Strange overdrive behavior in 99 LX (1 Viewer)

[jLB]

On the drive to Silverton, in our old 99 LX, I noticed an unexpected behavior when disabling overdrive...

• Overdrive Off switch illuminates the light on the dash as expected. (The light is "on" with the switch in the "out" position. The light is "off" with the switch in the "in" position.)
• Turning O/D off causes no change in RPM, and no engine braking (at highway speeds).
• Based on RPM (~2200RPM at 65MPH on ~32.8" tires. ~2400RPM at 75MPH on ~32.8" tires), it seems that O/D is working, and I believe I feel 3 distinct up shifts (4 gears) when accelerating.

The button seems to work, but the transmission or ECU seem to be ignoring the input...

The gas mileage was great through some of the downhill sections of the drive, but engine braking in 4th or 2nd (no ability to select 3rd) was a little hair raising at times.

Has anyone else observed this type of behavior before?

 

[jLB]

Follow up:

Early in our ownership of this vehicle, we picked up a spare ECU, from a 99 LX470 that was being parted out, to deal with the single valet key issue (w/o taking the vehicle out of service). When we swapped the ECU out, I was a bit puzzled at the ECU P/N difference, original = 89661-60610, "virginized" salvage = 89666-60170, but everything plugged up and seemed to work just fine.

On our July drive to Silverton Colorado, once we hit the mountains, we quickly figured out that the "OD off" was not working as expected. The switch and dash light worked as expected, but the transmission did not.

Wondering if it could be the ECU P/N difference, or if the salvage ECU might be a little less than perfect, I shipped the original ECU to @Mauser to get it "virginized". Once the original 89661-60610 ECU was re-installed the O/D is back to working as expected.

(Our 99 LX has a 10/98 date of manufacture)