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Chrysler, Plymouth, and Dodge Four-Speed Automatic Transmission
Torque Converter Disabling

Torque converter shudder is a very common issue with the A604 transmission. Previously I had designed a circuit to disable lock up operation with solves the shudder problem. You can see that article on this page, below. The only drawback to my initial design was that fuel economy suffered as well as transmission fluid temperature increased. I now have a new version that will eliminate the shudder, and still allow lock up to engage eliminating the drawbacks.

After much testing I discovered what really causes the torque converter shudder on or about 38 MPH. The torque converter lock up clutch is not getting enough hydraulic pressure to fully engage, and it slips at 38 MPH. The solenoid that controls it is continuously pulse modulated on and off keeping the average pressure low. It never allows full pressure when the speed of the vehicle is below 42 MPH. It then slips, and “shudders.” This usually occurs after the TC clutch is worn a bit. This symptom goes away at about 42 MPH or so when the computer decides to keep the solenoid on without pulsing it. The occurrence of this symptom is influenced by load, temperature, and wear of the TC clutch and other transmission components. This issue is caused by the Transmission Controller itself. Its behavior is contrary to what is described in the Tech Tran manual. Full lockup is never achieved below 42 MPH. It is a bug in the firmware, as confirmed by Mopar Tech Support. I confirmed that the transmission computer in my vehicle never engages full pressure to the lock up clutch until 42 MPH or so using a scope. I spoke to several people at Mopar about this, and they said it can be solved by a firmware update to the transmission computer. This option is not readily available to most people since a lot of dealers no longer have the necessary equipment to do it. It will also most likely cost you some money if your dealer happens to still have the necessary equipment. (I spent a lot of time on this issue, and finally gave up on the dealers in my area.)

My new circuit eliminates the shudder issue without a firmware update. The theory is simple. When ALL of the following conditions are met, the solenoid that controls the lockup is turned full on (not pulsed).

  1. Transmission is in third or fourth gear. ( OD pressure switch is closed )
  2. Lockup enable switch mounted on dashboard is “ON”.
  3. Either a pulsed or constant PWM signal is present on the LU/LR solenoid output of the transmission computer.

When driving at speeds between 38 MPH and 42 MPH (or so) the LU/LR solenoid output of the computer is pulsing on and off. This is detected by my circuit which then disconnects the LU/LR solenoid from its normal port on the computer and parallels it to the OD solenoid output port (that solenoid is conveniently being held full on while in 3rd or 4th gear) which is NOT pulsed. The lockup clutch is now given full pressure, and no slip. You may actually feel it engage, just like another shift point.

Note to the techies out there: The computer is always using PWM to control solenoid current. The “pulsing” I refer to is the PWM being turned on and off. This can be easily verified with a scope. NEVER ground any of the solenoid wires as it will burn it out. The solenoid coils are 2.5 ohms.

Building this circuit will require cutting into the harness that plugs into the transmission computer. You also must be willing to build and install this circuit entirely at your own risk. Neither Allpar nor the Author cannot be held liable for any damage or inoperability you may experience. This circuit has worked fine for me for two years now. I cannot guarantee you will have the same results. This circuit was built and installed on a 1994 Dodge Caravan with the 3.3L V6.

All of the parts for the circuit can be purchased at Radio Shack. The relays are 12V DPDT type. You should have a good understanding of electronics to build this, or be really brave. If you need help, find someone who have some electronics experience. I highly recommend that you read my previous article (which follows this one) so you can understand the theory of operation, and will understand why a third relay is used with only its coil connected. (I am sure the techie people out there will be asking…)

Simply connect all parts according to the schematic below. I recommend that all connections be soldered and heat shrink tubing used for all splices. Put the relays in a water tight box. The switch to enable/disable lockup is mounted on the dashboard with a simple toggle switch. When the switch is in the off position, lock up operation is prevented. The circuit behaves like my previous version. With the switch on, lock up is enabled, with no partial lockup. (partial lockup is the cause of the shudder)

chrysler automatic transmission torque converter repair diagram

You may occasionally notice some “harsh shift” symptoms when the transmission is up shifting. I have investigated this, and found out that it is normal and nothing to worry about. When conditions are just right, the computer may lock up the torque converter while in third gear when driving around 25 MPH or so. If throttle position is not changed and the vehicle eventually reaches 32 MPH or so, it will go into fourth gear while the torque converter is locked up. There will be a noticeable bump since there is no fluid coupling or slipping clutch in the torque converter. Without my circuit, you never notice this since the torque converter never fully locks up in third gear. This new behavior is not bad, but actually good since nothing is slipping any more.

