Transmission Overview
The 4T80E is a heavy duty transaxle appearing in Cadillac Northstar vehicles, but rated to handle the extra loads of motorhomes. This unit has two shift solenoids, a converter clutch solenoid, and a force motor to control line pressure. The solenoids receive power when the key is on; the computer supplies ground to energize the solenoids.

Terminal Identification
Here’s a breakdown of the individual connector terminals:

Electronic Pressure Control
To check the electronic pressure control signal, connect your high-impedance voltmeter or signal monitor to the terminals shown.

Checking the Shift Pattern
Here’s how to connect your signal monitor to check the transaxle shift signals:

And here’s the signal pattern you should see:

Forcing the Shift
Here’s the manual shift pattern you can expect with the transaxle in failsafe:

Here’s how to force the 4T80E transaxle to shift electrically:

First Gear
In first gear,only solenoid A receives ground.

Second Gear
To shift into second gear, the computer releases ground from solenoid A; neither solenoid is grounded.

Third Gear
To shift into third gear, the computer grounds solenoid B.

Fourth Gear
To shift into fourth gear, the computer grounds solenoid A; both shift solenoids are grounded.

Lockup
These are the connections necessary to apply the converter clutch.

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