Troubleshooting the Toyota A/T Temperature Light - ATRA

Troubleshooting
the Toyota A/T
Temperature Light
PCM
Most modern vehicles include
special features which are
supposed to enhance the
driver’s experience, sense of security
or enjoyment behind the wheel. When
something like an automatic transmission
goes bad, transmission professionals
like us get involved. Typically the
work we do turns a customer’s frown
into a smile again.
The purpose of this article is to help
you solve one problem that happens
on Toyota 4WD trucks and 4Runners.
These vehicles have an A/T warning
light system. If either the transmission
or the transfer case fluid gets too
hot, the warning light comes on and
TH01
TH02
Figure 1
Transmission
Temperature
Sensor
Transfer case
Temperature
Sensor
stays lit until the fluid cools down.
Unfortunately, the warning system isn’t
fool proof: Several ATRA members
have reported that after overhaul, the
warning light is on, even after they
verified the transmission and transfer
case fluids were operating at normal
temperatures.
Usually when this type of problem
happens you go right to your repair
database for troubleshooting information.
Unfortunately, all you may find is
a wiring diagram… and a poor one at
that. The next thing you might check is
a factory shop manual. Usually factory
manuals give you enough information
to solve the most difficult electrical
by Dave Skora
problems… but not
this time. The Toyota
shop manual barely
mentions the system,
let alone how to fix
it. So what’s the plan
of action? You could
just take the bulb out,
but that’s probably
not the best choice.
Here’s the right way:
First a little
background is needed
of how the system
is supposed to work.
If the transmission
fluid goes over 284º
F or the transfer case
fluid reaches 275º F,
the A/T warning light
will come on. The
warning light stays
on until the temperature
cools back down
by 30-40º F.
The tricky thing
about this warning system is that there
are two sensors. One is in the transmission;
the other in the transfer case.
To light the warning light on 1989-
95 models, the PCM grounds the circuit
when the transmission or transfer case
fluid gets too hot (figure 1). 1986-88
models are similar except they also
have a separate A/T warning light module
(figure 2). This module is connected
to a temperature sensor in the transfer
case. If the transfer case temperature
gets too hot, the A/T warning light
module also lights the A/T warning
light by grounding the circuit.
A/T Warning
Light
GEARS April 2006 35

Troubleshooting the Toyota A/T Temperature Light
TCM
A/T
Warning
Light
Module
Oil
2
1
3
4
5
B+
1986-88 Truck or
4Runner
If the A/T warning light continues
to stay on, locate the transfer case
module on the steering column support.
Disconnect the 5-wire connector. If the
light goes out, it indicates a transfer
case problem:
• the transfer case fluid is too
hot…
• the transfer case temperature
sensor is bad, or…
Transmission
Temperature
Sensor
Transfer Case
Temperature
Sensor
A/T Warning Light
Figure 2
• the A/T Warning Light Module
is bad.
To check the sensor and circuits,
see the A/T Warning Light Module pin
chart (figure 3).
If the A/T warning light stays on,
turn the ignition off. Disconnect the
TCM located at the right kick panel.
Then turn the ignition back on. If the
A/T warning light goes out now, it indicates
a transmission problem:
• the transmission fluid is too
hot…
A/T Warning
Light Module
Oil
TCM Connector View
• the transmission temperature
sensor is bad, or…
• the TCM is faulty.
If the A/T warning light is still on
with both A/T warning light module
and TCM disconnected, the warning
light circuit is shorted to ground. Look
for a short in the yellow/red wires
between the rear instrument panel connector
and either terminal 1 of the A/T
Warning Light Module or terminal OIL
of the TCM.
A/T Warning Light Module Pin Chart
Pin Function Condition Voltage or Resistance
1 Warning Light
Light Off
Light On
12 Volts
< 0.10 Volts
2 Ignition Key On 12 Volts
3 Transfer Case Sensor
Resistance between pins 3 5-20K ohms @ 68°F
4 Transfer Case Sensor
& 4
540-690 ohms @ 248°F
5 Ground Always < 0.10 Volts
Figure 3
Side
View
Transmission Temperature Sensor Transfer Case Temperature Sensor
Figure 4
36 GEARS April 2006

In case you haven’t found the
problem, let’s solve it from the other
direction.
• Turn the ignition off.
• Disconnect both the TCM and
A/T Warning Light Module.
• Turn the ignition on.
The A/T warning light should be
off. If it’s still on, fix the short to
ground in the yellow/red wire between
the instrument panel and either the OIL
terminal of the TCM or terminal 1 of
the A/T Warning Light module.
Assuming the A/T warning light is
off with both modules disconnected:
• Turn the ignition off.
• Reconnect the TCM.
• Turn the ignition on.
If the A/T warning light is on,
check the transmission temperature
sensor and circuit. See the A/T Warning
Light Module pin chart. If you don’t
find any problems, replace the TCM.
If the A/T warning light comes on
with the A/T Warning Light module
connected, check the transfer case temperature
sensor and circuit. See the A/T
Warning Light Module pin chart. If you
don’t find any problems, replace the
A/T Warning Light Module.
1989-95 Truck or
4Runner
If the A/T warning light is on, turn
the ignition off, disconnect the PCM,
and then turn the ignition back on. If
the A/T warning light is on, repair the
short in the wire between the warning
light bulb and the PCM.
If the A/T warning light is off,
measure the resistance across the terminals
at the PCM for the transmission
temperature sensor (Pin THO1 to
ground) and the transfer case sensor
(Pin THO2 to ground) (figure 4). The
sensor resistance should be 5-20 ohms
at 68° and 540-690 ohms at 248° F.
Check the circuit or sensor if the resistance
is incorrect. If the resistance is
correct, the PCM maybe faulty.
As with so many circuits, the A/T
warning light is a fairly simple system
to diagnose and repair, once you know
how it’s supposed to work.
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GEARS April 2006 37