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Automatic Transmission Slipping

by Larry Carley copyright

An automatic transmission should engage smoothly and firmly when you place the gear selector into Drive. If your transmission hesitates to engage when you put it in Drive, or it flares or slips when upshifting, or it slips while driving, it is not working properly.

Transmission slipping may be caused by several things:
* A low fluid level
* Low internal fluid pressure
* Faulty pressure regulation or control

Transmission Slipping Caused By Low Fluid Level

This is one of the most common and easily fixed problems that can cause transmission slipping. For the torque converter to transmit engine torque to the transmission, the converter must be filled with fluid. Also, there must be enough fluid to generate the internal pressure required to engage the various gears. If the fluid level in your transmission is low, the transmission may hesitate for a few seconds before engaging when you first put it into Drive or Reverse.

The first thing you need to do is to check the fluid level in the transmission. If you don't know where the dipstick is for your transmission, look in your Owners Manual.

To get an accurate reading, the fluid in the transmission must be warm. This may require driving your vehicle for a few miles to warm up the fluid.

Also, on most vehicles the fluid level must be checked while the engine is idling and the transmission is in PARK.
If you check the fluid level with the engine off, the dipstick reading will NOT be accurate and will read higher than normal because some of the fluid that would normally be circulating inside the transmission and torque converter will have drained back into the transmission pan. That's why the fluid level must be checked while the engine is idling and the fluid is circulating in the transmission. Set the parking brake and make sure the transmission is in Park before you check the fluid level.

If the fluid level is not between the ADD and FULL marks, add the specified fluid to SLOWLY bring the fluid up to the FULL mark.
Add only ONE PINT of fluid at a time before rechecking the dipstick reading. On many transmissions, it only takes about one pint of fluid to bring the level up from the ADD line to the FULL line. You do NOT want to overfill the transmission because this can cause fluid leaks and fluid aeration (that will cause additional shifting and slipping problems).

WARNING: Make sure you add the proper type of ATF that is specified for your transmission. According to Dallas Transmission Repair, using the wrong type of ATF can cause shifting and slippage problems, and may even damage your transmission!

The type of ATF specified for your transmission is often marked on the dipstick cap or the dipstick itself. The type of ATF required can also be found in your Owners Manual. If you need additional guidance, see this listing of Automatic Transmission Fluid Applications.

If adding fluid and bringing the fluid level up to the FULL mark fails to eliminate the slipping problem, the problem is one of the following causes.

Transmission Slipping Caused By Low Fluid Pressure

An automatic transmission must generate a certain amount of internal pressure to work properly. If the pump is worn or the fluid filter or pickup tube is obstructed, the transmission may not create enough pressure to engage and shift normally.

A worn pump is bad news because it means the transmission will need to be rebuilt or replaced.

A plugged transmission filter, on the other hand, can be remedied by removing the transmission pan, draining out the fluid and replacing the filter with a new one. Be sure to clean the pan thoroughly before it is remounted on the transmission.

If a filter and fluid change do not cure the slipping problem, your transmission may have a worn pump or a problem in the valve body that regulates pressure, engagement and shifting. This will require the skills of an experienced transmission technician to diagnose.

Transmission Slipping Caused by Faulty Pressure Regulation or Control

The internal pressure inside an automatic transmission is controlled by a pressure regulator or pressure solenoid valves. Part of the diagnostic procedure is to attach a pressure gauge to the transmission so control pressure readings can be taken with the transmission in various gear positions while the engine is idling. Lower than normal readings in any gear position will reveal which circuit is malfunctioning. The fix may only require replacing a pressure regulator valve or control solenoid or sensor. But more often than not, the fix will require rebuilding or replacing the transmission.

On newer electronic transmissions, the status of the various transmission control valves, solenoids and sensors can be displayed on a scan tool. This usually requires a high end (expensive!) scan tool or a factory scan tool that can show all of the transmission data PIDs. A scan tool is also helpful for diagnosing torque converter clutch (TCC) problems. It will show you if the clutch is engaged or not, and you can compare transmission shaft speeds against engine RPM to see if the torque converter clutch or transmission is slipping. A basic code reader or scan tool is all you need to check for transmission-related codes.

Automatic Transmission Repair Options

The average motorist is at a disadvantage when it comes to automatic transmission diagnosis and repair because transmissions are very complex and not well understood by the general public. In many instances, a repair facility will recommend rebuilding or replacing a high mileage transmission rather than attempting to repair it because they know from experience such repairs are often a temporary fix. Sooner or later you will be back with another transmission problem.

Don't waste your money on transmission fluid additives if you are having a transmission problem and are hoping for a cheap fix. It won't happen. The damage has already been done and you will probably need a new transmission. Additives can slow down fluid leaks in older transmissions. They can also provide additional wear protection for transmissions that are in good working condition. But there is no miracle cure in a can.

If your transmission has reached the end of the road and needs to be rebuilt or replaced, you have four repair options: a new transmission (very expensive and may not be available from a new car dealer), having your old transmission rebuilt, replacing your old transmission with a remanufactured transmission, or replacing your old transmission with a used transmission from a salvage yard.

A used transmission can save you money but you want to make sure the used transmission has been tested and comes with a warranty. Guarantees on used transmissions typically range from 30 days up to 1 year - but do NOT cover installation labor. If you are shopping for a good used transmission, try to find one from a low mileage vehicle that has been wrecked.

A rebuilt or remanufactured transmission should also come with some kind of warranty. Typically, these range from 90 days up to 3 years (longer is better!). If a shop is rebuilding your old transmission or replacing it with a reman unit, the guarantee will usually cover installation labor, too - which often costs as much if not more than the transmission itself.

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