Pressure Control Problem Diagnosis - ATSG AXOD Update Handbook

PRESSURE CONTROL PROBLEM DIAGNOSIS

To accurately diagnos and repair pressure control problems, it first must be determined whether the problem is a
malfunction that is inside the transaxle, or outside the transaxle. The pressure problems that we usually need to
address and repair are high line, low line, and/or inadequate line rise, and fluctuations as seen on a pressure
gauge while driving. One of the tools that can be used to determine inside transaxle or outside the transaxle, is
the "Schaffer Shifter", or the "Tran X 2000". However, a current draw test in the EPC circuit can also be
performed, while monitering the pressure changes on a 0-400 lb. pressure gauge.
NOTE: It is very important to perform this test with a 0-400 lb (or higher) gauge as this transaxle is
capable of producing over 400 PSI of line pressure, which could burst a gauge of a lower rating,
and possibly cause injury.
A current draw test in the EPC circuit will also require the use of a Digital Volt-Ohm Meter, that is capable of
reading Amps or Milliamps. When performing this test you will be looking at amperage changes in relation to
pressure changes, as seen on the pressure gauge. With a normally functioning EPC circuit, you will see about
150 PSI on the pressure gauge while in Park, on the initial start up, which is part of the computer strategy, and is
normal. After the vehicle is warmed up and/or is put into gear the idle pressure will drop immediately to
between 48 and 77 PSI. As the throttle is opened, the amperage should begin to drop, and the pressure should
begin to rise.
To begin the EPC circuit current draw test, the EPC ground wire that runs from the computer to the transaxle
case connector must be located, and the external wire color will be White with a Yellow tracer on both versions,
as shown in Figure 98. On 1991-1992 model vehicles the wire will be found in the top case connector and comes
from pin number 38 on the computer. On 1993-UP vehicles there is only one case connector and the wire still
comes from pin number 38 on the computer. Refer to Figure 98.
Connect the 0-400 lb. pressure gauge to the mainline pressure tap, as shown in Figure 100. Cut the White wire
with the Yellow tracer and hook the DVOM, in series, so that current flows through the DVOM, as shown in
Figure 99. Start the engine and let it warm up until the pressure stabilizes between 48-77 PSI. The amperage
reading at idle should be approximately 1 Amp. As the throttle is opened, amperage should begin to drop, and
pressure should begin to rise. Refer to the chart in Figure 99.
If the amperage drops, but no pressure rise, the problem is inside the transaxle.
Problems on the inside of the transaxle will include, EPC Solenoid failure or trash in the solenoid, boost valve
and/or sleeve damaged, pressure regulator valve damaged and/or sticking, blockage from gasket material (early
models), pump damage and/or pump slide sticking.
If the amperage does not drop, and the pressure does not rise, the problem is outside the transaxle.
The inputs that are used by the computer to determine pressure changes will now have to be checked. These
inputs include, Throttle Position Sensor, Mass Airflow Sensor, Power Steering Pressure Switch, Turbine Shaft
Speed Sensor, Vehicle Speed Sensor. Bad harness connections and wiring damage must also be thoroughly
checked and verified. After all sensors, connections and wiring have passed inspections and tested OK, the
computer may have to be replaced.
When pressures are seen to be fluctuating on the gauge, but the amperage readings are steady, then the problems
are on the inside if the transaxle.
If the pressures are seen to be fluctuating on the gauge, and the amperage readings are also fluctuating, then the
problems are on the outside of the transaxle and are electrical.
Technical Service Information
FORD AXODE (AX4S)
AUTOMATIC TRANSMISSION SERVICE GROUP
112
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