Two failures that feel like one
Almost every power window complaint is one of two entirely separate things, and the first thing to do is decide which.
Electrical. The motor is not turning. Something in the chain — switch, fuse, relay, module, wiring, motor — is not delivering.
Mechanical. The motor is turning and the glass is not going where it should. The regulator has failed.
The test takes three seconds: press the switch and listen with the door open.
Silence means electrical
No sound at all from inside the door, from either switch.
- Check the fuse, and check it with a meter or by swapping it rather than by looking at it. A cracked element can look intact.
- Try the other switch. If it works from the door switch but not the master switch, the master switch or its wiring is the fault, and the motor is fine.
- Check the door-jamb wiring. This is the one people miss, and on high-mileage cars it is extremely common. The harness passes from the body into the door through a rubber boot, and it flexes every single time the door opens. Conductors fatigue and break inside insulation that looks perfect from the outside. Peel the boot back and flex the loom by hand while the switch is held — an intermittent window that works when you slam the door is this, essentially always.
- Back-probe the motor connector while somebody operates the switch. Voltage present and no motor means the motor. No voltage means keep going upstream.
- On vehicles with a door module, the window may be commanded over the network rather than switched directly, in which case a scan tool tells you more in a minute than probing does in an hour.
Noise means mechanical
You can hear the motor spinning, or a grinding, or a rapid clicking, and the glass does nothing or moves crookedly.
Cable-type regulators — the most common design — use a steel cable running around a drum on the motor and over pulleys to a carrier that grips the glass. The cable frays with age. Once strands break, the cable bunches on the drum or jumps out of a pulley groove, and what you hear is grinding while the glass tilts, sticks, or drops.
Scissor-type regulators use a rigid arm assembly on a pivot. They fail at the plastic slides and at the rivets. The failure is more often a sudden bind than a gradual grind.
Glass in the bottom of the door is a regulator failure in either design, or the glass has come off its clamps. It is not a motor fault.
The intermittent ones
- Works when warm, not when cold. A tired motor drawing more current than it should, or grease in the runs stiffening. Check the current draw.
- Works with the door open, not with it shut. Almost always the jamb wiring, with the loom position slightly different when the door is closed.
- Slows down at the top. The felt runs are dirty or the glass is binding in the channel. Clean and lubricate the runs with a proper rubber-safe lubricant before condemning anything — this fixes a real proportion of "weak motor" complaints for the price of a can.
- Goes up an inch and comes back down. Anti-pinch. The module thinks it has hit an obstruction. Either the runs are stiff, or the module needs reinitialising.
Doing the job
- Lower the window part-way if you still can — it puts the regulator's fasteners where you can reach them.
- Door card off. Screws hidden under trim caps, in the handle recess, and behind the grab handle. Take your time on the clips.
- Peel the vapour barrier back carefully and keep it intact. It is a water barrier and a sound barrier, and a torn one produces a damp footwell and a rattle.
- Support the glass before undoing anything that holds it. Tape across the frame, or a helper.
- Undo the glass clamps, then the regulator fasteners, and manoeuvre the assembly out through the access hole.
- Fit the new assembly, connect it, and test before reassembly — this is the step that saves taking the door apart twice.
- Reinitialise the window: the up-and-hold, down-and-hold sequence for the vehicle.
- Reseal the vapour barrier properly with the correct butyl. Do not use tape.
- Refit the card, checking every clip is located before pressing it home.
Buying the part
Get the assembly with the motor already fitted, and get the right corner. Front left, front right, rear left and rear right are four different parts, and left and right are from the driver's seat looking forward.
Some vehicles also differ between two-door and four-door, between manual and power, and between vehicles with and without one-touch. The build date matters more often than it should. Where a listing gives a part number, that is worth more than a vehicle description.
Questions people ask
- Should I buy just the motor or the whole regulator?
- The whole assembly, in almost every case. All of the labour is in getting the door card and the vapour barrier off; the parts cost difference is small next to doing that job twice. A new motor bolted to a regulator with a frayed cable is a repeat visit.
- Why does auto-up not work after fitting a new regulator?
- Because the module has lost its learned end positions. Almost every vehicle with one-touch or anti-pinch needs a reinitialisation — typically holding the switch up for several seconds past the top stop, then down past the bottom. The exact sequence is vehicle-specific and it is in the owner's handbook more often than people expect.
- One window works from its own switch but not the driver's master switch. What is that?
- That points at the master switch or the wiring between the two, not the motor — because the motor has just proved it works. Check the master switch first; they wear on the driver's door because that one gets used for every window.