A/T - Diagnostic Test Lights

Shift Problem: Trans or Control Unit?



What's the first question that comes to mind when you have a shifting problem on an electronically controlled A/T? Is the cause mechanical, hydraulic, or electrical? If there's a problem code in the control unit, that helps, but where do you start if there's no code? Check the control unit signals while you drive the car.





First, you need to make two 12V LED test lights. We used two Radio Shack LEDs (P/N 276-011A) with terminals (P/N 07JAZ-001190A) from our Automobile Terminal Pin Repair Kit. Label one test light "A" and the other "B."





Connect your Test Harness (T/N 07LAJ-PT3010A) to the car as described in the appropriate S/M. Connect LED A to terminals A5 (+ RED) and A25
(- YEL). Connect LED B to A3 (+ RED) and A26
(- YEL).






Test drive the car with an assistant to watch the LEDs. When driving in D3 or D4, the LEDs should go on and off as the trans shifts, as shown.



When LED A comes on, the control unit is supplying voltage to shift solenoid A. Likewise, when LED B comes on, shift solenoid B is being supplied voltage. If the trans doesn't shift, but the LEDs show the right shift signals, the control unit is OK. Test the shift solenoids themselves next. If they're OK, there's an internal transmission problem.