Transmission Problem Guide
     
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Just like that of a manual transmission, the automatic transmission's primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds. An automatic transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios. In an automatic transmission; the same set of gears produces all of the different gear ratios. The planetary gear set is the device that makes this possible in an automatic transmission.

 

Transaxle:

Basically a transaxle is a fully automatic electronically controlled front wheel drive transaxle with four forward gear ranges including overdrive and one reverse gear range. A variable displacement vane type oil pump supplies oil pressure. Two shift solenoids, operated by the Power train Control Module (PCM), control shift points and TCC apply rates, which are influenced by various sensor inputs. The transaxle unit includes a three element hydraulic torque converter and lockup clutching element, four multiple disc clutch packs, roller clutch, sprag clutch, two bands and a compound reaction planetary gear set. Power is transmitted to the drive wheels from the planetary gear through a final drive gear set and differential assembly.

 

Transmission:

The normal automatic transmission is fully automatic with four forward and one reverse speed. It consists of a lockup torque converter assembly and a three unit planetary gear train. Electronic controls for the transmission are integrated into the on-board Power train Management systems. Sensors provide inputs to the Power train Control Module (PCM). The PCM electronically controls the transmission hydraulic system for gear selection and automatic shift control. Torque converter operation is controlled by the PCM. These controls, along with hydraulic controls in the main control body, operate a piston plate clutch in the torque converter that eliminates torque converter slip when applied.


Testing:

There are several ways to test an automatic transmission. There are some that can only be done by a shop that has the proper equipment, and others you can do at home.

 

Air Pressure Tests

To test the bands and servos inside the transmission, air pressure can be substituted for fluid pressure. When compressed air is applied to certain ports, it will apply the particular band or servo. When a band or clutch engages a dull thud will be heard. There should be no hissing heard while band or servo is engaged. This can only be done by a repair shop.

 

Line Pressure Test

Transmissions need to operate with the proper fluid pressure. If the fluid pressure is too low or too high, shifting can be too soft or too hard. Different parts of the transmission operate with different fluid pressures and a transmission pressure gauge can be attached to a transmission to check these pressures. These readings can be compared to a chart that will pinpoint the most likely cause of a problem.

 

Shift Point Road Tests

Each shift in a transmission is supposed to occur at a specific time under certain throttle conditions. For example under light throttle a 1-2 shift will take place sooner then a 1-2 shift under heavy throttle. By knowing when the shifts take place and when they are supposed to take place you can figure out what is wrong with it.

 

Transmission Fluid Leak Tests

This one is kind of self-explanatory. You can use a special dye to find a transmission leak. Oil-soluble aniline or fluorescent dyes premixed at the rate of one half teaspoon of dye powder to one half pint of transmission fluid may be useful in locating the source of a leakage. If the transmission fluid in the radiator cooler leaks into the coolant system, a black light must be used to see the fluorescent dye solution.

Fortunately there are not too many places an automatic transmission can leak.

 

To do a through check:

Check speedometer cable connector at the transmission. Replace rubber 0-ring, if necessary.

Check oil pan gasket. Tighten attaching bolts to proper torque and/or replace gasket, if necessary.

Check fluid filler tube connection at transmission case.

Check filler tube 0-ring seal for damage. If leakage is found, install new short oil inlet tube and 0-ring seal or clean the area and apply sealer around the tube.

Check fluid lines and fitting between transmission and cooler in radiator tank for looseness, wear, or damage. If leakage is found, tighten fitting to proper torque, or replace the damaged parts.

Check engine coolant in radiator. If transmission fluid is present in coolant, the cooler in the radiator is leaking.

If leakage is found at the downshift control lever and manual lever shaft, replace seal.

Inspect pipe plugs on sides of transmission case. Tighten plug or replace, if necessary.

 

Fluid leakage from the converter housing may be caused by engine oil leakage past the rear main bearing, by oil galley plug leaks, or by power steering fluid leakage from steering system. Determine the exact cause of the leak before beginning service procedures.

