Knowing when a replacement transmission is needed, and when repairs could be made to prevent replacement.

Let’s set the stage. You’re driving down the road listening to your kids’ favorite…scratch that…You’re cruising down the road with the windows down listening to the sweet sounds of the cat-back exhaust you installed on your vehicle, when out of the blue, you stop moving, no matter how many times you hit the engine rev limiter the vehicle is not moving…NOW WHAT? Well, here are some items to look at, and most of this can be done on the side of the road.

Visual inspection

One thing you will hear over and over is the importance of a good visual inspection. None of the inspections listed below help if the vehicle's drive shaft/axles are broken and hanging from the bottom of the vehicle. Open the hood, get under the vehicle, look around, and make sure things are dry and in place. There should not be fluid leaking and spraying around. Make sure there is nothing obvious like loose wiring. Do a wiggle test on the wiring and hoses. If anything feels loose secure it and retest.  

Moves freely in neutral

You must make sure the brakes are not locked, or something seized up like a wheel bearing or axle. If you are on an incline, simply put the vehicle in neutral, then release the brakes for a second. The vehicle should roll downhill. If you are on flat ground, you should be able to push the vehicle with the transmission in neutral and the brakes released. It only has to move a foot or so.

Holds in the park position

Contrary to rolling in neutral, the driveline needs to keep the vehicle stationary in the park position. In Park ”P” position try pushing it back and forth or see if the vehicle stays stationary on a hill with the brakes released. It should move a few inches in each direction but not more than that. What we are checking here is that components after the transmission such as the axles, differential spider gears, ring and pinion, and transfer case internals are connected. *Note that FWD transverse mount transmissions will have the differential built into the transmission and some AWD components are built internally as well, if they are damaged, internal transmission repair is needed.

If the vehicle does roll freely in the park position, have someone sit in the driver’s seat, block the wheels, set the parking brake, start the engine, place the transmission in gear and look under the vehicle. You may observe the drive shaft/cv axle spinning without vehicle movement, you could have broken spider gears in the Diff, stripped splines of the axle or CV shaft. This damage requires driveline repairs. The transmission is not faulty in this situation and replacing it will not solve this issue.

Shifter Cable

If your vehicle still has a physical cable connecting the shifter to transmission this is a possible source of your “No Move” condition. When moving the shifter through its range, you should be able to feel the detents of the rooster comb inside the transmission, so if you don’t feel these, it is a dead giveaway of a cable/linkage issue. Next would be to isolate if it is inside or outside the transmission. Typically going directly to the manual linkage on the side of the transmission is the easiest way to do this. To start, remove the cable from the linkage, most times it is a ball and socket and is easily popped off with a small pry bar, or a screwdriver in a pinch. Now you can rotate the linkage by hand and feel for the detents *** DON’T ATTEMPT THIS WHILE THE ENGINE IS RUNNING *** THE WHEELS MUST BE PROPERLY CHOCKED WITH THE PARKING BRAKE SET OR THE VEHICLE WILL TAKE OFF POSSIBLY ROLLING OVER YOU. ***

If the linkage feels good it is likely not the cause but is still possible, removing the pan would be needed to inspect further, but it is not something to attempt on the side of the road.

Transfer case shifter position

If your vehicle has a selectable 4wd transfer case, there is a possibility that the t-case has inadvertently shifted to neutral. In this situation the vehicle will not hold in park with the brakes release. On the surface this could look like a transmission failure. This could be a mechanical connection or an electrical actuator like the ones pictured here.

If it’s mechanical the fix could be easy, first try to move the lever through its ranges, if that doesn’t solve it just try the same thing you did with the transmission linkage. Pry the rod off and move it manually to see if you can shift the transfer case back into gear. If it is an electric switch, it is a bit more complicated to diagnose roadside, but we recommend at least trying different transfer case ranges to see if it will go into gear.

Noise when shifting to park

If you hear a Grinding/Whining/Ratcheting noise coming from the driveline when you shift into park while the vehicle is not moving, what is likely happening is the transmission output shaft is spinning after placing the transmission into gear and you are trying to stop that spinning shaft with the park pawl. This is a clear indicator of a drive line issue post transmission like the transfer case scenario described above. This will need further diagnosis to determine where the disconnect is.

