How can I motorize 42077 Technic Rally Car with LEGO Power Functions?
With some Power Functions elements and a few extra pieces, it's pretty easy to turn the Rally Car into an RC car.

Here's a quick demo video of my design in action.



And here are the instructions I made for it.

Required extra pieces

First you are going to need some extra pieces:




Power Functions:


1x - Servo Motor
1x - IR Receiver
1x - Battery Box
1x - L-Motor (alternately you can use an M-Motor but it is not as powerful)

Technic:


1x - 5L beam
2x - 2L thin beam
4x - Axle Pin with Friction Ridges
2x - 3L axle



In addition to that, you will need a Power Functions remote, and possibly some extra pieces to build the remote. If you don't have a remote already, there are instructions for one at the end.

Disassembly

To add power functions, you will need to disassembly it a bit to remove the engine block and manual steering system.


Remove doors.





Remove roof sides.





Remove roof gear and pull the blue pins out one step.





Take the center of the roof off with the front window frame.





Detach the trunk.





Detach the hood.





Remove seats.





Remove fire extinguisher.




Remove manual steering.

Removing the steering system is a little tricky, but doable.




Locate the place where it attaches to the back.





Push up on the grey piece to detach it from the small black pieces, so you can slide it back and remove all the yellow pieces.




All of these pieces should be removed.



Also remove the black and blue pieces that hold it.



After they are removed, the engine block will be easier to remove.



Removing engine block


On the back, locate this area.





Partially detach it temporarily.





Remove the gear and axles.





Reattach the section.





From the underside, locate where the engine block attaches at the back.





Push up to detach the engine block and the white pieces and pins that attached it.




Check alignment

At this point, you going to want to make sure the front gear attached to the gear rack for steering is perfectly vertically aligned like so. If not, slide the pin out and adjust it. It needs to be vertical for the servo motor to attach to it.



Locate pieces

In these sections you removed, you will find some other pieces you will need.



Disassemble them to find these pieces.



Add drive motor


Build this.





From the underside, position it to slide in the back into the differential area.




In this area, you are going to need to add the black gear.




Hold a black gear in place, then slice the axle and motor into the holes.






Add steering motor


In the front, flip up this white panel.







Slide the dark grey in the gear out one stud.





Build this bracket.





Insert it here, then slide the red pins out one stud.







Through the seating area, insert the servo motor into the bracket and push the red pins back in.







Now if the axle is correctly aligned, you can just slide the axle into the motor.





Flip the panel back down.




Adding Battery Box


Build the following.





Slide the batter box into the back here.







On the sides, attach it with the pieces you just built.








Re-assembly


Reattach seats.





Reattach hood.





Reattach roof center, and slide the blue pins back in.







Reattach the trunk to the blue pins.







Hold the trunk in the up position, and reattach the roof sides.







Reattach the doors.




IR Receiver


Attach black pins to the receiver, and attach it to the spoiler.







It's tricky, but run the wire through the crack down to the batter box.









Now run the wires from the motors up through that crack, and attach to the IR receiver, servo to blue, engine to red.






Turning the car on.

Turning the car on/off is as simple as opening the hood and switching the orange switch on the battery box.



Remote

You can build the following remote with a few extra pieces.






Power Functions:


1x - IR Remote Control

Technic:


2x - Axle Pin 3L with Friction Ridges
2x - Axle and Pin Connector Angled #1
2x - Axle 2L
1x - Liftarm 1 x 4 Thin with Stud Connector (or the version without the stud connector)
1x - Wheel 30.4mm D. x 20mm (or some other wheel rim)


Build sticks.







Attach sticks.





Build arm.





Attach arm.





Attach wheel.




Happy building!