My 99 S80 2.9 has a problem shifting from third to fourth gear. The check engine light has come on and the message center has a message transmission service urgent. I took it to the dealer and they said the transmission has a vacuum leak. Is this a large repair bill as the dealer does not fix transmissions? The problem is continuing and I do not want to ruin the transmission. Thanks

no engine told. size, this tells what transmission sites behind it.
1999 Dodge RAM 1500????? not VAN. 1500
2wd or 4wd'/
5 engines. then.
check all fuses, some have stand alone TCM brain (tranny)
scan the PCM for P07xx error (tranny)
check ATF levels hot, park ,idle, only do not overfill it ever.
ATF+3 (not sure but do check)
they used 3 transmission then
4-speed 42RH-RE automatic
4-speed 46RH-RE automatic
4-speed 47RH-RE automatic

42RE , is electronic shifted,
OD gear is dead or just the TCC lockup deaD
best is count the gears more carefully to see truth.
yes 4th gear is OD Geared. 3rd gear is 1:1 lockup.
.74-to-1 for first, 1.54-to-1 for second, direct 1.00-to-1 for third and 0.69-to-1 for fourth.(OD)
scab the PCM
to see if errors from the transmission show up.
I bet they do.
the 3>4 shift solenoid must work, .and no pressure errors.
the schematics show an OD switch if not closed OD does not happen, forget about that switch>
if car is 4wd and is in 4wd, many cars/ and truck all shut off
OD in 4wd, by logic inside the PCM (pure) my not be true on the biggest engine not told.

Question edited for clarity and autocorrect typo.
Question moved to a G6 GT category.

This is probably your best bet.

https://www.google.com/search?q=transmission+specialist+near+me

But there are videos here as this seems to be a known issue.

https://www.google.com/search?q=Pontiac+G6+GT+3.5+won%27t+shift+into+fourth+gear

..

Which transmission does yours have 4T65-E or 4T80-E ?

4T80-E
No Fourth Gear

Checks

Causes

1-2 Shift Solenoid Valve
?€¢
Inoperative
?€¢
Debris
?€¢
Pinched wires

3-4 Shift Valve Bore Plug

Misassembled

4th Band

Burned, slipping, missing

4th Servo Cover

Cracked

4th Servo Pin

Broken, seized

4th Servo Piston

Damaged

4th Servo Seals

Rolled, cut or damaged
4T65-E
Slipping or No Fourth Gear
Slipping or No Fourth Gear Checks
Cause

DEFINITION: A large engine RPM increase is noted in fourth gear, Drive range, with little or no vehicle forward speed increase. First, second, and third gear operation are normal. Shifts into fourth gear are soft or do not occur. Fourth gear ratio is 0.705:1.

Automatic Transmission Fluid Level
Low fluid level

Transmission Main Line Pressure
Low transmission main line pressure

Oil Filter Assembly
• The oil filter assembly (100) is missing, plugged, or damaged.
• The oil filter seal assembly (101) is missing or is damaged.

Control Valve Body Assembly
• The 3-4 shift valve (362) or the 4-3 manual downshift valve (360) is stuck or binding.
• Debris is found in the passages of the control valve body (301) or of the control valve body spacer plate and gasket assembly (396)

Case Cover Assembly
Distortion or damage occurs at the manual valve (404) or at the case cover (401), causing PRN oil leak.

Driven Sprocket Support Assembly
A plugged third clutch oil passage in the driven sprocket support assembly (609)

Fourth Clutch Assembly
• The clutch plates (500-502) are worn, damaged, or misassembled.
• A missing or not seated fourth clutch spring retaining ring (601)
• A damaged spring assembly (602) or piston (603)
• Worn or damaged fourth clutch piston seals (604, 605)

Third Clutch Sprag Assembly
• The sprag (720) was misassembled or backwards on the inner race (661).
• A damaged third clutch sprag assembly (653, 661, 719-721)

you may try reving engine alittle higher try shifting to fourth if that works your third gear sincroniser is shot

Here is what the service manual says about the EPC solenoid, the processor looks at allot of different inputs to command the solenoids, such as Throttle Position Sensor, Vehicle Speed Sensors, Transmission Range Sensor and RPM signal, The PCM controls the shift patterns for the transmission. Electronic Pressure Control (EPC) Solenoid
The electronic pressure control (EPC) solenoid is located on the main control valve body and is controlled by the PCM. The EPC controls line pressure used to apply and release clutches and bands in the transmission.
The 2-3 shift solenoid is located on the main control valve body and is switched on and off by electrical signals from the PCM. The 2-3 shift solenoid activates the 2-3 shift valve which controls the automatic shifting between second and third gear. Second gear is established when the 2-3 shift solenoid is switched on. The 2-3 shift solenoid is switched off when vehicle speed increases and requires a second to third gear upshift.
The 3-4 shift solenoid is located on the main control valve body and is switched on and off by electrical signals from the PCM. The 3-4 shift solenoid activates the 3-4 shift valve which controls the automatic shifting between third and fourth gear. Third gear is established when the 3-4 solenoid is switched off. The 3-4 shift solenoid is switched on when vehicle speed increases and requires a third to fourth gear upshift.

BEST TO CODE SCAN VECHICLE COULD HAVE STUCK SECOND AND THIRD GEAR.OR FAULTY OVER DRIVE SOLENOID.CHECK TRANSMISSION FLUID LEVEL IT COULD BE A LITTLE LOW ON TRANSMISSION FLUID.

transmission shifter module. could also be the final shifter solenoid.

Check transmission lock up selenoid.

transmission is gone

You have an overdrive relay issue. The tansmission is really a four-speed box, but the OD selector either locks our fourth gear or allows the car to shift normally into fourth. I use the OD on my volvo all the time, because the revs drop in half when the OD is activated, The relay is located behind the glove box, toward the center console. $10 buck on Ebay and your transmission should shift correctly again. I would try that BEFORE more expensive fixes, like OD solenoid $200 or transmission control module. Just for kicks a picked up a spare ECU on Ebay for $10 because these get costly if and when they go. Easy fix, though, a couple of plugs.