740/1740 Codes and
Chrysler Converter Flow
Sometimes the reason for the 740 code is
not found within the transmission or torque converter. A
restricted cooler or line is a good example. Using a fluid
without the correct frictional qualities (the wrong fluid) may
cause a 740 code. The hydraulic integrity of the valve body
(worn bores) and the pump (excessive clearance or cross leaks)
are two possible culprits that also must be eliminated. If all
of these possibilities check out OK, it’s time to take a
closer look at the torque converter.
Converter charge oil enters the converter
through the input (turbine) shaft. When this oil exits the end
of the input shaft, its path around the splines (between the
shaft and turbine hub) is blocked by a seal. The only remaining
path is through the grooves on the outside of the turbine-hub
bushing. The oil then passes between the turbine hub and cover,
fills the area in front of the lockup piston and eventually
spills around its outside edge, where it continues to fill the
rest of the converter before exiting as lube oil.
The action of the converter charge oil
between the piston and cover is what keeps the piston from
dragging on the cover when the converter is in the TCC-release
mode. When the switch valve is stroked to the TCC-apply
position, the converter charge oil switches and goes straight
to the cooler. The residual pressure between the piston and the
cover (release oil) then is vented off through the slot in the
plate but first goes past the switch-valve spool. This allows
the oil between the lockup piston and cover to exit the
converter through the input shaft so that the piston can
the flex plate is bent, it could allow the
converter to move rearward and restrict the flow of oil between
the input shaft and cover. The example shown in Figure 2 was
close enough to cause a 740 code.
The turbine-hub bushing is the next element
that may cause a restriction. When the bushing is installed
properly, the top of the bushing is flush to 0.020 inch up from
the top of its bore. The bore for the bushing is in the pilot
area of the cover, and the top of the bushing bore is recessed
about 0.028 inch below the thrust area for the turbine hub. The
cutaway in Figure 3 shows a bushing that was installed 0.080
inch (twice the thickness of a dime) below flush. You are
viewing the bushing from the bottom side, so you can see that
flow was completely restricted. This, too, caused a 740 code.
The next elements that may cause a flow
restriction are the thrust washers or bearing that handle the
axial thrust forces between the turbine hub and cover. Since
all the oil that enters or exits the converter through the
input shaft must pass through this area, the
torque-converter rebuilder must be aware of
the differences. Hold the turbine hub in your hand with the
cover side facing up. Now, place whatever you are using between
your thrust surfaces onto the hub (for example, a thrust washer
or bearing), and, last, put a turbine-hub bushing onto the
turbine hub.
Imagine the oil flowing through the grooves
on the outside of the bushing toward the thrust washer or
bearing. The tolerance between the inside diameter of the
washer or bearing and the outside diameter of the turbine hub
usually is close enough to prevent much flow between them. The
cover does not have any passages for oil flow that extend
beyond the grooves on the outside of the bushing. If you use a
solid washer in this application (brass, fiber or phenolic),
the only flow that you will have is the oil that squeezes past
the internal clearances that you have built into your
converter. This also will usually set a 740 code.
If your thrust washer has grooves on only
one side, you also need to pay attention to this. When you
place the thrust washer on your turbine hub with the grooves
facing up (toward the cover) you can see that there are
passages for oil flow, but if you can place the thrust washer
on the turbine hub with the flat side up, you will see that
when the oil exits the grooves on the outside of the
turbine-hub bushing, it has no place to go. This also may cause
a 740 code. Some bearings are well equipped for oil flow (the
604, for example). Other bearings may restrict the flow of oil.
Place your bearing on your turbine hub and make sure that it
will not restrict flow.
Chrysler Front-Wheel-Drive Transmissions
The 518 and 604 converters are first
cousins, hydraulically speaking. They share many of the same
flow characteristics and several of the same problems.
The input shaft of the 604 has more
clearance at the cover, but clearance probably would not be an
issue anyway, because of the stronger flex plate. The
turbine-hub bushing on the 604 looks like a smaller version of
the 518 bushing with one less groove on the outside. When the
bushing is installed to its proper depth, it is flush to 0.020
inch up from the thrust surface of the cover. This is well
above the 0.012-inch flow-relief step at the top of the bore.
The machined angle at the bottom of the bore for clearance is
greater than its sister angle on the 518 converter and is more
flow-friendly.
Special attention should be paid to the
position of the bushing. Since the inner race of the OE bearing
is only 0.015 inch below the thrust surface of the outer race
(see Figure 4), it may contact the bushing if the bushing is up
0.020 inch above flush as shown in Figure 5. The 0.005 inch of
wear shown in Figure 6 is the result of that contact. This is a
bad thing for two reasons: First, the material from the bushing
that was worn away by the bearing race has to pass through the
bearing, and second, the pinpoint contact of the inner race of
the bearing to the bushing may cause the race to flex and
eventually fail. Either of these two concerns may help to
explain the failure rate of the bearing.
As in the 518 converter, a solid thrust
washer may cause flow restrictions and 740/1740 codes. The
position of the bushing is also important when you’re
using a grooved thrust washer, but not for the same reasons
that were important with the bearing. The bushing may keep the
thrust washer from contacting the thrust surface of the cover.
The distance that the thrust washer is held above the cover
translates into extra and unwanted internal clearances,
including clutch-release clearance. Since the damper springs
have no permanent retaining device, as the clearance between
the TCC piston and turbine increases the chances of a spring
walking out of place also increase. A small amount of extra
clutch-release clearance will allow the spring to walk up the
turbine only enough to push the clutch forward, making it drag
on the cover. A larger amount of clearance will allow the
spring to come completely out of the piston’s spring
retainer, causing catastrophic results.
Checking Chrysler’s clutch-release
clearance is time-consuming at best, and some rebuilders are
relying on the “use thicker friction material and
hope” method. Making sure that the turbine-hub bushing is
not creating extra clearance may help to eliminate some
problems. 
Ed Lee is a technical specialist for Sonnax
Industries Inc.
©2005 Transmission Digest