Questions and Answers

Ford Explorer DTCs 632 And 646 Q. My 1995 Ford Explorer 4X4, 4.0 liter V-6 OD switch just started blinking on the dash without any warning. The engine and transmission was running fine. The light just came on, and I noticed the RPM is running high when shifting into second gear. If I accelerate and then take my foot off the gas, it's shifts easily into second gear. All the other gears are fine.

This hard shifting into second gear, just started after the OD switch started blinking on the dash without warning. I bought an OBD-I code reader and found the following codes. KOER:

Code 646 - Incorrect Gear Ratio obtained for second Gear - Second Gear failure. Engine Running
Code 632 - O/D Cancel switch, Overdrive cancel switch did not change state during KOER.

I disconnect the battery for two minutes and then reconnect it and the OD blinking light went out. I drove the car and it shifted fine in all the gears even second, however, about 30 minutes into the driving I went up a steep hill and turned to go down another hill and the OD started blinking, about 25 to 30 mph. I disconnected the battery again and manually turn off the OD using the OD switch on the gear shift and the transmission shifts fine.

Questions:

1) What can I do to fix this problem and stop the OD switch from coming on?
2) What do I need to do to correct these two codes listed above?
3) Should I put a new transmission in? I call a Ford dealer and explained the problem, and without even looking at the truck, the Service writer advise, whenever the OD switch starts blinking, it's a serious transmission problem, which would require them to GO INTO THE TRANSMISSION. I was quoted a price for a "fix", new transmission, $2,500.00 to $3,000.00. Note: Before the OD started blinking, the transmission was running fine.
4) How bad is it to drive with the OD switch manually turned OFF below 40 mph and then manually turning it on over 40 mph using the gear shift OD/ button? If there is a second gear issue, the OD would still kick in over 40 mph?

Any help appreciated. Thanks in advance for your assistance.

Cecil

A. DTC 632 is possibly the result of the test being done incorrectly. When you do a KOER test, you need to do a Dynamic Response Check is used to verify operation of the Throttle Position Sensor (TPS), Mass Air Flow (MAF), Manifold Absolute Pressure(MAP), and Knock Sensor (KS) during a brief wide open throttle condition. The famed "Goose test".

To properly test for DTC 646.

NOTE: Obey all traffic laws.

Drive Cycle:

• Clear continuous memory.
• Gear position in DRIVE range, press the Transmission Control Switch (TCS, the Transmission Control Indicator Lamp (TCIL) should light, and moderately accelerate from stop to 40 mph. This will allow the transmission to shift into THIRD gear. Hold speed and throttle opening steady for a minimum of 15 seconds, 30 seconds for altitudes above 4,000 feet.
• Press TCS, the TCIL should turn off, and accelerate from 40 mph to 50 mph. This will allow the transmission to shift into FOURTH gear. Hold speed and throttle steady for a minimum of 15 seconds.
• With transmission in FOURTH gear and maintaining steady speed and throttle opening, lightly apply and release brake, to operate stop lamps. Then hold speed and throttle steady for at least an additional five seconds.
• Brake to a stop and remain stopped for a minimum of 20 seconds with transmission in drive. Repeat drive cycle at least five times.
• Rerun Key On Engine Off (KOEO) Self-Test and record all Continuous Memory DTCs.

If you still get the code, then the possibilities are an intermittent harness connection, damaged Transmission Range (TR) sensor, damaged shift solenoid or, and the only internal cause, worn friction elements.

Each one of these can be tested individually and proven to be good or bad.