Good luck and if you have any questions, you can contact me by clicking here.

How To Disable Torque Converter Lock Up For The A604 (original version)

From what I have been reading, torque converter lock up shudder seems to be quite a common issue for A604 transmissions. I had it happen to me as well, a year after I had the torque converter replaced. It was out of warranty, and I was not going to pay another $400 to have it replaced yet again. I went through all the usual methods people try to solve it, such as changing the fluid, adding friction modifiers, etc, but they were all temporary fixes at best. I asked several mechanics at different transmission shops if there was a way to disable torque converter lockup on the A604, and they all said it was impossible. They all said that the computer would detect it and go into "limp mode." Having an electronics and computer background, I could not accept that as a final answer. I knew that there must be a way.

I knew nothing at the time about the inner workings of this particular transmission, other than what I have read on allpar.com. I knew that there were solenoids. I got out my meter and started probing around. I found an eight pin connector on top of the solenoid pack, and started to check it out. Using my meter, I found that there were four solenoids, and three switches with "pull up" resistors inside the pack. At this point, I assumed that one of the solenoids must be for lock up control. One by one, I disconnected each solenoid (one at a time) and took the vehicle for a test drive. I did this for each of the four solenoids. Unfortunately, just as the mechanics told me, the transmission went into "limp mode" when any of the solenoids were disconnected. I did not want to give up, so I ordered the "Chrysler A604 Techtran Manual" from the ATSG. I found this great resource on the allpar.com web site.

This technical manual was the best investment I had ever made. Using the manual, I designed a circuit to disable lockup of the torque converter, and it has worked perfectly for me for about a year now. For those interested in the theory of how my circuit works, there is an explanation at the end of this article.

If you wish to try it, do it at your own risk. I highly recommend that you have a transmission cooler installed before you do this. When the torque converter does not lock up, additional heat is generated. Your gas mileage may go down. I found the mileage loss to be negligible. You must also be willing to hack into the eight pin connector that plugs into the top of the solenoid pack. It will require cutting three wires. You will splice into two of them (attach an additional wire to the existing wire) and interrupt the third.

I do not recommend that you attempt this project if you do not have experience working with relays and soldering. You should know how to identify the coil wires on the relays, and the N/C, N/O, and COM connections. If you are not sure what these terms are, stop now and get someone with electrical/electronics experience to help you.

chrysler torque converter wiring diagram I

The pins on the connector you will be splicing are:

3 OD pressure switch (orange)
4 ATX 12V supply (pink or red)
7 L/R solenoid (light blue)

The wiring colors listed is what I found used in my vehicle. I cannot guarantee they will be the same colors on your vehicle, but chances are they will be. It's best to take apart the connector and verify it.

Below is a diagram showing pin location and numbering. This view is from looking down at the connector while it is plugged into the transmission. You are in front of the vehicle. You are looking at the wire side of the connector. You probably will want to unplug the connector and remove the plastic protective cap on it see the location of each wire. You can remove this connector with a 5/16" nut driver.

Parts Needed:

(2) Relays – 12V SPDT, or DPDT type. Radio Shack part number 275-206 is fine.
The Radio Shack relay has two sets of contacts. We will only be using one set. This relay comes with a socket.

(1) project box to put it all in.
(1) toggle switch to be used as a bypass for disabling the circuit.

Hardware, wires and connectors will also be needed. I recommend that all connections to the relay socket be soldered.

If you are using the Radio Shack relay, you will notice that each relay has two commons, two N/C connections, and two N/O connections. It's OK to connect like terminals together on the same relay. Another words, you can join both N/C contact, join each N/O contact, and connect the two commons together. This will give you a bit more redundancy.

I am now going to explain how to hook it all up. It's up to you to engineer the physical placement of the relays, routing and dressing the wires, etc. Each vehicle is different, so you will have to use your best judgment. Be sure that the relays cannot get wet (seal the box you are putting them in) or let it get too hot.