If you have leakage from the converter housing, here's what you can do to isolate the cause:

Fluid leaking by the front pump seal lip will tend to move along the impeller hub and onto the back of the impeller housing.

Fluid leakage by the outside diameter of the seal and front pump body will follow the same path as the leaks from the front pump seal.

Fluid that leaks from a front pump-to-case bolt will be deposited on the inside of the converter housing only. Fluid will not be deposited on the back of the converter.

Leakage by the front pump-to-case gasket may cause fluid to seep down between the front of the case and converter housing.

Fluid leakage from the converter-to-flywheel stud weld will appear at the outside diameter of the converter on the back face of the flywheel, and in the converter housing only near the flywheel.

 

The following areas of possible leakage should be checked to determine if engine oil is leaking.

Leakage at the rocker arm cover may allow oil to flow over the converter housing and cylinder block, causing oil to be present in or at the bottom of the converter housing and cylinder block, causing oil to appear at the bottom of the converter housing.

Oil galley plug leaks will allow oil to flow down the rear face of the block to the bottom of the converter housing.

Leakage by the crankshaft seal will work back to the flywheel, then into the converter housing

Fluid leakage from other areas, such as the power steering system forward of the transmission, may cause fluid to appear around the converter housing due to blow-back or road draft. The following procedures should be used to determine the cause of leakage before any repairs are made.

   


Diagnosis By Symptom

I saved this one for last because as a DIY, this is how you will most likely diagnose a problem with your transmission. Probably the most common way to diagnose a transmission is by seeing what it does. Specific symptoms have specific causes and a good transmission man will know what parts are bad by taking the vehicle on a road test. Here is a list of some of the more common symptoms and possible causes, including some that a DIY can handle at home.

Remember that these are general causes for these symptoms and some may not apply to your vehicle. For more detailed information pertaining to your specific vehicle, consult an appropriate repair manual.

I dare say that unless you know your way around the inside of a transmission, some of these terms will be unfamiliar to you. It's a good bet that if you don't know what it is, you should take it to a shop

 

As you can see from looking at this list there are a number of things that can cause the same problem. Most of them can only be repaired by a shop with the proper tools. This list is to give you an idea what may be wrong with your car so you will be more informed when you have to take your transmission in for repair.

Before condemning the transmission because of a problem, I always recommend that the fluid and filter be changed. Not a transmission flush, just the filter and whatever fluid comes out when you drop the pan. This could fix a number of symptoms that may seem like a big problem, but are caused by a clogged transmission filter.

One very important thing is the Automatic Transmission Fluid (ATF). Different ATF can change the way a transmission shifts or cause it not to shift at all.

Your owners manual will specify what type of transmission fluid you should put in your transmission. If you have any doubts, go to your local Dealers parts department and they will have the proper fluid for your transmission.

The modern automatic transmission is a very sophisticated piece of machinery. For it to do what it does in such a small package and to do it for mile after mile is nothing short of wondrous. But because of its complexity, they require special skills and tools to properly repair or rebuild one.

 

Delayed Or Soft Forward And Reverse

Improper fluid level.

Fluid contamination.

Improper band/clutch application pressure.

Filter damaged or plugged.

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Main control solenoid stuck or damaged.

Pump assembly bolts out of specification.

Pump gears damaged or cracked.

 

Delay Or Soft Reverse

Improper fluid level.

Filter damaged or plugged.

Transmission shift cable and bracket damaged or misadjusted.

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Main control solenoid stuck or damaged.

Overdrive one-way clutch damaged.

Forward clutch piston check ball not seating.

Forward hub seal rings damaged.

Forward clutch piston seals cut/worn.

Rear one-way clutch damaged.

 

Engine Will Not Crank

Electrical component failure, connection, or wiring harness damage to:

Transmission Range (TR) sensor.

Powertrain Control Module (PCM).

Transmission shift cable or bracket damaged or misadjusted.

TR sensor misadjusted.

Front pump seized.

Flywheel damaged

 

Harsh Forward And Reverse

Improper fluid level.

