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These recommendations do not cover all possibilities. They are some things to check before you have your transmission rebuilt. Always check transmission fluid level first. Always run a scanner on any computer controlled transmission. On modulated transmissions make sure they have enough vacuum.

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4L60E / 4L80E
The transmissions above, are rear wheel drive 4speed automatics that are all computer controlled. Common problems are: Check engine light comes on. The transmission shifts erratically, slips or does not shift at all.

These problems can be caused when the external sensors on the vehicle go bad. The input and output speed sensors, throttle position sensor or vehicle speed sensors need to be checked. Always run a scanner first to see if a sensor is causing your problem.  
The 4L60E transmission also has a problem with a hard 1-2 shift when hot, and a 1870 code. When rebuilding the transmission, I recommend that you always replace the valve body plate and install a sonnex Valve Kit TCC Regulator W/Sleeve. This is to help against having these problems in the future. On the 4L60E make sure you always us a AC Delco hardened shell when rebuilding the transmission.

The 4L80E transmission has a lot of problems with the input and output speed sensors on the side of the transmission. They should always be checked first if you are having a shifting problem.

 An ignition switch that goes bad can cause a false shift solenoid code that comes up on a scanner. So be careful.

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4T40E / 4T60E / 4T80E 
The transmissions above are 4 speed automatics that are computer controlled. The most common problems are: Your check engine light comes on. The transmission starts shifting erratically or not at all. Make sure you or a shop runs a scanner on the vehicle.  You want to see if a sensor has gone bad causing your problem.

Always check your modulator line on the 4T60E transmission to see if there is fluid inside. If fluid is inside the line, then the modulator is no good.

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125
This transmission is a front wheel drive, 3 speed automatic.  Common problems are: The transmission shifts soft and early. It shifts hard and late.  The engine stalls as you come to a stop when the engine is hot.

Always check the throttle pressure cable that goes from your transmission to your carburetor or throttle body depending on the configuration of your engine. This controls the shift points and sometimes the heat causes the little plastic connector that holds it on to become brittle and break, causing erratic shifting.

Now to fix the engine stalling problem. Check your lock-up solenoid in the transmission side pan. The majority of the time it sticks when the transmission becomes hot.

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2004R / 700R4
The transmissions above are rear wheel drive, 4 speed automatics. The shift timing is controlled by a throttle pressure cable that connects from the transmission to the carburetor or throttle body depending on the configuration of your engine. Always check these items first before you purchase a transmission.

Also, make sure your clutch fan on the front of the motor is not starting to seize up. This causes the engine to sound unusually loud, almost like the transmission is not shifting into 4th gear. On the 700R4 make sure you always us a AC Delco hardened shell when rebuilding the transmission.

Also, If your engine stalls when you come to a stop, check the lockup torque converter clutch solenoid. They often stick when hot.

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200C
This transmission is a rear wheel drive, 3 speed automatic.  Common problems are: The transmission shifts early and soft. It shifts late and hard. It does not shift at all.

Always check the throttle pressure cable that goes from your transmission to your carburetor or throttle body depending on the configuration of your engine. This controls the shift points and sometimes the heat causes the little plastic connector that holds it on to become brittle and break, causing erratic shifting. This style transmission also has a problem with the reverse clutches burning up at about 80,000-120,000 miles. make sure you replace the little seal in the case.

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T250 / T350 / T400
The transmissions above are rear wheel drive, 3 speed automatics.  Common problems are: The transmission shifts erratically. Loosing transmission fluid without a trace of a leak.

The Turbo 400 has a modulator on the passenger's side of the transmission pointing towards the passenger's door. The Turbo 350 has a modulator on the rear of the transmission.  Pull the rubber line off the modulator and see if transmission fluid comes out of the line.  If fluid comes out, the modulator is no good and can cause the above problems.

You should check your fan clutch from time to time. I see a lot of them seize up confusing people into thinking they need a transmission. So be careful.

When you pull the shifter from 3rd to 2nd, you should be able to feel what you would call engine braking. Now if you don 't, and if you are driving a vehicle with a T400 transmission, there is an accumulator piston, which is made of plastic, that often breaks. If you have your transmission rebuilt, always make sure they put an aluminum accumulator piston in. They don't break.

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3T40E     This transmission is a 3 speed front wheel drive automatic that is all computer controlled.  If your transmission starts shifting erratic or not at all, make sure that you or a shop run a scanner on the vehicle.  You want to see if you have a sensor that has gone bad, causing your problems.

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440T4 / 325 4L
The transmissions above are front wheel drive, 4 speed automatics. Shift timing is controlled by a throttle pressure cable that connects from the transmission to the carburetor or throttle body, depending on the configuration of your engine. Check this cable first. Make sure the cable is not out of adjustment.

The 440T4 also has a modulator that controls shift firmness. If this modulator goes bad it can shorten the life to your transmission. Pull the vacuum line off at the transmission, if there is fluid in the line, the modulator is no good.

If you have a 440T4 transmission, and your engine stalls when you come to a stop, make sure you check the lockup torque converter clutch solenoid, they often stick when hot. 

If your vehicle is equipped with a 440T4 transmission, the engine must be in good engine tune. If the engine is running rough or needs tuned up, it can shorten the life of your transmission.

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4L30E

The transmission above is a rear wheel drive 4 speed automatic, that is all computer controlled. It uses a microcomputer to judge operating conditions while monitoring the throttle position sensor and vehicle speed sensor and many others.  Common symptoms are trans shifting erratic, slipping or not shifting at all. external sensors on the vehicle like the winter mode switch, throttle position sensor and vehicle speed sensor, quite often go bad causing these symptoms.

Always run a scanner first to see if a sensor is causing your problem. I recommend that you change the transmission fluid on this style transmission about every 12,000 to 15,000 miles.


 

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