The two relays are going to be referred to as relay #1 and relay #2 from this point on. Relay #1 is going to be used to interrupt the LR/LU solenoid, and relay #2 is going to be used to trick the transmission computer into thinking the LR/LU solenoid is still connected when relay #1 is engaged.

Each relay has two connections for its coil. I am going to be referring to each relay's coil connections as A and B. It does not matter which terminal you name A or B, but be consistent. Polarity does not matter with relay coils.

1. Cut the wire going to pin 4 (pink or red wire) of the transmission connector. Make sure you cut it at a point giving you several inches of wire left going into the connector. Prepare a piece of red wire long enough to go to your relay box. Splice all three ends together. You have now tapped into the switched 12V supply. DO NOT USE ANY OTHER SOURCE OF 12V FOR THIS PROJECT OR IT WILL NOT WORK.

2. Connect the other end of the red wire you just added to the "A" coil connection on both Relay 1, and Relay 2.

3. Prepare an orange wire long enough to get from the transmission connector to your relay box. Just as you did in step one, cut and splice this wire to pin 3 (orange wire) of the transmission connector. You have just tapped into the OD pressure switch.

4. Connect the other end of the orange wire to the "B" coil connection on Relay 1. Do not connect it to Relay 2.

5. Now comes the fun part. Cut the wire going to pin 7 (blue) of the transmission connector. Unlike the previous steps, you will not be splicing into this wire, but extending both ends. You are going to wind up with two wires going to your relay box from the point where you cut it. Prepare a yellow and a blue wire and route both of them to your relay box. Splice one end of the yellow wire to the wire going to pin 7 of the transmission connector. Splice one end of the blue wire to the wire that used to go to pin 7. You are connecting the blue wire to the wire going back to the transmission controller. Do not connect the other ends going to the relay box just yet.

6. Install a simple on/off toggle switch at a convenient location inside the vehicle. This switch is going to be the bypass, to enable normal lockup operation for highway driving. Using a blue wire connected to one terminal, and yellow on the other terminal of the toggle switch, then route both of them to your relay box.

7. At the relay box, you should now have two blue wires and two yellow wires that are not connected. Prepare a short piece of blue wire, long enough to reach from relay 1 over to relay 2. Connect one end of the short blue wire along with the other two blue wires to the "B" coil terminal of relay 2. You should now have three wires connected at the B coil terminal. You will not be using any other connections on relay 2, other than the two coil terminals.

8. Connect the other end of the short blue wire (coming from coil terminal B of relay 2) to the common connection of relay 1. If you are using the recommended Radio Shack relay, you can connect the wire to both of the common terminals on relay 1.

9. Connect both of the two remaining yellow wires to the N/C contact on relay 1. As above, if you are using the Radio Shack relay, you can join both N/C contacts together along with the two yellow wires.

10. Seal everything up, and you are done.

Below is the electrical schematic. Please note for simplicity I am showing only one set of contacts of relay 1 in use.

torque converter schematic

If the bypass switch is turned on, normal lockup operation will occur. Only turn on the bypass switch if you intend to allow lockup when you are going to do highway driving. It's best to do this immediately after starting the vehicle. Please be aware that once the computer detects that the torque converter is not locking up (when the switch is off and you are driving above 38 MPH), an error code will be stored in the computer and the computer will not attempt lock up again even if you turn the switch back on. You must cycle ignition power to reset the computer to re-enable lock up.

The switch can be turned off (to disable lock up) at any time if torque converter shudder occurs. I usually leave the switch in the bypass, or on position, and at the first sign of shudder, I turn it off.

If you have any questions, you can send me an e-mail

Theory of operation:

This is quite techie. I wrote it assuming the reader has a knowledge of electronic theory and basic DC circuits.

The A604 transmission controller uses four solenoids to control the transmission's functions. One of the four solenoids has two functions. It is called the LR/LU solenoid. Its first function is to engage the low/reverse clutch for first gear, and its second function is to engage torque converter lockup when not in first gear. Its second function is only available in 2nd, 3rd, or 4th gear. The function that it controls is decided by a hydraulic switch valve inside the transmission. If you wish to have further detail information on how all that works, refer to the ATSG Techtran Manual for the A604, available from J.C. Whitney.

