A typical system is similar in many respects to a manual window-winding mechanism, except that the manual winder handle is replaced by a motor.
The system usually consists of a two-way control switch in the dashboard or centre console, wired via two circuits to a motor in each door. One switch position and circuit drive the motor one way to wind the window up; the other switch position and circuit wind the window down.
The switch is wired to the battery via a relay and a fuse or a circuit breaker.
Although they can be a real convenience (no more leaning over the passenger
seat to open or close the window) they can also be a considerable annoyance
when they stop working. Fortunately, however, both the mechanical and the
electrical parts of the system are fairly straightforward for the advanced DIY
mechanic to work on.
Identify the fault
The most obvious fault with electric windows is that they fail to operate.
If the problem occurs on all windows you can be pretty sure that the fault is
in the main wiring circuit, and this is also the region to check first even if
the problem is restricted to just one window. Try listening to see if you can
hear the motor operating – if it is, but the window does not move, the problem
lies in the mechanical linkage.
Slow or jerky operation and mechanical noise point to a problem with the
mechanism, window guides or drive motor of the window concerned – the fact that
the window is partially working shows that the electric circuit is
Similarly jamming part way through the motion is probably due to the
mechanism or the window guides, and if there are rattles from the door the
window guides may be loose.
Keep it safe
As with all electrical equipment you need to take care when making tests
to ensure that you do not short the power supply to earth. The circuits for
electric windows are designed to carry very high currents so a lot of
damage could be caused before the protecting fuse blows.
To avoid the risk of accidentally operating the winder motor while you
are dismantling the window mechanism, disconnect the battery leads before
you start work.
If you need to operate the window as part of your checks, take
particular care to make sure that nothing gets trapped — the scissor action
of a typical lever mechanism could easily remove the end of a finger.
Erratic operation, with the window working perfectly sometimes and then
playing up, is most likely due to an intermittent electrical fault, such as a
Check the fuse
Most window circuits are protected by a fuse or cutout in the supply line
and this is the first place to check if the windows refuse to operate. With
some arrangements a single fuse in the main supply line is used to protect the
complete window system so a failure here affects all the windows. Other cars
have individual fuses for each window motor so failure will only affect the one
In some cars the fuse is in the main fusebox but many makers use in-line
fuses so check with your manual to find where the fuse is and replace it if
blown. Instead of a fuse the system may be fitted with a circuit breaker.
Then test the window. If it works properly the fuse probably blew (or the
cutout tripped) due to a temporary overload. However if the new fuse blows
immediately you will have to investigate further.
Power supply check
Generally the power for the window circuits (which draw a large current) is
supplied direct from the battery through a relay (or relays) actuated by
turning the ignition on. Find the relay position from your car handbook.
Use a test lamp to check that current is reaching the relay. With the
ignition turned on check for current at the relay control terminal. If there
isn’t any the problem lies in the wiring from the ignition switch. Then test
the output side of the relay – current means the relay is faulty and should be
replaced, otherwise the problem lies in the control switches, the wiring or the