Transmission Diagnostics, Larry Carley, ImportCar, January 2001

When a customer comes to you with an automatic transmission problem, what do you do? Refer him to a transmission shop or do you tackle the job yourself?

Common problems include things like slipping, delayed engagement when the transmission is put into gear, delayed shifts, harsh shifts or engine flair during shifts, unusual noises, or loss of one or more gears. In some cases, the underlying cause might be something as simple as a low fluid level in the transmission or a badly plugged filter. Other times, you might find a leaky or loose vacuum line, or a broken or misadjusted throttle cable.

On electronic transmissions, the interaction between the transmission and engine control electronics can produce some really weird symptoms. For example, transmissions that suddenly go into a "limp in" mode and stay there for no apparent reason (this is often due to a bad speed sensor or wiring problem that affects communication between control modules).

Diagnosing transmission problems requires special expertise as well as the right tools. For one thing, you need a good transmission pressure gauge — and the pressure specifications for the vehicle you’re going to diagnose, along with any adjustment procedures or other service information that may be required to fix the problem.

If you’re working on a late-model import with an electronic automatic, you’ll also need a scan tool with the right adapters and software that can access transmission codes if the transmission control module doesn’t provide LED codes. You’ll also need the appropriate diagnostic charts for the vehicle so you can follow up with additional tests as needed after you’ve pulled one or more codes.

CHECK THE FLUID FIRST
Anytime a customer complains of slipping, delayed engagement or erratic shifting, the first thing to check is the fluid level in the transmission. On most vehicles, the fluid must be hot and the engine idling with the transmission in park or neutral to get an accurate reading.

If the fluid level is low, it probably means the transmission or transaxle is leaking. Check the tailshaft output seal if it’s a rear-wheel-drive vehicle, or the transaxle halfshaft seals if it’s a front-wheel drive. If there’s nothing wrong with the transmission, adding the proper ATF should restore normal operation. But if the leak isn’t fixed, sooner or later your customer will be back — and maybe with even worse transmission problems because the fluid level got too low.

Note the condition of the fluid. If the fluid is badly discolored, it indicates oxidation and a need for replacement. A burnt odor would also tell you the fluid has gotten too hot and needs to be replaced.

A quick way to test for worn transmission fluid is to do a "blotter" test. Place one or two drops of ATF from the transmission on a paper towel and wait 30 seconds. If the spot is widely dispersed and red or light brown in color, the fluid is probably OK. But if the spot does not spread out and is dark in color, the ATF is oxidized and should be changed.

If the fluid needs to be changed, a simple drain and fill (dropping the transmission pan and replacing the filter) is better than no maintenance at all. But a simple drain and fill procedure leaves up to three-quarters of the old fluid inside the torque converter. A better approach is to use flushing equipment to achieve a total fluid exchange.

Follow the vehicle manufacturers’ recommendations as to the type of ATF to use. Honda, Toyota and Mitsubishi all specify their own special fluids (Honda Genuine, Toyota Genuine and Mitsubishi Diamond fluids).

PRESSURE TESTS
If the fluid appears to be fine, the next thing to check would be the operating pressure inside the transmission or transaxle. The Automatic Transmission Rebuilders Association (ATRA) says many common transmission problems can be quickly diagnosed using a pressure gauge. But many technicians don’t use a gauge to check operating pressures because they don’t know how to interpret the readings. Even so, pressure readings can provide valuable clues as to what’s going on inside a transmission.

One way to learn how to use a gauge is to hook up one to your own vehicle, then watch the gauge readings while you’re driving to and from work to see how they change. The same thing can be done with other vehicles that don’t have transmission problems to develop a feel for what "good" gauge readings look like. Once you’re comfortable with using a pressure gauge, you can then use it to diagnose vehicles that are experiencing transmission problems.

Before you take any readings, you should first look up the reference specifications for the transmission in a shop manual or your electronic database, along with any illustrations that might be needed to help you locate the correct connection for your pressure gauge.

When doing pressure tests, ATRA says to start with mainline pressure. Mainline pressure should be checked in each range: Park, Reverse, Neutral, Drive and each individual gear (1, 2 and 3). Each range, except Park and Neutral, should be checked under three conditions: slow idle, fast idle and wide-open throttle. Write down the readings for later reference.

