A Ford Transmission Discrepancy
Robo Tech was called to a shop for a 2004 Ford Explorer with a transmission complaint.
The customer complained of transmission slipping between gears, making a clunking, rattling,
grinding noises when accelerating and suddenly having trouble shifting between gears. It
had 97,000 miles on it. The check engine light had came on the last month and customer was
told he needed a full transmission rebuild because the bands were worn out. Well, Robotech
didn't think so. I was then determine to find the real reason for the troubles.
I started my diagnostics by performing a full visual inspection. Then I proceded to do a
scan test, retrieve codes and a PID analysis. There were codes P0700, general transmission
control fault, P0741 Torque Converter Clutch Circuit Performance, or Stuck Off, and P0780,
Shift Malfunction . The customer did mentioned that he had at one time disconnected the battery
for 10 minutes and the issue had gone away. Then, 8 months later the issue came back.
That in itself was a major clue. By disconnecting the battery you are actually resetting the
KAM or keep alive memory. This is an adaptive memory that records sensor wear and tear.
So, the issue was possibly a sensor failure passed the normal wear point. But what could
ithad been?
A quick analysis of the situation was needed. O.B.D. 2 is an analytical diagnostic system.
It relates to the relationships between components. I had ageneral transmission code P0700, that
was understandable. But, what did 741 and 780 had to do with each other? What did the
torque converter and shifting issue had to do with it?
For that we need a little overview on transmission control. The T C M computer makes a continuous
comparison between I S S and O S S or input and output speed sensors. A discrepancy will
cause general type D T Cs. Again, the here is that no specific codes are set. An example
of a specific code would be a short or open circuit. But the codes in this vehicle were
generalized. I knew then that the issue was not a hard fault, but a discrepancy that was
out of the programmings threshold. But what could it be?
A certain clue was the association of torque converter and shifting error. You could have
a faulty transmission but not necessarily a converter issue. There were two possibilities
here, either the torque converter was faulty, or one of the speed sensors was shot.
I proceeded by doing a waveform analysis test on the I S S and O S S sensors to check for
a sudden drop in signal. I did the test on a rough road to wiggle the wires and sensor.
After half hour of testing, it all checked out fine. Not a hitch on the signal, which
was recorded over time. So, there was only one last test to do here and it was a roug
one. I had to do a torque converter stall test. This is done by manually actuating the
T C C solenoid, with the engine running at idle in drive. If the torque converter is
good, the engine should stall. The T C C solenoid meshes the engine and wheels together at higher
speeds to save gas. By forcing a mesh at the T C C the system is stressed. I hot wires
a ground to the T C C solenoid, and in drive, at idle, grounded the T C C. The engine R
P M dropped a bit, but did not stall. This torque converter was faulty, and was causing
a discrepancy between I S S and O S S signals, hence the problem would disappear when the
battery was disconnected. I ordered the torque converter replaced and the vehicle was never
seen again.
This case illustrates the importance of D T C analysis. One of the greatest assets in
O B D 2 is that actual code definitions. Almost all codes now have a variation between electrocal
hard fault or performace. Remember that this is always a clue in itself. Hope you learned
from this one.