I have a 98 Bravada.
The Transmission failed and I had another "good" one swapped in.
Now it works fine when it is just started, but within 10-30 minutes I lose all gears.
Shutting the engine off for about a minute will let it work again for a minute or two.

Any clues ?

I only use this vehicle for light towing, but I have nothing else that will handle a trailer and would prefer not to have to shoot it.

 

If you are loosing all gears forward & reverse, you must be loosing line pressure. If you are loosing the upshifts in drive or overdrive ranges only, I would look at either a defective MPS (manifold pressure switch) or loss of power to the EPC (electronic pressure control) solenoid. The shift solenoids are both off in 3rd gear which is limp mode. Limp mode is 3rd. gear starts and a manual 2nd gear. This allows the driver to get it home or to a shop for repair without further damage to the transmission. more information is needed. does it have any codes for transmission failures? you made need a better quality scan tool to communicate with the transmission side of the PCM to retrieve transmission codes.

 

I will check further, but I am pretty sure I am losing everything.
I can tell that it is getting ready to go.

I will check codes.

Are the codes persistent or do I have to get it to fail to check them ?

Thanks.

 

Codes come in three ways;

History (Permanent), Current & Pending here is a reprint of ALLDATA explanations

A DTC is directly related to a fault discovered during a diagnostic test. The PCM records a DTC based on a failed test during one or more trips. Certain failures must occur during two consecutive trips before a DTC is recorded. When a fault is detected, the related DTC is classified as one of four types: Type A, Type B, Type C, and Type D.
Any of the four DTC types set when the test enable criteria is met, the test is run, and it results in a detected failure. However, Type B DTCs require two consecutive trips with a failure detected to set a DTC and turn on the MIL. Some failures that are typically type B DTCs have certain conditions when they will illuminate the MIL and set a code on the first trip.
Type A

1. Emissions related.
2. Illuminates the MIL on the first trip, flashes the MIL if it is catalyst damaging.
3. Stores a freeze frame of the first failure if memory is empty.
4. Stores in history on the first trip (some models).
5. Stores a fail record.
6. Updates the failure record each failure.

Type B

1. Emissions related.
2. Pending after one trip failed.
3. Pending code disarmed after one trip passed.
4. Illuminates MIL after second trip failed.
5. Stores freeze frame data of the first trips failure, if freeze frame is empty.
6. Stores in history on the second trip failed (some models).
7. Stores a fail record on the first failure.
8. Updates the fail record each failure.

Type C

1. Failure does not cause emissions greater than 1.5 times FTP standard.
2. Illuminates a service lamp or message (not the MIL) on the first trip.
3. Does not store a freeze frame.
4. Stores in history on the first trip (some models).
5. Stores a fail record.
6. Updates the fail record each failure.

Type D

1. Failure does not cause emissions greater than 1.5 times FTP standard.
2. Does not illuminate any service lamp or message.
3. Does not store a freeze frame.
4. Stores a fail record.
5. Updates the fail record each failure.

Diagnostic Executive Or Task Manager

The diagnostic executive or task manager is the computer programming that contains the strategy for the following tasks:

1. Determines whether the enabling criterion necessary for a monitor to run exists. Checks for conflicting situations, conflicting situations include component faults and other monitors presently running.
2. Commands the comprehensive component and emission systems monitors.
3. Sets diagnostic trouble codes (DTCs) in memory according to code classification and status. Decides whether the malfunction indicator light (MIL) is off, on, or flashing.
4. Prioritizes freeze frame data storage and erasure (including failure record management on some vehicles).

Bi-Directional Communication

Bi-directional communication enables a technician, using a scan tool, to cause the vehicle computer to give a selected command to an actuator. This feature can be very helpful when diagnosing. For example, commanding an EGR on and watching the HO2S response can tell you not only that the EGR is opening, but also that the EGR port is not clogged. In another example, commanding the secondary air injection on and watching the HO2s response can tell you that the airflow is actually being delivered to the exhaust.
Freeze Frame & DTCs

While some vehicle’s PCMs will record certain sensor and actuator operating conditions when a type A or B, emission related, DTC is stored. Most PCMs only store one freeze frame record. The freeze frame data stored is of the first failed test that set the DTC and illuminated the MIL. The freeze frame is not updated, or refreshed, by succeeding failures of the same test. Fuel trim and misfire DTCs have priority and overwrite the freeze frame data in memory, unless a fuel trim or misfire DTC is already stored. When the freeze frame is overwritten, the previously stored data is lost.
Fail Records & DTCs

Some systems have fail records. To some extent, fail records make up for the limitations of the freeze frame. Fail records help diagnosis when more than one DTC is set or when a DTC is not emission related. The fail records contain the same data as a freeze frame, one for each DTC. However, even fail records are limited; there may not be fail records for every DTC if there are more DTCs than the fail record can store. On some vehicles fail records are stored any time a DTC is set, this includes first trip type B DTCs. Unlike freeze frame data, fail records are not limited to emission failures. Type A, B, C, and D DTCs are stored in fail records. Fail records are assigned a space and updated, or refreshed, with new data each time the test fails. When different DTCs are set, additional fail records are assigned. When multiple DTCs are set and all fail records space is used, the oldest fail record is replaced with the new fail record.
DTC Status

The status of an OBD II code is classified as history, pending, or current.
History DTCs

Whenever a fault is detected, whether it is a Type A, B, C, or D, the DTC is stored as a history code. The fault could have occurred in the present or any previous trip, but must have occurred at least once since the codes were last cleared. A history DTC remains in memory until a specified number of ignition cycles occur or a technician clears the codes with a scan tool.
Pending DTCs

Typically, these are type B codes that have not been validated by a second test, possibly because the second test has not yet run. This may be because the criteria for the test has not been met. Pending codes only indicate that a fault has been recently detected. They do not indicate that the fault is present, nor do they indicate that the fault is gone.
Current DTCs

A current DTC indicates that during the last ignition cycle a test related to the DTC has failed. The scan tool message may display ‘‘INT’’ for intermittent if the test has failed and passed during the last ignition cycle. If this is a type B DTC it is also a pending DTC until the test is failed a second time, passed three times, or until the DTCs are cleared. However, if the DTC is a type C or D, this DTC clears on the next ignition cycle. Some manufacturers give additional status definitions such as: failed since cleared; failed this ignition cycle; not run since cleared; not run this ignition cycle; last test passed; test ran and past