If all pressures are within the manufacturers’ specifications at slow idle, the pump and pressure regulator are functioning properly.

If all pressures are low at slow idle, it indicates a potential problem in the pump, pressure regulator or filter, a low fluid level or internal leakage. To isolate the problem, increase engine rpm to fast idle and recheck the pressure readings in each gear range again. If all the pressures now read normally, it usually indicates a worn pump, but the problem could still be attributed to internal leaks.

Internal leaks will usually show up in a particular range. For example, a forward clutch leak would have normal pressure in Park, Reverse and Neutral, but have low pressure in all forward ranges. A direct clutch leak will show a pressure drop when the transmission shifts to third and low pressure in reverse because, in most cases, the direct clutch is on the third and reverse gears.

A restricted filter will usually show up as a gradual pressure drop at higher engine rpm because the filter cannot pass as much fluid as the pump is trying to draw.

A stuck pressure regulator valve will show up as fixed line pressure, which means the same pressure all the time. The pressure may vary with engine rpm, which means low pressure at slow rpm and higher pressure at higher rpm. There will be no boost in pressure from the throttle valve or modulator system and no reverse boost.

If pressures are high at slow idle, it indicates a pressure regulator or throttle pressure problem. On most cars, the modulator controls throttle pressure. If the transmission has a throttle pressure tap, it will tell you if the throttle pressure circuit is the problem. On vehicles without a throttle pressure tap, remove the throttle valve plunger. If line pressure is now normal, then it is a throttle valve problem. If pressure is still high, it’s a pressure regulator problem.

Pressures also need to be checked at stall or wide-open throttle (WOT). Before you do a stall test, inspect the vehicle for broken motor or transmission/transaxle mounts. Also, check the brakes. The brakes must be strong enough to hold the vehicle while the engine is revving with the transmission in gear. Set blocks or chocks in front of the wheels, set the parking brake for added insurance and make sure no one is standing in front of the vehicle when you’re doing the test.

To do a stall test, put the gear selector in the range to be tested. With one foot firmly on the brake pedal, press the accelerator to the floor and note your pressure reading. Some technicians will pull off the vacuum line or pull the throttle valve cable with the engine at fast idle. This is not a normal operating condition and will prevent you from detecting problems that may be due to trapped vacuum or a misadjusted cable.

If all pressure readings at stall speed are low, then you should pull the throttle valve cable to maximum, or disconnect the vacuum line. If the pressure readings are now within specifications, the problem is in the cable or vacuum system. If idle pressures are normal, then the problem is in the throttle system only.

The reverse stall test is also a maximum pump output test. If you suspect a weak pump, this test will help you confirm your diagnosis. Often this will show up as low pressure at reverse stall, but all other pressures (including idle) will be normal.

If the transmission has an internal problem and you end up replacing or overhauling it, don’t forget to flush the ATF oil cooler lines — especially if the old transmission has failed completely or the fluid is contaminated with debris.

TORQUE CONVERTER CHECKS
Torque converter problems may be confused with transmission problems and vice versa, so it’s essential that you check this important component, as well.

One of the classic symptoms of a bad torque converter is a vehicle that is sluggish or lugs when starting out from a dead stop. The vehicle may feel as if it is in second or third gear rather than first gear. What’s happening is the torque converter is failing to provide any torque multiplication because the one-way stator clutch is not holding, allowing the stator wheel to slip.

To test the torque converter, you need to perform a stall test. This will reveal the maximum engine rpm the converter can handle with the brakes applied.

Before doing the test, look up the stall speed specifications for the application and follow the same precautions as when doing a stall test to check transmission line pressures.

Connect a tachometer or scan tool to the vehicle to monitor engine speed, then start the engine and place the transmission in Drive. While pressing firmly on the brake pedal with your left foot, press down on the accelerator pedal to increase engine speed until engine rpm will go no higher. Then release the accelerator pedal and allow the engine to return to idle. Note the maximum stall speed rpm (typically 1,500 to 2,200 rpm) and compare to specs.

