​Auto Transmission Diagnostics

​If your automatic transmission is acting erratically, be it shifting too hard, shifting too late, shifting too early, etc. try the diagnostic to isolate the problem. The diagnostic, like the main computer diagnostic, may show an error code if it reads a sensor or circuit that is not within factory specs. If the problem is mechanical, the diagnostic won't show it. If it's electrical, then the diagnostic gives a general idea where to start looking to correct the performance.

Procedure:

1. Have the engine at normal operating temperature, in P with the overdrive switch on and then turn OFF the engine.
2. Turn ON ignition but do not start engine. The AT light (1.) should go out after 2 sec.
3. Turn the ignition OFF.
4. Move selector to drive D by using the Push Release button (2.). If not equipped, use the brake pedal
5. Set overdrive switch to OFF.
6. Wait 2 seconds and turn ignition switch ON but do not start engine. 
7. Wait 2 seconds and move selector to 2.
8. Set overdrive switch to ON.
9. Move selector to 1.
10. Set overdrive switch to OFF.
11. Depress accelerator pedal fully and release it.
12. The codes will begin to flash. There’s one long flash that signifies the start of the sequence. Following should be 10 short flashes. Then it repeats.

Figure 1: AT Light

Figure 2: Release button

Error Codes:
All 10 flashes the same = No error codes detected. 
1st flash longer than others = Revolution sensor is shorted or disconnected. 
2nd flash longer than others = Speed sensor circuit is shorted or disconnected. 
3rd flash longer than others = Throttle sensor circuit is shorted or disconnected. 
4th flash longer than others = Shift solenoid "A" circuit is shorted or disconnected 
5th flash longer than others = Shift solenoid "B" circuit is shorted or disconnected 
6th flash longer than others = Overrun clutch solenoid circuit is shorted or disconnected 
7th flash longer than others = Lock-up solenoid circuit is shorted or disconnected 
8th flash longer than others = Fluid temperature sensor is disconnected or A/T control unit power source circuit is damaged. 
9th flash longer than others = Engine revolution signal circuit is shorted or disconnected 
10th flash longer than others = Line pressure solenoid circuit is shorted or disconnected 
All the same with no long start flash = Battery low, been disconnected, or control unit has just been reconnected.

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