I found this while surfing the web: (it was on a forum for diesels, but it sounds like it might work for all trucks)
Post#1
I put the LED in so I know when it's locked. The light starts out dim, then gets brighter as it locks up. When the LED is on, I feel better about towing a heavy load (horses). I then put the switch in so I can lock it during the times the PCM doesn't.... like going up hill at 35 MPH or so.... I don't see a need for all that slipping. I would rather have all that torque pass thru my driveline, and not just dissipate "into" it, creating a bunch of heat, wear, and other losses.

Post #2
Locate the T/C lockup wire, either at the tranny, or the harness plug, and tap into it with a switch to manually force ground, which will lock the T/C.

The wire is Violet/Yellow.
Spot #1. On the passenger side of the tranny. Black plug.
Spot #2. Passenger side wheel well. Forward most connector. On 1999 to 2001, it is pin #6. Not sure is 2002 and 2003 are the same, but more than likely they are.

Use a scotch lock connector and tie into the Violet/Yellow wire, (DO NOT CUT IT, TAP IT!), with a 20 gauge stranded, and run it inside the cab.
Connect it to one side of a SPST switch, (on-off), and connect the other side of the switch to ground. When ever the switch is "ON" the T/C will lock.
CAUTION! Never lock the T/C below 15 mph or you can stall the engine.

Now my question is this ok to do? I mean if the converter stays locked up then it is not slipping and adding excessive heat.

I currently turn O/D off will towing. Is this the right thing to do? I like the gas savings, but not at the cost of the tranny life. I tow a 14' boat (1970 McKee Craft ~1800lbs) about 60 miles round trip when I go out. I have bypassed the stock cooler and added an aftermarket cooler (20,000 gvw Jegs part # 400-3820). Do I need to do anything else for cooling? I live in Charleston, South Carolina. The truck is a 2000 F150 XLT 4.2 V6, 4R70W tranny, 3.55 locker rearend, and is bone stock (except the tranny coooler). BTW any advice on recommended mods is more than welcome.

Thank you,
Caddpro93

 

You're not going to get lockup to hold fully on a stock converter. They were designed to slip a certain amount.
Alan

 

You bring up an interesting subject and i'm not going to attempt to say if it is the wise thing to do or not.
However, here are some things to consider.
The lock up is not meant to hold/pass large amounts of torque but with a v6 it may not be that much of a problem with the light weight you pull.
Going out of lock increases the engine rpm thus the engine torque production for a given pulling load and road situation.
Granted that this produces high trans heat from fluid shearing but unlock operation should be well within the temp of the fluid's rating versus life of the fluid and the trans clutches, bands and lockup parts. An aux cooler certainly helps this out.
Not many here realize that the lock up function comes on and off more than they realize while just cruising along. Just letting up on the throttle momentatly releases the lock up by design in the PCM.
This is because on a braking situation you don't want the weight of the truck and engine combination pushing when you need braking power for fast braking reactions. This operating feature has been a part of the lockup type in Ford transmissions for quite a long time now such as the lighter Ranger/Broncos with the A4LD trans.
The lock/unlock can be tested/seen in a truck that has a tach by accelerating lightly at the same time tapping the brake pedal. You usuallly see a momentary tach increase than back again as lockup re-engages.
I might add that with the heavier weights of these trucks this electric lockup is responsible for some of the higher brake pad wear on the fronts. There is less engine braking taking place so brake application become much more needed to haul down road speed when the trans goes out of lock.
The difference in trans types is how overdrive lock up is accomplished. In these AODE types, the lock up is done in the converter while in the older AOD's it is done differently and is not an electric lock up operation.
So the answer to what you do is up to you and the results you feel you get in return. And yes the lockup has to be turned off a low road speeds
For me, I tow a good 6000 lbs plus a 5400 lb truck and could not pull in a forced lockup condition at the lower rpm with a 4.6L engine yet on the hills that have to be climbed.
I have no idea of how the shifting would work with forced lockup and under PCM control because the PCM has a fault recognition pattern it uses to compare with to see when an operation has gone faulty, then lights the lamp and sets a code based on what the pattern failure is.