45RFE Simplifying Sol`s Codes

Simplifying Solenoid Electrical Codes
by Jesse Zacarias
Valve Body Pro
Research and Development
www.valvebodypros.com
D
ealing with solenoid electrical
problems sometimes can be
confusing, especially when you
read the possible causes on the factory
diagnostic tree.
For example, on a code P0755 for
a 545RFE, which is described as a 2C
solenoid circuit, the possible causes are
said to be one of these conditions:
• 2C solenoid control short to
other circuits
• 2C solenoid control circuit open
• 2C solenoid short to ground
• Transmission solenoid assembly
• Power control module (PCM)
The diagnostic procedure provided
isn’t very helpful.
Let’s simplify things a little. With
this in mind, I’ve redrawn the solenoid
electrical circuit (figure 1) to keep it
simple and easy to understand.
30
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C4
11
LP
10
8
6
4
2
1
LR/LU
UD
2C
MS
4C
OD
18
RY1 Transmission Control Relay
Figure 1
When you turn the ignition on or
start the engine, the PCM provides
voltage to the transmission control
relay through pin 18. The transmission
control relay, represented by RY1 in
the drawing, is grounded to the body;
this causes the relay to close, which, in
turn, provides system voltage to all of
the solenoids.
Then the PCM runs a solenoid
integrity test by grounding each shift
solenoid. If all the solenoids’ circuit
tests are approved, the PCM will
keep the transmission control relay
energized; from there it’ll run the
integrity test periodically.
If the test fails on any of the
solenoids, the PCM will de-energize
the transmission control relay, which
cuts off the supply voltage to the
shift solenoids and puts the vehicle in
failsafe mode.
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Simplifying Solenoid Electrical Codes
Let’s apply this information to a
2004 Dodge Ram 2500 4x2 with a
545RFE and code P0755. This truck
was brought in after the technician
replaced the solenoid pack and the
code persisted.
Since this is an electrical
code, you’ll want to check both
the voltage supply and ground at
the same time. By backprobing the
solenoid in question, in this case
solenoid 2C at the PCM connector
C4 pin 6, and backprobing any other
two solenoids on the solenoid pack
for reference, in this case solenoids
OD and 4C, you can begin the test
Channel 1 (yellow trace) shows the 2C solenoid control
at C4 connector pin 6; channel 2 (green trace) shows the 4C
solenoid at pin 2; and channel 3 (light blue trace) is the OD
solenoid at pin 1.
The scope is grounded to the battery to provide a good
ground source. Figure 2 shows the signal zoomed out 16x to
reveal the whole 16 seconds.
Let’s take a closer look using
a one-to-one scale (figure 3). At
cursor 1, both the OD solenoid (light
blue trace) and the 4C solenoid
(green trace) passed the integrity
test, since both dropped to 0.54V
when grounded. But the 2C solenoid
failed the integrity test since the
voltage never dropped; it remained
at 12.29V.
This test proved there were no
open circuits on the 2C solenoid
circuit from the C4 connector to the
transmission control relay, because
system voltage was there when the
relay closed.
And there was a good ground at the PCM because
the solenoids that did pass the integrity test dropped to
0.54V (540mV). That leaves only the driver inside the
PCM, represented by the S4 on the drawing, as never
switching closed.
We replaced the PCM, reprogrammed the Vehicle
Identification Number (VIN) and ran the test again (figure 4);
this time all three solenoids passed the integrity test. Cursor
1 shows the 2C solenoid integrity test and cursor 2 during the
OD and 4C solenoid integrity test.
If the test fails on any of
the solenoids, the PCM will
de-energize the transmission
control relay, which cuts off
the supply voltage to the shift
solenoids and puts the vehicle
in failsafe mode.
Figure 2
Ignition turns on at about two seconds; the engine started
at cursor 1 and you can see the PCM run three integrity
tests on the solenoids before it finally went in failsafe mode
because solenoid 2C failed to pull to ground.
Figure 3
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Figure 4
As you can see, this test procedure can help you
diagnose future solenoid electrical codes easily and without
the confusion of the manufacturer’s diagnostic tree.
Jesse Zacarias is a consultant for Snap-on and
part of the Research and Development Department
of Valve Body Pro.
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