Improper band/clutch application pressure.

Filter damaged or plugged.

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Main control solenoid stuck or damaged.

Pump assembly bolts out of specification.

Pump gears damaged or cracked.

Overdrive one-way clutch damaged

 

Harsh Shifting Felt

Improper fluid level

Improper band/clutch application pressure.

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Main control solenoid stuck or damaged.

Intermediate band improperly adjusted.

Overdrive band not properly adjusted.

 

Harsh 1-2 Upshift

Intermediate band adjustment incorrect.

Intermediate band seal or piston damaged.

Intermediate band friction element or lever rod damaged.

Direct clutch drum damaged, outer diameter out of specification.

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Main control solenoid stuck or damaged

 

Harsh 2-3 Upshift

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Main control solenoid stuck or damaged.

Improper band/clutch application pressure.

Center support seal rings or groove damaged.

Center support outside diameter or case bore, center support damaged or leaking.

Direct clutch assembly friction elements burnt or missing.

Direct clutch check ball missing, not seated.

Direct clutch brake drum, intermediate band improperly adjusted or damaged.

Intermediate servo piston, spring, lever, or strut damaged.

 

No Forward

Transmission shift cable broken or misadjusted.

Improper fluid level.

Improper band/clutch application pressure.

Fluid filter damaged or plugged.

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Pump assembly damaged or leaking.

Forward clutch assembly burnt, damaged, or leaking.

Rear one-way clutch worn/damaged.

 

No Forward In Drive And No Reverse

Improper fluid level.

Improper band/clutch application pressure.

Filter damaged or plugged.

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Main control solenoid stuck or damaged.

Pump assembly bolts out of specification.

Pump gears damaged or cracked.

Overdrive one-way clutch damaged

 

No Reverse

Improper fluid level.

Transmission shift cable broken or misadjusted.

Improper band/clutch application pressure.

Fluid filter damaged or plugged.

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Pump assembly bolts out of specifications.

Reverse clutch burnt, worn, or leaking.

Direct/reverse servo piston seal cut/leaking

 

Noise Or Vibration In Forward Or Reverse

Improper fluid level.

Transmission shift cable and bracket damaged or misadjusted.

Transmission Range (TR) sensor out of adjustment.

Improper band/clutch application pressures.

Pump internal leakage, cavitation.

Pump bolts out of torque.

Overdrive one-way clutch damaged, worn, or improperly assembled.

Rear one-way clutch assembly damaged, worn, or improperly assembled

 

Shift Timing Erratic Or Hunting

Improper fluid level.

Fluid contamination.

Filter damaged or plugged.

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Main control solenoid stuck or damaged.

 

Transmission Overheating

Main control assembly gaskets damaged or bolt out of torque.

Main control separator plate damaged.

Main control solenoid stuck or damaged.

Faulty Powertrain Control Module (PCM).

Seized torque converter one-way clutch.

Excessive slip.

Restriction in oil cooler or tubes.

Improper band/clutch application pressures.

 

Transmission Noisy, Other Than Valve Resonance

Check for gear noise to verify if within normal range.

Check linkage for proper adjustment, wear, or damage.

Check fluid for proper level and/or contamination.

If fluid level is low, add specified fluid to bring level within cross-hatched area at operating temperature.

If fluid is contaminated, disassemble, clean, and service transmission, flushing torque converter and cooler.

Perform Stall Test. If noise does not stop,check extension housing bushing,seal or propshaft. If noise stops, proceed as follows:

Run transmission in all gears and check for noise.

If noise does not stop in any gear, remove speedometer gear and check for noise. If noise stops, replace speedometer gear.

planetary and/or one-way clutch.

If noise stops in 2nd, High, and R only, service reverse planetary.

If noise stops in High only, service both planetary sets.

 

 

This Transmission problem guide was written by Vincent Ciulla, who has been a professional automotive technician for over 30 years and is a certified master technician.

Vincent writes and presents various auto related articles at www.autorepair.about.com

 
 
   
 

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