A500 Four Speed A/T - Service Procedures: Overview

88chrysler09

Models


1988 B- & N-Truck


Subject

A500 Four-Speed Automatic Overdrive
- Service Procedures


Index


TRANSMISSION


Date

July 18, 1988

No.


21-06-88

P-2835 (C21-10)
This bulletin provides a description of the A500 four-speed automatic overdrive transmission as well as diagnosis and repair procedures to service this transmission --- initially released for use on the subject model vehicles.

FIGURE 1:

The front portion of this unit is a modified A999 three-speed loadflite automatic transmission. The rear unit, or overdrive, replaces the extension housing and provides a fourth gear with an economy gear ratio (.69 to 1.0) (Figure 1).
A lockup torque converter which is electronically controlled and hydraulically activated, will also be used with this transmission. Fourth gear (overdrive) and lockup will only occur during certain conditions determined by the SMEC (Single Module Engine Controller).

When the vehicle is traveling in third gear over 25 miles per hour, the SMEC uses the following information to allow the shift: The SMEC checks the coolant sensor signal for a 60- degree fahrenheit minimum temperature. It also checks the engine speed sensor, the vehicle speed sensor, the throttle position sensor, and the map sensor.
There is an overdrive off switch located on the instrument panel. The switch is marked O/D OFF and incorporates a pilot light to indicate when the switch has been activated. When activated, the switch will prevent shifts into overdrive and locktip. This is desirable when towing, driving in hilly terrain, or any other driving conditions that make overdrive unsuitable. Activating the switch again will restore the automatic overdrive operation. The switch will reset for operation when the key is turned off.

Other features in conjunction with this unit include:

^ The output shaft of the three-speed section will now be referred to as the intermediate shaft.

^ The three-speed section rear drum is retained on the support with a snap ring.

^ The governor and speedometer drive have been relocated to the rear of the output shaft.

^ The overdrive case now contains two output shaft bearings. One is the output shaft front bearing, and the other is the output shaft rear bearing.

^ There are no rotating seal-rings or pressurized oil for the overdrive and direct clutches in the overdrive housing. The governor is the only component receiving pressurized oil through slip-fit tubes. Pressurized oil for the overdrive lubrication circuit is supplied through the intermediate shaft.

^ Governor pressure and overdrive pressure taps are provided in the rear of the main case for in-vehicle transmission pressure testing.

^ The valve body has been modified by adding several new valves. There's an overdrive solenoid, a 3-4 shift valve, a 3-4 timing valve, a 3-4 accumulator, and a 3-4 shuttle valve. Once in fourth gear, the lockup solenoid, lockup valve, and lockup timing valve accomplish the hydraulics to lock the converter turbine to the torque converter housing.

^ The direct drive and overdrive gear ratios are supplied by a third planetary gear set, a direct clutch, an overdrive clutch, and an overrunning clutch. A very strong spring, rated at up to 800 pounds (5,516 KPA), holds the sun gear to the annulus for direct drive. For coasting or reverse gear, power flows only through the direct clutch.