Feature Article
How to Add a
Torque Converter Mystery Switch
Issue 12, Spring 1996
I think you will agree with me when I categorize Turbo Diesel owners as independent people who are not afraid to try something new. You are an ingenious membership who reinvents and improves a product to make it better serve your needs. You show a strong willingness to share your 'Shadetree Solutions.' With your input each quarter, we will publish the 'Member2Member' exchange to give you a forum to tell other members how you solved a problem.
Let's move into our 'M2M' column with follow up information on comments in Issue Eleven covering control of the torque converter lock-up on post '94 trucks. Also in this issue you will find helpful information on diagnostic codes straight from a wonderful little publication - your Dodge truck service manual.
TCC MYSTERY SWITCH
Robert, since I had my powertrain control module (PCM) changed (see Issue Ten, "Member2Member," page 29) I have reworked the electrical application of the "Mystery Switch." Now I can lock the torque converter clutch (TCC) as well as unlock it or leave it automatic (controlled with the computer). Now it is officially my TCC switch. This is a much better way to descend the mountains and hills. The first test and evaluation run that I made after the modification to my TCC switch was to climb from 8000 ft. over the top of Mount Baldy Pass, 10,700 ft. and descend to 6500 ft. What a joy this was. Instead of the TCC dropping out every time I bad to brake for the many sharp bends, the TCC stayed locked all the way down, super!
When to use? When you are about to descend an incline (hill or mountain), select the appropriate gear (4th or 3rd) engage the switch and monitor the tachometer. Don't let the engine over rev (2500 rpm max) and make sure the switch is returned to auto operation at 1000 rpm or slightly below. You MUST do this to return the transmission to normal operation. In essence, you have to sit up straight, clear your head and pay attention when manually locking the TCC. This modification should work especially well with those of you who have a '94 or '95, automatic transmissions and have an exhaust brake. When you engage the TCC switch (manually lock) you can take your foot off the accelerator pedal and get complete engine braking. No free wheeling. But do NOT forget to return the switch to normal at the bottom of the hill, or if you have to shift to another gear. Again, the internal hydraulic and electrical make-up of the transmission will not allow it to shift if the TCC is locked. You have removed the computer from the circuit when the TCC is manually engaged. Be careful and good luck. I have included a How-to Sheet and Photos if anyone is interested in installing the switch.
How To Add A Torque Converter Control Switch -- 1994 or '95 with an Automatic Transmission
Under the hood on the right side, remove the cover of your air cleaner. (Cover it to keep stuff out of turbo.) The power control module wire connector is behind the air cleaner mounted on the fire wall. See Photo.
Raise 2 each tabs at top of connector to open and gain access to torque converter control wire. See photo below. Note: Photos 3A and 4A were taken with air cleaner removed but it is not necessary to do this to identify and modify wire.
The wire in question is orange with a black stripe and is in cavity no. 54, which is the seventh cavity from the left on the bottom row (left looking from front of vehicle). See photo 4A. Also note that some cavities are blank you must count them too.
After you locate the proper wire, open up the plastic split wire loom and make the cut and splices about 3 or 4 inches up in the loom. Use two different colored # 18 gauge wires because you must know which wire goes where when you make the switch connection. This is very important!! I used one brown and one orange wire. Splice the orange wire to the upper (farthest away from computer) wire, and splice the brown wire to the computer side. You must make a good splice but keep it small. I soldered the wires and covered the splice with a small length of heat shrink tubing.
Run the two new wires along the wire loom to the left side of the cowl and through one of the two rubber wire feed troughs. You can do this by spraying WD-40 on an awl and make a hole through the rubber.
Inside the cab, remove the drivers side knee bolster to gain access to the area where the wires come through the firewall. Pullout the drink tray and remove the two phillips head screws, one on each side of tray bracket, the tray and bracket can now be removed. Remove the ash tray and remove two phillips head screws, remove ashtray bracket. If you have a tilt steering wheel, lower it to bottom position.
Set emergency brake and put the shift lever in first gear. Gently pull dash bezel towards you from right side to left. Disconnect cigarette lighter and power station wires, remove bezel and set aside. This will allow you to remove the O.D. switch bracket, 3 phillips head screws. Gently pull bracket towards you. Unlatch and disconnect O.D. cable (connector).
Now you must find a suitable switch. You will need a double pole, double throw, off in the center switch (DPDT 3 space position); finding the right switch took me longer than performing the modification because of the place where the switch is mounted. It must either be less than one inch wide to recess into the bracket hole, or it can be about one and one quarter inch wide if the mounting threads are at least one half inch long. A Big A Auto Parts switch p/n 27-176 or 27-159 will barely work. (I'm still looking for a better switch.) Drill appropriate size hole in bracket blank and mount switch. I mounted a Radio Shack 12v lamp (p/n 272-331, red) in the O.D. switch bracket and connected it to come on when the torque converter switch is in the applied (TCC locked) position. One side of the switch is used only for this function. My light is mounted to one side of bracket; if I were to do it again it would be in the center. It is very important that you add the indicator light as you will disable the co mputer's ability to shift the transmission when you have the new TCC switch in the manual (locked) position. It will NOT shift from 4th to 3rd or 3rd to 4th if the torque converter clutch is manually engaged. It also should not be locked in 1st or 2nd gear.
Make the connections and carefully mount your switch and run the two wires, being careful to avoid sharp edges or moving parts. Use tie raps to contain the wires and make it look nice. Put everything back together in the reverse order.
After your truck is all back together, put your TCC switch in the center position and drive down a back road. Flip your TCC switch to the up position and accelerate to a high enough speed to verify that your trans works normally. (Shifts through all four gears and locks up the TCC) This is just to verify that the switch is connected properly. The indicator lamp should NOT be illuminated. You can check the lamp before you start your truck. It should light with the key on and the TCC switch placed in the down position. Again, be careful and aware of the switch operation. You should do most of your driving with the switch in the up (normal) position. If you are reluctant or nervous about manually locking the TCC, you can replace the DPDT switch with a simple single pole single throw (SPST) switch and forget the light. This will allow you to unlock the TCC at will, and there are no side effects. This will give you more zip in the 55-65 mph range as well as smooth out the shudder between 40-50 mph in O.D. if you haven't had your PCM updated.
Good luck and have many fun trips.
Russell Ward
Kayesville, UT
In issue Eleven, page 38, we showed a picture of Russell's mystery switch. Several members wrote in with guesses as to it's function. Now we know. Russell and John Holmes (Issue Eleven, page 33) both have discovered techniques to control the lock characteristics of the 47RH automatic transmission.
As TDR editor I've got a dilemma on my hands; member input versus factory recommendations. Dodge is strongly against the use of lock-up to aid in braking the truck Converter clutch wear will be greatly accelerated as you are now putting 160 hp in -reverse - of the design application. Use of lock-up in the above manner may fall under the "abuse" terminology if you were to have a lock-up failure during your warranty period. Once the truck is out of the warranty period or if you choose to forego the warranty provision, by all means feel free to hook up a TCC switch. Russell has certainly provided detailed instructions. More next issue? Stay tuned.