If the engine stall speed is higher than specifications, the transmission is slipping (probably a holding clutch or band). If the engine stall speed is lower than specifications, the torque converter is defective (stator is slipping), or the engine is not producing normal power (engine problem).

Note: The duration of a stall speed test should not exceed five seconds. During this test, the churning of the fluid inside the converter produces a lot of heat, so let things cool down for a few minutes before repeating the test (if necessary). For this reason, some vehicle manufacturers do not recommend performing a stall test.

Sometimes a stator wheel will hang up and remain locked at high speed. If this happens it will act like a speed brake, causing the torque converter to fight against itself. The car may start out and drive normally at low speed but, as speed starts to build, the engine will strain harder and harder to overcome the resistance. If the engine won’t rev much beyond 3,000 rpm in neutral, the stator is definitely hung up.

Another driveability problem that may be blamed on the torque converter is a driveline shudder. A shudder may occur if the torque converter lockup clutch has problems engaging, usually because of low apply pressure. In most causes, the low pressure is an internal transmission problem and not a bad torque converter.

If a vehicle experiences a vibration or rattling sensation while driving, the torque converter may be transmitting an engine misfire through the driveline. The problem here isn’t a bad torque converter but, rather, whatever is making the engine misfire.

If a vehicle’s fuel economy suddenly drops three to four miles per gallon, but the engine is running normally (no misfire or loss of power), the problem may be a lockup torque converter that isn’t locking up. If the torque converter clutch (TCC) fails to engage, the vehicle will still drive normally but with increased slippage between the engine and transmission. The underlying cause is usually a bad TCC solenoid, but loss of the vehicle speed input signal to the computer may also prevent the computer from energizing the TCC solenoid.

One way to see if the computer is energizing the TCC solenoid is to connect a scan tool to the vehicle and take it for a test drive. The TCC on/off command should change as the vehicle speed changes. No change? Then check for a vehicle speed signal.

To test for proper lockup, connect a tachometer to the engine or use a scan tool to monitor engine rpm. Then take the vehicle for a test drive on a flat level road. At 50 mph, lightly apply the brake pedal — not enough to actually slow the vehicle, but just enough to momentarily disengage the torque converter clutch. You should see about a 150-250 rpm increase in engine speed if the TCC is working properly. The rpm should then drop the same amount when you take your foot off the brake and the clutch re-engages. No change in engine speed would tell you the converter isn’t locking up.

Sometimes a torque converter clutch will fail to release. When this happens, it can stall the engine when the vehicle comes to a stop. The problem here is usually a defective TCC solenoid.

Noise is another condition that may be blamed on a bad torque converter. But unless the mounting bolts have worked loose, the most common cause of clunks and rattles is either a broken flexplate, or loose motor or transmission mounts.

Fluid leaks in the vicinity of the torque converter are usually due to a bad front transmission input shaft seal. A common mistake that can cause this to happen is allowing the torque converter to hang on the transmission input shaft when replacing an engine. The weight of the torque converter can deform the seal lip and cause it to leak. Of course, old age can make a seal leak, too. Fluid leaks may also occur if the torque converter housing is cracked or damaged.

Another problem that affects the operation of the torque converter is "transmission drainback." One-way check valves normally keep the torque converter full of fluid when the engine is shut off. But if the transmission input shaft bushings are worn, it can create a path for fluid to drain out of the converter back into the transmission. The loss of fluid from the torque converter can make the transmission feel as if it is slipping on initial take-off for about five to 10 seconds.

If a torque converter fails a stall test or is leaking, it must be replaced. Many experts also recommend replacing the torque converter if an automatic transmission or transaxle has failed and is being replaced. Why? Because the torque converter acts like a garbage pit and traps a lot of the debris that results from normal wear. The debris may then contaminate and damage the new transmission. Why not just rinse out the old torque converter with solvent? The design of the wheels and large surface area inside the converter make it very difficult to clean.

What’s more, by the time most transmissions need to be replaced, the friction linings inside a lockup converter are reaching the end of their service life. Reusing a lockup torque converter with worn linings is asking for trouble. And since there’s no easy way to open up and clean or rebuild the internal components (doing so requires cutting open the housing on a lathe and rewelding the housing to seal it back up), replacement is your only service option.