98 Windstar strange overdrive behavior

So I have a 1998 Windstar 3.8L, pre-9/9 transmission, and it does not behave well. I know, surprise, right?

So what happens is that I get what seems like indecisive overdrive engagements after driving at high speed for a while. The transmission will turn overdrive on and off while cruising at constant throttle. After a couple fits of this, the transmission will go into "gimp mode" and flash the overdrive light until the car is shut off and turned back on again.

It behaves great at low speeds and low overdrive usage. No apparent slippage or violent shifts.

If I disable overdrive, I can drive just fine and not get any indecisive gear shifting. And it never goes into gimp mode.

Any ideas? I know there's a couple sensors that the transmission uses to make its decisions, is there anywhere I should start?

Thanks!

Welcome Kaniff82,

Please elaborate. We can help.

Drive for "a while"?
Gimp Mode = "Get you home" mode. I understand that.

What failed for me first? The VSS (Vehicle Speed Sensor).

Karl.

Welcome Kaniff82,

Please elaborate. We can help.

Drive for "a while"?
Gimp Mode = "Get you home" mode. I understand that.

What failed for me first? The VSS (Vehicle Speed Sensor).

Karl.

"a while" seems to be enough that the transaxle comes up to operating temperature and drives at highway cruising time for 10-15 minutes

Gimp mode is the overdrive light flashing and the transmission won't engage overdrive. I've heard of another gimp mode where the transmission won't go out of second gear, this doesn't seem to be that mode.

Karl, what were your symptoms when the VSS went?

Also, I've not had any problems with cruise control or ABS which I've seen indicated as common failures related to some of the speed sensor failure.

If the O/D light was flashing, then there should be a transmission code stored.
You might get lucky...and have a OBDII "P" code that can be read at your local auto parts store (get the actual numeric code), otherwise, you will need to take it to a shop to have the more advanced reader used to read the proprietary code(s).
Doing this will get you a better idea of what is going on.
If it is some "indeterminate" type of code.....your Transmission Range Sensor (TRS) aka Neutral Safety Switch could also be causing problems.
But.....without codes......we are guessing.

Another thing to look into is how fresh the transmission fluid is.
It needs to be changed at LEAST every 30K miles......
This would be all 12.25 quarts.......not just the roughly 6 quarts that you get with a pan drop.

If the O/D light was flashing, then there should be a transmission code stored.
You might get lucky...and have a OBDII "P" code that can be read at your local auto parts store (get the actual numeric code), otherwise, you will need to take it to a shop to have the more advanced reader used to read the proprietary code(s).
Doing this will get you a better idea of what is going on.
If it is some "indeterminate" type of code.....your Transmission Range Sensor (TRS) aka Neutral Safety Switch could also be causing problems.
But.....without codes......we are guessing.

Another thing to look into is how fresh the transmission fluid is.
It needs to be changed at LEAST every 30K miles......
This would be all 12.25 quarts.......not just the roughly 6 quarts that you get with a pan drop.

Thanks for the recommendations. A few days ago, I changed ATF using the alldata recommendation drain 3 quarts by running, add 10 quarts and drain 10 quarts by running, and top off. The fluid looks cleaner and it shifts a lot better than it did previously.

I'll see what I can get from the OBD readout.

Sounds like the new fluid may be cleaning out the internals.....specifically the fluid valves....etc
You may need to change the fluid again....an early change may help you to get the last of the buildup out of there.....leaving you with nice clean fluid.
Maybe.....2K on this fluid....and then repeat what you did?

Sounds like the new fluid may be cleaning out the internals.....specifically the fluid valves....etc
You may need to change the fluid again....an early change may help you to get the last of the buildup out of there.....leaving you with nice clean fluid.
Maybe.....2K on this fluid....and then repeat what you did?

Thanks again, I used Mercon for the last change as that is what is indicated in the manual, should I use Mercon V for next change?

Btw, no P codes from OBD :(

Sounds like the new fluid may be cleaning out the internals.....specifically the fluid valves....etc
You may need to change the fluid again....an early change may help you to get the last of the buildup out of there.....leaving you with nice clean fluid.
Maybe.....2K on this fluid....and then repeat what you did?

The comment about fluid got me thinking. I think it may be possible that the TC clutch is failing to lock up. Is my drain/fill change method fully flushing the torque converter?

I didn't change the filter, as the fluid didn't show any debris, just looked old and dirty. Perhaps I should?

I resistance and ground tested the TCC, EPC and shift solenoids via the top connector and they all appear to be OK. But the problem doesn't show up until the car is driven for a while, so perhaps testing the solenoids after driving it to the point of failure might show different results?

The method that you used ran the new fluid through the torque converter.
Back in the days......when one only needed to change the transmission fluid if they were doing something out of the ordinary....otherwise....it was fine for life.......they had drain plugs in the torque converter.
Now......when we MUST change the fluid regularly.....there is NO drain plug in the torque converter......so the only way to change the fluid in the whole transmission.....is to run fresh fluid through to flush it out.

I would use Mercon V for the next change.
If you want to get carried away (I am anal about this because I shelled out just over $2800 for a FORD remanufactured unit at about 100K...and don't want to do that again).......you could put 9 ounces of AutoRX in www.auto-rx.com and drive for 1000 to 1500 miles.....and then change the fluid.
The idea being to clean the hydrolic part of each solenoid out......particularly that torque converter one....as it requires a pretty complete tear down of the transmission to get to.
No certainty that this will solve the problem.....if the parts are worn......then it will not "unwear" them.....but for $25 for the AutoRX....plus new fluid after the 1000 mile run.....it is worth a try versus tranny replacement.

One fluid that you might look into is Mobil 1 ATF....easy to find, and a good ATF.
It is now Mercon V rated.