Fluid level/condition

While you are under the hood, inspect the fluid level. Check it with the engine running, and not running, if it is an older Chrysler (ex. 46R/A518), make sure to do this while the transmission is in neutral. *Note if the fluid level is higher with the engine off and it drops when the engine is started, this indicates the pump is picking up fluid from the pan.

If fluid is not being picked up (fluid level is the same, running or not) it could be a bad pump, damaged filter, filter fell out of the inlet, or the filter o -ring/gasket has failed. The hydraulic principle is the same as a drinking straw. If that straw has a crack in it above the fluid level, when you try to get liquid up all you get is air, but if you seal off the crack it will pull fluid up the straw.

Over fill

If you feel that the issue is a filter concern, you could overfill the transmission for testing purposes and get you off the road in a pinch. Typically, 3qts will be sufficient to get the fluid level above the filter and submerge the pump pickup in fluid. If the vehicle starts to move after adding this fluid the transmission may have been low on fluid or the filter has come loose or has damage.

Shift by wire

Most transmissions today don’t have a traditional shifter handle, physical cable, and manual linkage configuration. A lot of manufactures have ditched these items to go with switched input to a module that in turn sends outputs to solenoids in the transmission. These solenoids operate several functions, first there is a solenoid that operates the park pawl. The park pawl’s job is very important, it locks the output shaft and prevents the vehicle from moving. The second thing the solenoids do is apply clutch elements needed to engage the transmission in gear. This is important because, unlike earlier model transmissions that fail safe to reverse gear and one forward gear, a loss of power to the transmission from bad electrical connections, or improper signals from the control module could lead to the vehicle being stuck in park or possibly a loss of engagement (neutral).     

DTC’s

Regardless, if a check engine light is on you should always check for codes. You need to scan all modules before condemning the transmission. Today’s transmissions are influenced by other modules and inputs more than ever. The shifter position may be provided from the shifter, or in a lot of newer vehicles (as stated above) a rotary dial or push button. The input needed by the TCM (Transmission Control Module) may come from the vehicle’s BCM (Body Control Module) that is why we check all modules.

You may or may not have means to do this roadside, however, because they are inexpensive and useful, some keep a ELM327 interface to check codes.

There are a bunch of these on the market, and it is a buyer beware situation. I have had personal experience with some of the cheaper ones that do little more than turn on an LED light in the device, good luck getting anything useful out of them. We don’t have an in-depth list of devices that do and don’t work so we recommend you research them before deciding on one. (If you have a tool you would like to send for testing, we’d be happy to review it, feel free to reach out by email).

Speedo/Scan tool data

If you have access to scan codes, you may also be able to see trans data. If you can, monitor the speed sensor data parameters. What is available will vary by make & model of the vehicle, transmission type, and scan tool/diagnostic software. What you are trying to see is the Input Speed Signal (ISS) and Output Shaft Signal (OSS) here are some possible outcomes and how to interpret them *All of these scenarios are only valid without speed sensor codes present.

·        ISS reads the same or close to the same as engine RPM at idle in park or neutral and drops to zero in reverse and drive with OSS reading zero = The transmission is engaging in gear and stopping the Input shaft from turning. The vehicle should start to move if you give it throttle. If the ISS jumps up, and you don’t move, then the transmission is slipping. If the RPM goes up slightly with throttle and sounds like it is struggling, look for something binding up like a brake caliper/stuck parking brake.

·        ISS reads the same or close to the same as engine RPM at idle in any range, and increases/decreases with engine RPM, and the OSS increases without vehicle movement. = There is a disconnect in the drivetrain (transfer case/drive shaft/CV axle/differential).

·        ISS reads the same or close to the same as engine RPM at idle in any range, and increases/decreases with engine RPM, and the OSS remains zero with no vehicle movement = Transmission slipping or no engagement  

·        ISS reads zero all the time in any range regardless of engine RPM = Input shaft is not spinning. It could be a broken input shaft, the flex plate could be broken, or missing the TC hardware. *If present remove the inspection plug to see if the torque converter (TC) is spinning, if it’s not, suspect a flex plate issue.

Summary

While it is never fun to be stuck on the side of the road, there is a certain satisfaction in understanding how it all works and being able to diagnose the issue. There is even more satisfaction when you are able to fix it yourself and continue on your journey.