard shift-related complaints after installing a reman unit. The technician had discovered issues with the aftermarket trim solenoids, which he felt was leading to the shift complaints he was receiving of:
- Ultra-hard Reverse engagement
- Hard 2-1 downshift engagement
- Slide bump shifting into 4th
This past week, I visited a reman facility, “Browns Transmissions,” and spoke with Tom Hess, the technician who had discovered the issue. Tom has been around for a long time and is as sharp as a tack, so I paid close attention as he showed me what he had found. The “new aftermarket trim solenoid B” was in the ‘closed’ position. As I mentioned above, trim solenoid B is a ‘Normally Open’ solenoid. This could certainly account for the issue, so I asked Tom what he is doing to address the solenoid issue.
He uses a 2mm Allen wrench to change the screw position located at the end of the solenoid (Figure 1). He adjusts the screw until a .035″ gap is present between the valve land and the solenoid body (Figure 2). This typically involves rotating the screw counterclockwise until it bottoms and then turning it counterclockwise three more times. Tom inspects all the aftermarket solenoids he receives and has found that, in most cases, the new trim B solenoid requires adjustment. When I inquired if this was fixing the issues, Tom indicated that he builds a lot of 1999-2005 1000/2000 series units, and this attention to detail has led to great success and customer satisfaction with the rebuilds they provide.
Thanks again to Tom and Browns Transmission for sharing the information. Well, that’s about all the time we have for now. Until next time, remember, “Never drive faster than your guardian angel can fly.”
