Third Clutch Failures

TECHNICAL TRAINING
Shift Pointers
•Author:
•Subject:
•Essential Reading:
Jesse Zacarias
Erratic 2-3 shift, code P0780
Rebuilder
•Unit:
Shop Owner
BAYA 5-speed automatic
Center Manager
•Vehicle Applications:
Diagnostician
2003 Honda LX
R&R
What’s Causing Third-Clutch
F
or a while now I have been
interested in finding out what
is causing third-clutch failures in Honda five-speed transmissions. An opportunity finally
presented itself when a customer
brought in his 2003 Honda LX with
a 3.0L engine and a BAYA fivespeed automatic transmission. We
had overhauled this transmission
three years ago when the vehicle
had 60,000 miles. Now with
134,000 miles the vehicle had an
erratic 2-3 shift and code P0780
“Problem in Shift Control System.”
When we disassembled the unit
the only problems we found were
burnt third clutches (Figure 1) and
badly burnt fluid.
The customer was not too
happy with a vehicle that had experienced two transmission failures in 134K miles – the original
one at 60K and now the rebuilt that
lasted 70K. So I bought the vehicle
because I thought it would be perfect for finding the cause of the
third-clutch failure, since we knew
the vehicle’s history.
I wanted to put the transmission
as close as possible to the condition
in which it came in but in a workable state to obtain the readings we
wanted, so we repaired the transmission only. That is, we replaced
the third clutches and steels with
Honda original equipment, installed an aftermarket rebuilt converter and thoroughly cleaned the
valve body. We did this to find the
original cause of the failure.
When we rebuilt the transmission three years ago, we installed
an overhaul kit with Honda original clutches; aftermarket rebuilt
50
Failures in Honda Five-Speeds?
converter; new linear solenoids A,
B and C; and new shift solenoids
A, B, C and D. We did not replace
the bushings in the mainshaft or
the countershaft, since at that time
we did not have the tools to replace them. We also did not re-
1
3
flash the PCM to the latest software or replace the pressure
switches.
I connected our test equipment
to the vehicle (Figure 2) to obtain
as much data as possible. I obtained main line-pressure readings
2
During 2-3 shift
Channel 1 main line pressure, Channel 2 1st-clutch pressure
Channel 3 2nd-clutch pressure, Channel 4 3rd-clutch pressure
Jesse Zacarias
Transmission Digest
along with the pressure reading for
each application clutch, amperage
reading at linear solenoids A and
B, and voltage readings from each
shift solenoid and the third and
fourth pressure switches. I even
monitored the duty cycles of linear
solenoids A and B during upshifts
and downshifts.
I looked for any pressure lost in
the third-clutch circuit while the
transmission was hot, and as
Figure 3 shows, third-gear oil pressure was within the 10-psi difference allowed between apply
pressure and main line pressure. In
this case it was equal to main line
pressure, proving no pressure lost
in circuit 30, which leads to the
third-clutch pack. This is interesting, because as we mentioned at
the beginning we did not replace
the bushings in the countershaft or
mainshaft, and by this time they
had 134,000 miles of wear.
After obtaining all this data, I
sat down for hours analyzing it. I
noticed something peculiar when
looking at the recordings of the
pressure readings taken during upshift and coast downshift. On the
third clutch, pressure was present
during a forced 4-2 downshift but
not as much during the 5-2 downshift (figures 4 and 5).
When I looked at the recordings
made with the third pressure
switch connected to the scope, I
noticed that the pressure present
was enough to close the third pressure switch (Figure 6).
This third-clutch pressure was
present from when fourth pressure
was connected to CPC A pressure
(Channel 3), through 2nd gear
(Channel 1) and until third gear
was fully applied (Channel 2), for a
total of about 2 seconds and reaching at times 50 psi. This thirdclutch pressure reaches its highest
when second-clutch pressure is
fully applied at 125-130 psi. That
could not be good for the thirdgear clutches, but was it a cross
leak? It was not present during all
the upshifts or coast downshifts
and not even during the 5-2 forced
November 2011
downshift. So that almost ruled
out a cross leak, but how was this
oil pressure getting into the thirdclutch circuit on a 4-2 downshift?
4
To eliminate the possibility of a
software update that might solve
this problem, I had the PCM recontinues next page
During forced 4-2 downshift
Channel 1 2nd clutch, Channel 2 3rd clutch
Channel 3 4th clutch, Channel 4 5th clutch
Jesse Zacarias
5
5-2 downshift
Channel 1 2nd-clutch pressure, Channel 2 3rd-clutch pressure
Channel 3 4th-clutch pressure, Channel 4 5th-clutch pressure
Jesse Zacarias
51
Shift Pointers
flashed to the latest software per
Honda’s bulletin. When I ran the
same pressure test, no difference
was apparent other than crisper
6
shifts, so that ruled out a correction with a re-flash.
After studying the hydraulic
charts I discovered the reason for
During forced 4-2 downshift
Channel 1 2nd clutch, Channel 2 3rd clutch
Channel 3 4th clutch, Channel 4 3rd pressure switch
Jesse Zacarias
the presence of this oil pressure.
Remember that the PCM modulates linear solenoids A and B to
control the gradual engagement
and disengagement of the clutches
that are coming on and going off. It
does this by slowly increasing or
decreasing the amperage. For more
details see “A View into the Honda
Five-Speed’s Shifting” in
Transmission Digest, August 2011.
