Why a Car's Automatic Gearbox Slips and What Causes It

An automatic transmission that slips is showing you the beginning of a significant fault, and it is worth acting on early because the repair cost rises sharply once it becomes a complete failure. There are only a few causes, and the pattern in which the slipping appears identifies which one is in play.

This guide covers what slipping looks like, the causes, and what to do about it.

What Slipping Feels Like

Slipping means the transmission is not transmitting the full engine torque to the wheels. The symptoms are distinctive when you know to look for them.

The revs rise without the road speed rising proportionately, particularly on a hill or under load. The vehicle takes longer to accelerate than it should. There may be a slight surge or shudder during the acceleration. In a torque converter automatic the same symptom is the converter slipping rather than the clutch packs, and it feels similar from the driver's seat.

A distinction worth making early: a gear selector that slips out of a gear position on a manual or an automatic selector is a completely different fault, and it is mechanical rather than hydraulic. On an automatic selector, worn detents or a worn selector mechanism produce a click rather than a transmission slip, and the fix is entirely different.

The Causes

Low Transmission Fluid

The first thing to check and the cause of a surprising proportion of slipping complaints. The fluid is a hydraulic fluid as well as a friction material, and it transmits the pressure that applies the clutches and the bands. When the level is low, the pressure falls and the clutches cannot hold their torque. When the fluid is contaminated or burnt, it loses its friction characteristics and cannot make the clutches grip.

Two symptoms point at low or degraded fluid: slipping that appeared gradually, and slipping that is worse when the transmission is cold and improves as the fluid warms. Where the fluid is badly degraded it will often smell burnt, and that is a significant finding. Checking the level and the condition of the fluid is inexpensive and it is the first step in any diagnosis. The interval and the specification are in our guide to transmission fluid.

A Worn Torque Converter

The torque converter multiplies engine torque at low speed and locks up above a threshold to improve efficiency. A converter that cannot lock transmits power through fluid friction rather than mechanically, which means significant energy loss, and the symptom is slipping that is present from a standstill and most noticeable on a hill or at low speed in heavy traffic. A damaged converter also produces a shudder on a light throttle from rest, and it may produce a whine. A converter that is damaged is normally replaced with the transmission rather than separately, and it is the reason a torque converter transmission rebuild costs what it does.

Worn Clutch Plates and Bands

The internal friction elements are consumable items with a defined life. When the friction material on a clutch plate wears through, the plate cannot transmit torque, and the transmission slips in the gear that uses that clutch. Worn bands and worn servo pistons produce the same thing. The pattern of which gear slips identifies which clutch or band is involved, and a vehicle that slips only in third and fourth has a different cause from one that slips in all forward gears.

A Faulty Valve Body or a Solenoid

The hydraulic control that applies the clutches and the bands is operated by solenoids that are commanded by the control module. A solenoid that has failed, or a valve body passage that has worn or a gasket that has failed, means the pressure does not reach the clutch. The symptoms are abrupt: harsh engagement, hunting between gears, and slipping in one gear rather than a gradual decline across all gears. A shift solenoid failure is much cheaper to fix than a worn clutch, which is why the diagnosis matters.

Internal Leaks

The pumps, the valves, and the clutches all leak internally to some degree, and the seals that stop them deteriorate with age. An internal leak means pressure is lost without any external sign, which is why the fluid level can be correct while the transmission still slips. Internal leakage increases with wear and with the pressure that the transmission produces, so it is a progressive fault. This is one of the cases where a rebuild is the only answer.

Incorrect Fluid or a Failed Cooler

A transmission running on the wrong fluid will slip, and it is not unusual for a vehicle to have been serviced with the wrong specification, particularly where a vehicle has been owned by several people. A failed transmission cooler or a blocked cooler, on a vehicle with an external cooler, allows the fluid to overheat, and overheating causes slipping as well as accelerating the wear of everything else. Overheating is the single worst thing that happens to an automatic and it is what turns a repairable fault into a total failure.

Why It Is Urgent

Slipping produces heat. A slipping clutch pack generates heat in the fluid, the fluid degrades faster, degraded fluid means less friction, and less friction means more slip. That cycle runs fast, and a transmission that slips for a few thousand kilometres can go from a solenoid replacement to a complete rebuild.

There is also a threshold where the friction material is worn through and the steel plates begin to wear, and once that happens the damage is not reversible with a fluid change and a solenoid. Heat also cooks the internal seals, which is what turns a slipping transmission into a failing one.

Stop driving the vehicle if the transmission is slipping badly or if it has started to shake. Recover it rather than driving it, because the difference in the repair cost is substantial.

Diagnosing It

Establish the pattern: which gears slip, whether it is worse cold, whether there is a shudder, whether there is a harsh engagement, and whether it is present at all speeds or only at a particular speed range. Then check the fluid level and condition, which is the cheapest test and the most informative.

Read the fault codes, because a transmission control module on a modern vehicle records the conditions and the solenoids involved, and that immediately distinguishes a hydraulic control problem from a friction element problem. Where the vehicle has a service history with dates and mileages, check whether the fluid has been changed, because a transmission that has never been serviced is the most likely to be slipping.

Pressure testing confirms whether the hydraulic system can produce the pressure it should, and a stall test confirms the clutch holding capability. Both are specialist tests and both are what distinguishes a solenoid from a worn clutch.

Preventing It

Servicing the fluid on the interval the specification requires, with the correct fluid, is the entire preventive measure and it is the one most commonly skipped. The second is avoiding conditions that overheat it: towing beyond the vehicle's rated capacity, carrying heavy loads on long climbs, and using the vehicle exclusively in stop-start traffic. The third is not ignoring it: a vehicle that slips in one gear on a Tuesday may not start on Monday.

Those items and the rest of the routine are covered in our vehicle maintenance guides, and the arithmetic on whether a rebuild is worth doing is in what a transmission rebuild involves. If this guide was helpful, you may also want to read why a car loses power on hills for the related complaint where the slope makes it obvious, and why a car loses power in low gears for the low-speed version of the same symptom.