46RE/47RE Transmission Control Relay Coil Power Circuit
By Jon Glatstein, ATSG Technician
46RE and 47RE units
use an interesting method to energize the coil of
the Transmission Control Relay. As we all know, the switch part of the Transmission Control Relay is what
provides battery power to the solenoids when its coil is energized.
The PCM controls the Transmission Control Relay by turning the relays coil ground on and off. What is
interesting is the other side of the coil where power comes in.
We are all familiar with the Accessory power buss. All of the circuits that become energized only when the
key is in the ACC or RUN position. We also know about the ignition power buss which is all the circuits that
are energized only with the key in the RUN position. Of course there is the battery buss which is on all the
time. Well, in the 46RE/47RE units there is another power buss. It is not documented as a power buss, so it
does not have an “official” name. Let’s call it the “Engine Running” buss.
Battery power to the Engine Running buss is provided by the PCM at pin 25 of connector 3 (see figure 1)
ONLY when the engine is running. This is labeled “Generator Source” because it actually is the “Source”
voltage for the generator’s field winding. There are two other devices connected to this same wire, the Leak
Detection Pump in the EVAP system, and the Transmission Control Relay.
The Electronic Voltage Regulator circuit inside the PCM energizes connector 3 pin 25 only when the engine
is running. The EVR controls the output of the generator by varying the pulse width of the generator field
winding ground through connector 2 pin 10 which they call “Generator Field Driver.” This driver, like many of
the drivers in this PCM, controls ground.
This Generator Source voltage goes through two splices before it gets to the generator. At splice 116 it forks
off to the Transmission Control Relay pin 86, which is one side of the relays coil. The other side of the relays
coil goes to pin 85 and then connects to the PCM at pin 30 on connector 2 which is labeled “Transmission
Control Relay Control.” This pin controls the Transmission Control relay by turning the coils ground on and off.
The second splice that the Generator Source wire goes through is splice 180 where it is tapped of to provide
power to the Leak Detection pump in the EVAP system. Electrically speaking, this pump operates the same
way as the Transmission Control Relay coil. Engine Running power comes in at pin 2, and pin 3 connects to
the Leak Detection Pump Solenoid Control at connector 3 pin 10 of the PCM.
So, if you do not see power to the Transmission Control Relay, or the solenoids, with the key on, try starting
the engine. The same thing applies when trouble shooting open solenoid codes. If you see problems with
the charging system, the evap system, and the tranny at the same time, this “Engine Running” buss is the
common element.
One additional note. In every wiring diagram I have ever seen for this circuit the Generator Field Winding is
labeled incorrectly. The Plus and Minus signs are backwards. Generator pin 2, which leads to the Generator
Source pin at the PCM should be labeled as “+” while generator pin 1, which leads to the Generator Field
Driver pin at the PCM should be labeled “-.” Often this plus and minus designation is incorrect in the PCM
part of the wiring diagram as well.
Jon Glatstein is a regular contributing author to Transmission Digest and is a technical consultant and
electronics specialist for ATSG. He is also the ATSG Webmaster.
(and some 42/44RE units after 1996)
