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Craig Sherman's guide to fixing torque converter lockup problems - Version 2
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).
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)
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.
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.
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.
(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.
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.
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
If you have any questions, you can send me an e-mail
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
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
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.
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. 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.”
We strive for accuracy but we are not necessarily experts or authorities on the subject. Neither the author nor Allpar.com / Allpar, LLC may be held responsible for the use of the information or advice, implied or otherwise, on this site. This page is offered “AS IS” and without warranties. By reading further, you release the author and Allpar, LLC from any liability.
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