94 grand am tranny problem

I have a 94 pontiac grand am. I can put in into reverse but it just reves and does not move. It also has started to do this in drive somtimes. When I get it into drive and drive it some it shifts very smoothly. why could it be doing this?

There are several things which happen when the selector is placed in a gear (such as reverse). The mechanical linkage moves the arm on top of the transmission to the appropriate position. This moves the spool valve connected to the other end of the selector shaft to the correct position for the range selected. At the same time, a switch on top of the transmission moves to indicate the position of the gear selector. The ECM needs to have the switch inputs in order to operate the correct solenoids in the transmission in the correct sequence (in Reverse, both solenoids "A" and "B" must operate, just as in 1st forward range). If the linkage (cable) is loose, worn, or improperly adjusted, either or both of these functions may not be happening as intended. If the linkage is out of position another indicator would be that the back-up lights on the rear of the vehicle do not light in reverse. If this is the case, start looking at the linkage and switch.

If the linkage is correct, the transmission still requires oil pressure to operate the clutches and bands in order to engage the gear train and move the vehicle. Having clean transmission oil at the correct level, and a clean filter for it to pump through is critical. If the fluid is low or the filter is clogged the pump cannot develop the necessary pressure. If the fluid is very dirty or contaminated the pump may not develop pressure or if it does the valving and solenoids may not operate correctly, becoming stuck by the contaminants. If the fluid level is correct and the condition seems acceptable, it is possible that the pressure is still low due to a stuck pressure regulator valve or worn/sludged pump.

In short, regular fluid and filter changes are required to maintain the transmission.

Another item which affects transmission oil pressure is the vacuum modulator. It is mounted on the front of the transmission and requires a vacuum line connected to the intake to move the valve in unison with engine load. Usually, these modulators fail in a default condition which causes higher oil pressure which would not cause your symptoms, but they can stick and fail in a low pressure mode, causing slipping and poor engagement. Again, this is most common with dirty or contaminated transmission oil. In any of those cases, there is a pressure test port on top of the transmission to which a pressure gauge can be connected for diagnosis.

If all that is in good working order, there is a servo which actuates the reverse band and it may have a seal or pin problem. It is accessible from the outside of the transmission. It can be removed and inspected without the expense and hassle of removing the transmission from the vehicle.

Further, the shift solenoids and TCC pressure modulation solenoid can be tested at the electrical connector at the front of the transmission with the unit in the vehicle.

In short, it is important to understand that a good technician can perform a fairly comprehensive analysis without removing the unit, saving considerable expense. Some of the repair can also be accomplished without removing the transmission. I simply wanted to point that out before you accept the typical "It needs to be rebuilt" line from any repair shops. If they don't perform some diagnosis, they cannot be certain of that. One test you can perform to indicate whether the line pressure is low or clutches are weak is to remove the vacuum line from the modulator and cap it. If engagement is improved by doing this it may be indicative of a control or pump problem or weak clutches. Don't plan on operating the transmission with this line disconnected for more than a short test drive since further damage can result from continued operation at full pressure.

If it has to be removed, it might as well get a full rebuild, since a good portion of the labor required is in the removal and installation of the transmission, and the cost of normal rebuild parts is usually less than $200 - The labor is the largest portion of the expense. If a solenoid has failed completely, or valve body is worn, or clutches are leaking or glazed, the transmission must be dropped from the engine cradle to even access the valve body. There are many other internal problems which can cause the symptoms you reported, but most of those will require removal and disassembly of the transmission for diagnosis and repair.