Hi, Ty...For those unfamiliar with the Sonnax upgrades for RWD Chrysler transmission on Jeep and Dodge Ram trucks, see my article: http://www.4wdmechan...nsmissions.html. I'm very pleased with the Sonnax upgrades for our Ram 3500 48RE transmission, which include the manual valve, line pressure valve, the detent ball/sleeve and a piston upgrade—each without removal of the transmission or a major teardown...If you want your RE or RH transmission to survive, read the article at the "Automatic Transmission Workshop"!
Your A-518 automatic is the first 727 Torqueflite-derived RWD Chrysler platform. Chrysler introduced the A-518 in 1988 and used it through 1996 on some applications. (42RE and 46RE were essentially the replacements for the A-518 in trucks.) The A-518 has many similarities in design and parts layout to both the A-727 Torqueflite 3-speed and the later RE 4-speed overdrive transmissions. Jeep 30RH and 32RH transmissions are also similar, but like the lighter A-500, their platform is the Chrysler 904/999 three-speed.
The A-518 and later RE transmissions have the 4th gear overdrive built into the transmission unit and cases, and control for overdrive is solenoid based. As for stamina, the A-518 four-speed overdrive became a muscle car retrofit when built to performance standards. This replaces an A727 three-speed, modifying driveline length and the rear motor mount to fit.
JVX Racing specializes in this conversion and makes a range of upgrades for the 904/999, 727 and overdrive derivatives like the A-500 and A-518. The JVX catalog will be very useful for reference, Ty, and they also have a manual or controller shifting mechanism to replace the stock shift-to-overdrive system: http://jvxracing.com/jvxcatalog.pdf. Download the catalog to better understand the performance uses and upgrades for your transmission and modifications available.
All of these overdrive derivatives rely on the stamina of the original A-727 or 904/999 three-speed designs. Overdrive becomes the wild card, yet these units hold up better with each new design iteration over the years (42RE through 48RE in Dodge Ram trucks). Our 48RE is allegedly the "best" of these units, the last iteration, anyway, and late enough to have many of the weaknesses found in earlier RE overdrive units worked out.
In your case, you have a "factory" A-518 installation, with all of the stock controls for overdrive. Here is the "factory" explanation for the 3-4 Shift on a similar 42RE transmission. I have bolded the points of interest to your problem, Ty. This should help clarify:
"3-4 SHIFT SEQUENCE...The overdrive clutch is applied in fourth gear only. The direct clutch is applied in all ranges except fourth gear. Fourth gear overdrive range is electronically controlled and hydraulically activated. Various sensor inputs are supplied to the powertrain control module to operate the overdrive solenoid on the valve body. The solenoid contains a check ball that opens and closes a vent port in the 3-4 shift valve feed passage. The overdrive solenoid (and check ball) are not energized in first, second, third, or reverse gear. The vent port remains open, diverting line pressure from the 2-3 shift valve away from the 3-4 shift valve. The overdrive control switch must be in the ON position to transmit overdrive status to the PCM. A 3-4 upshift occurs only when the overdrive solenoid is energized by the PCM. The PCM energizes the overdrive solenoid during the 3-4 upshift. This causes the solenoid check ball to close the vent port allowing line pressure from the 2-3 shift valve to act directly on the 3-4 upshift valve. Line pressure on the 3-4 shift valve overcomes valve spring pressure moving the valve to the upshift position. This action exposes the feed passages to the 3-4 timing valve, 3-4 quick fill valve, 3-4 accumulator, and ultimately to the overdrive piston. Line pressure through the timing valve moves the overdrive piston into contact with the overdrive clutch. The direct clutch is disengaged before the overdrive clutch is engaged. The boost valve provides increased fluid apply pressure to the overdrive clutch during 3-4 upshifts, and when accelerating in fourth gear. The 3-4 accumulator cushions overdrive clutch engagement to smooth 3-4 upshifts. The accumulator is charged at the same time as apply pressure acts against the overdrive piston."
All of the information is useful; however, your problem is likely localized to the control switch, the solenoid and the wiring interface between these devices and the PCM (powertrain control module, also called the "computer", ECM or ECU). So, troubleshooting should involve the control switch for overdrive, the solenoid and the wiring.
Let me know if you need procedures for testing...Pleased to help here.