Let’s look at the PCM solenoid
strategy during a
4-2 downshift, using the sweep
reading of the scope in Figure 7 as
an example. As you can see it takes
three steps to accomplish a 4-2
downshift.
We start with all solenoids off in
fourth gear.
1) Solenoid C (blue) is turned on
for about 0.3 second. This action
moves shift valve C, connecting
fourth-clutch-circuit oil to CPC
A oil pressure, already at high
amperage (high pressure), and
makes the solenoid shifting
strategy the same as for a 3-4
shift: SSA off, SSB off, SSC on.
7
Shift solenoids A, B & C on a 4-2 downshift
Channel 1 solenoid A, Channel 2 solenoid B
Channel 3 solenoid C, Channel 4 3rd-gear switch.
Jesse Zacarias
2) Next, as solenoid C (blue) is
being turned off again, solenoid
B (green) is being turned on at
the same time for about 1 second. This gives us the solenoid
strategy for the 2-3 shift: SSA
off, SSB on, SSC off. This solenoid strategy moves shift valve
B and connects fourth-clutch
pressure that is now CPC A oil
pressure to exhaust port H4X at
shift valve B.
3) Finally, solenoid A (yellow)
goes back on, giving us the solenoid strategy in second gear:
SSA on, SSB on, SSC off.
Now let’s look at what is going
on in the hydraulic circuit during a
4-2 downshift (Figure 8). When
SSC was turned on for 0.3 second,
CPC B oil pressure that was beginning to modulate to lower amperage (Figure 9) increased in
pressure. Because of the way the
shift valves are placed by the solenoids, this CPC B pressure starts to
enter third-clutch circuit through
52
Transmission Digest
8
SSA off
9
SSB off
SSC on
3-4 shift strategy
4-2 forced downshift
Channel 1 linear solenoid A, Channel 2 linear solenoid B
Channel 3 3rd-gear switch, Channel 4 4th gear
Jesse Zacarias
shift valve C circuit 5C, through
shift valve B, then through shift
valve A and finally through circuit
30, third clutch.
When SSC was turned off again
and SSB was being turned on (0.7s
on the time scale in Figure 7), CPC
B oil continued uninterrupted into
circuit 30 third clutch, this time
through circuit 5D at shift valve C,
through shift valve B, through shift
valve A and finally to circuit 30 at
shift valve A (Figure 10).
Finally, solenoid A comes on,
giving you second-gear solenoid
strategy (SSA on, SSB on, SSC off).
Third-gear oil exhausts at H3X
through shift valve A when shift
solenoid A is turned on (Figure
11).
What makes matters worse is
that, as figures 6 and 7 show, the 23 shift can start immediately if the
gas pedal is backed off slightly, by
turning solenoid C on. This does
not give the CPC B oil present in
circuit 30 enough time to exhaust.
Because of the 2-3 shift strategy
CPC B oil starts to enter again into
circuit 30, or third clutch.
On the basis of this finding, it is
my opinion that this CPC B oil
pressure present on a 4-2 forced
downshift is dragging the clutches
and contributing to an early thirdclutch failure. If we keep in mind
that the third clutches are also applied momentarily during the shift
continues next page
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Circle No.
November 2011
10 on Reader Card
53
Shift Pointers
10
SSA off
SSB on
SSC off
2-3 shift strategy
SSA on
SSB on
SSC off
2nd-gear strategy
11
12
Neutral to Drive
Channel 1 main line pressure, Channel 2 1st-clutch pressure
Channel 3 2nd-clutch pressure, Channel 4 3rd-clutch pressure
Jesse Zacarias
from Neutral to Drive (Figure 12)
and it requires only 36 psi to engage, we can imagine what 50 psi
can do with the second clutch on
with up to 130 psi. When these vehicles are being driven in fourth
gear, it also does not take much
throttle increase to get a downshift
to second. On my many road tests
on this car, 4-2 downshift was a
normal shift when I entered the
freeway.
It is also my opinion that by increasing the size of the third piston
in 2004, Honda made the problem
worse, since now you have about 1
square inch more application area.
I believe the solution would require a software change for the
PCM or else the third clutches
would have to be made to take this
kind of abuse without sacrificing
the 2-3 shift comfort.
In the past four months I have
been trying to find a solution. In
looking into the PCM strategy, it
would have to be programmed not
to lower the amperage on linear solenoid B during a forced 4-2 downshift (Figure 8). In other shift
situations linear solenoids A and B
are modulated independently of
each other. There may be other reasons why Honda has decided to
program it this way, but trying to
talk with the company did not produce any results.
My next approach was through
the clutches. I tried talking with
some clutch manufacturers, but the
only one that took the time to talk
was Raybestos. The company’s
GPX plates seem to be the best
choice at the moment, because of
their unique material and special
grooved pattern that makes them
run cooler when third clutches are
being dragged. Raybestos told me
that with this new information
they’ll look for options that may
result in even more durability. I
still have more testing to do, so my
research continues. TD
Jesse Zacarias is the owner of Elec-Tran
Diagnostics (www.electrandiagnostics.com) in
Gilroy, Calif.
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Transmission Digest