5R55S o/d light blinking light | Ford Explorer Forums - Serious Explorations

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5R55S o/d light blinking light


Elite Explorer
August 27, 2015
Reaction score
City, State
Ann Arbor, MI
Year, Model & Trim Level
2010 XLT 4WD
2002 XLS 2WD
Had the O/D light flash on my '02 XLS 4.0 for the first time on a 150 mile trip this weekend. Higher highway rpms confirm that the overdrive is not engaging at highway speeds (e.g., 3000 rpm @ 73 mph instead of 2200 rpms). Here's the background:

  • 185k miles on the transmission (but see below). New fluid and filter 6k miles ago, with no metal found in pan (just fine grey dust on magnet). The transmission has been shifting flawlessly, with overdrive, forthose miles;
  • @179k miles I swapped engine(and re-did all timing chains). While I had the engine out, I upgraded the transmission pump with a valve upgrade;swapped in a torque converter from a 116k miles XLT that was built the same month as the XLS, and swapped in the XLT's solenoid pack as well. All ran great;
  • This weekend, no problem for the first 100 highway miles.Weather in Michigan relatively cool (65-70 degrees). Then O/D light started flashing, and rpms went up. Never did this before;
  • On way back from family visit, the transmission operated fine (again) for first 80 miles or so, then again with the O/D light flashing and loss of overdrive; and
  • No symptoms or "flaring" with other shifts.
I' ve done a lot of searching/reading on these issues. Based on frequency of "fixes" it seems to me that the problem could be (in order of likelihood):
  1. Bad solenoid pack;
  2. Bad torque converter;
  3. Overdrive servo boreissue/repair;
  4. metal clogging up a valve in the valve body.
Solenoid pack replacement or servo bore fixes are each about $200+ and do not require removal of the transmission. Torque converter or full rebuild obviously would require this (major hassle, as I have no lift or garage or even paved driveway).

I have not yet pulled the codes, if any, but will do that today ortomorrow. Again, the car runs and shifts great, but no overdrive/lockup after 45 minutes or so of driving. My questions:
  • what is best diagnostic order of attack (to avoid "throwing parts" at this w/o actually fixing the problem)? and
  • How important is it to stop driving the truck while I figure this out? (I have anothervehicle.)

Here are the DTC's I just pulled (fortran lite), in order:
  • P0735 -- Transmission Gear #5 incorrect ratio
  • P0775 -- Pressure Control Solenoid B malfunction
  • P1738 -- Transmission Overtemperature condition
No DTC's mentioning the Torque Converter explicitly (though I know there are several of those that exist)--so that's encouraging.

I'm surprised by the P1738 (overtemperature condition). I had monitored in real time the transmission temps on a 3,000+ mile trip down to Austin and to Atlanta this past July--ambient temps routinely over 90-degrees F and crowding 100--and did not seen any temps on the OBDII monitor of over 200 degrees. This has me wondering if either of the following has occurred:
  • corrosion/blockage of flow in the transmission cooler, or
  • an undetected leak that has caused the transmission fluid to drop below specified level. (I doubt this second possibility, b/c when I initially drove truck last summer I'd failed adequately to fill the trans over only a 5 mile route and I experienced hard clunky shift engagement from full stop--no flashing 0/D light-- which clunking quickly went away when I topped off the trans fluid. I am not getting thatsympton now.)
I saved the transmission cooler from the 116k XLT, and could flush it and swap that for the older cooler on there now with labor and some Mercon V cost.


Is there a way to diagnostically test the solenoid pack w/o dropping the pan? If so, is that test reliable? If yes to both, and the diagnosis is bad solenoid based on that diagnosis, should I start by replacing the solenoid pack with a new ($250) or quality rebuild ($170) solenoid pack? Thanks, again.

Question: Do these DTC's suggest replacing the solenoid pack first?

Replace the 2 servos with aftermarket ones which have O rings. Check the internal wiring harness in the transmission for damage. The harness should be tested with a continuity tester from point to point except for the temperature sensor which is set in the ohm range. Make sure that the bulkhead connector doesn't have bent or corroded pins.