Each of the four solenoids has one end of its coil connected to a 12V supply. (called EATX) The computer "sinks" or grounds the other side of the solenoid's coil using PWM (pulse width modulation) to engage it. The end result is that the coil is pulsed on and off at a very high frequency (fast enough that the plunger will not vibrate.) This is used to control the average current through the coil. Each coil is only 3 ohms, and if you directly ground the switched side of the coil, you will burn it out. The 12V EATX supply to the solenoids is provided to pin 4 of the 8 pin connecter above the solenoid pack This is a switched supply, controlled by the computer via a relay.

The transmission controller periodically checks the continuity of all the solenoids, about every ten seconds or so. It does this by sending a quick grounding pulse to each solenoid and looks for an inductive kickback, or spike. If it does not see any inductive kickback or spike, it assumes the solenoid being tested is open or the wire going to it is disconnected. This condition will make it go into limp mode. Limp mode is only second gear and reverse. As you can now see, we cannot simply cut the wire going to that solenoid and leave it disconnected. The computer would detect this. Also, first gear would not be available. The solution is to open the circuit for the solenoid only when we are not in first gear and somehow trick the computer into thinking that the wire is still connected.

Remember, when the transmission is not in first gear, the solenoid's function becomes torque converter lock up control.

I solved this problem using two relays. The first relay opens the connection to the LR/LU solenoid ONLY when the transmission is in third or fourth gear. (it does not disable lockup in second gear, more on that later) The second relay serves as a decoy to make the computer think that the solenoid is still connected. Its coil is connected to the computer all of the time so the controller thinks the solenoid is still connected when it does its continuity test. The computer sees the coil of the relay, and it has similar electrical characteristics of a solenoid. We cannot substitute a resistor for the coil since the computer actually looks for an "inductive kickback" when it checks the solenoids.

The first relay is a single pole double throw type. This has two coil connections, and a common, normally closed, and normally open connections. The common and normally closed contacts are put in series with the wire for the LR/LU solenoid. When the relay is not energized, the solenoid is connected. Connect the common terminal on the relay to the wire going to the harness going to the TCU, and the N/C (normally closed) terminal to the wire coming from pin 7 (light blue) from the connector on top of the solenoid pack. The relay coil is connected with one connection going to 12 volts (switched battery) (pin 4) and the other connected to the OD(3-4) pressure switch (pin 3). When the transmission is in third or fourth gear, the pressure switch closes which grounds pin 3 as well as the other end of the coil of the first relay. This makes the relay pull in and open the connection to the LR/LU solenoid, thereby preventing lock up.

The second relay's coil is connected with one end going to 12 volts (switched battery pin 4) and the other end connected to the wire going to the computer that controls the solenoid. (the wire that used to go to pin 7) (Please note that the wire going to the computer was previously interrupted by relay 1. The coil wire needs to be connected to the common contact on the relay 1. You will wind up with two connections at that point. ) Do not make any other connections to this relay. ONLY the coil is being used.

You may be wondering about second gear. The transmission computer is programmed to allow torque converter lock up in second gear only if the engine is excessively hot. In all my years of owning my vehicle, this has never happened. If the transmission controller does want to do lock up in second gear – for example you are pulling a trailer up a hill, chances are that it will not shudder since the engine RPM will be quite high which would increase the line pressure eliminating any shudder.

When you install this circuit, the transmission controller will detect that the torque converter is not locking up. It will not go into limp mode. It will store a code in the TCU indicating loss of lockup control. This is code 38, and is normal.

One more method (from Andrew Dodd)

There's an easier way to implement lockup disable. Most lockup torque converters disable lockup when the brakes are applied, so a safer way to disable TC lockup is to fool the TCM into thinking that the brakes are applied. Instructions on how to do this on some A604-equipped vehicles can be found here. Rather than splicing into your TCM's solenoid harness, you splice a toggle switch into one wire. I'll be installing this in my LeBaron over winter break (Not because of shudder, but for better around-town acceleration when I want it.) This also should work on A413 lockup-equipped vehicles.

Important note: Craig wrote, “I have tried this method before with my own vehicle and two other people's, and found that it did not work for me. After 30 seconds or so of the brake override switch (fools the computer that the brakes are on) the lockup engaged anyway. Not to mention that I was told my brake lights were also coming on when I flipped the switch. I am not saying that his method will not work at all, it simply did not work in my three cases. I am not looking for a war, or anything like that, but just want to inform people that his method, just as mine, does not apply to all vehicles.”

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