Hi. I am trying to sort out a problem with my '91 F150 4x4 with a 6 cyl. and auto trans.

The problem is, the transmission will not shift into overdrive unless the hazard switch is turned on. Very strange. I have checked the fuses and most of the wiring under the dash and leading to the transmission. It does look like the hazards and speed control share the same circuit. Maybe that has something to do with it. This truck does not have cruise control through.

Any ideas? Thanks for the help.

Jeff

 

sounds like someone played a joke on you, just kidding. this one has me stumped too. just keep checking you wiring harneses. is your OD switch on the dash, as on a '90 or is it at the end of shift lever?

 

The OD button is on the dash. I have been chasing and testing all the wiring and everything is checking out. Any other ideas?

 

Check that your brake lights work properly. They should be off with the key on and your foot not on the pedal. They should both come on when you depress the brake. If the brake lights are mucked up, you may get some interesting transmission symptoms.

The PCM gets the BOO input from the brake light switch (Brake On Off). This is one of several inputs it needs to control O/D shifting, converter clutch lockup, etc.

If both rear lamps are burned out, or the brake light switch is stuck on all the time, the BOO input won't work right. It is possible to get an interaction through the hazard flasher, since it independently feeds power to the brake lamps and indirectly back to the BOO input.

Check the O/D cancel switch LED. When the PCM recognizes the switch as being depressed, it toggles an internal state and lights (or extinguishes) an LED in the switch. (The switch is just a momemtary contact, not a latching switch). There is a separate wire for the switch input and the lamp output, so you get a nice closed loop with the PCM. Push the switch, light comes on. Push it again, light goes off. Any behaviour different from this signals problems in the O/D cancel input to the PCM.

Run a KOEO test. I don't expect to see any static code, but see if there are any continuous memory codes relating to the VSS input, BOO input, etc.

Run a KOER test. The PCM will check the BOO circuit and the O/D cancel circuit. (You need to step on the brake and push the O/D cancel button at the appropriate point in the test). See what sort of codes you get, if any. Repeat with the hazard flasher on.

The only commonality I see between the "speed control" and the hazard flasher is through the brake pedal switch. Fuse 1 supplies power to said switch, the output of the switch (BOO) goes to the cruise control, multi-function switch (including the hazard flasher) and thence the brake lights, and to the PCM.

 

Thanks for the thoughts. I have ran KOEO and KOER tests. I didn't see any codes. I didn't think to try it with the hazards on. Good idea.

That OD button seems to function fine.

This is crazy. No one believes me that this problem is real. I even gave up and took it to the Ford dealer. They replaced the MLP sensor, but that didn't change anything.

I did create another way to create the OD shift other than turning on the hazard switch. Lightly applying the brakes will shift to OD for a couple of seconds until the trans shifts back to 3rd because of the extra load.

Someone has been into the truck messing with wiring. There must have been an add on cruise control system at one time, but I think I have cleaned up all of the wiring cutting they did (VSS, brake switch, coil power, TPS). I still suspect something related to this, but can't find any problem areas.

How about the TCC signal? That is my next area of investigation.

 

TCC is torque converter clutch. Should be independent of selecting O/D. I believe TCC is only supplied in 3rd and O/D, and only when the trans is warmed up and the load is "appropriate". I believe that the computer unlocks the torque converter as soon as you touch the brakes -- that's the primary purpose of the BOO signal.

If there is something squirrely with the wiring, like TCC shorted to BOO, then TCC might be involved where it otherwise wouldn't.

The main consequences of applying the brakes is creating some drag and asserting the BOO signal. This should unlock the converter and/or take out out of O/D, not the opposite.

Try adjusting the brake light switch so the brake lights stay on all the time. See if the trans will stay in O/D -- or at least your little trick with the brake pedal changes in character.

If it isn't the brake lights, try creating some drag with the parking brake. This will isolate the brake lights from the brake action. See if that causes the O/D shift.

It can be difficult to tell the difference between the torque converter locking and entry into O/D from third. The RPM drop for either transition is about the same. Are you sure you know which is happening (or not happening)?

Finally, check all of the grounds. Battery, engine, frame. Ground return issues and the attendant voltage shifts can cause strange interactions.

 

If the CCS (coach clutch solenoid) wire is grounded for some reason, it will override the O/D system and leave you stuck in 3rd.

According to the E4OD TSB, the computer controls for
4th gear are:

SS0 = off SS1 = off CCS = off

For 3rd gear with O/D cancel NOT active

SS0 = off SS1 = on CCS = off

For 3rd gear with O/D cancel active

ss0 = off ss1 = on ccs = on

The solenoids are turned on when the PCM grounds them. So ON means a low voltage and OFF means a high voltage. The block of solenoids is supplied with 12 volts via a shared wire.

It seems to me that you or a dealer could jack up the rear of the truck. Warm it up. Put it in gear, depress the accellerator, watch the tranny shift. When she won't go into 4th gear, look at the tranny signals and find out which one is missing.

 

I broke down and tore into the trans. After pulling the shift sol. module, I found that a trace on the circuit board that connects the sol. was fried. I am going to replace this and hope it works. The resistance between pins wasn't within spec. Hope this works!

Thanks for the help.

Jeff