Troubleshooting the electric circuitry is often feared but is very straightforward in reality.
Start with a copy or printout with the circuit diagram in question. We take an example here, the driver's side electric window of a Series 1 XM. A printout or a copy will enable you to take extensive notes on the diagram. Create as many as there are actual operating states of the circuit. In the case of the electric window, this might be:
- going up
- going down
Let's deal with going up here. You know that whatever happens, the current has to flow from the battery to the earth, going through the load (a window lifter motor here). Draw all switches in their operating position and try to find out how the current has to flow. It will necessarily be the shortest path between the two endpoints, going through all relays, switches and the load itself:
Don't be shy, grab a marker and draw it onto the copy.
The basic tenet of troubleshooting is very simple: halve the circuit and measure in the middle. If you have voltage there, the upper part is OK, you have to go further down. If you don't, the upper part is failing, you have to go further up. Repeat this process for the half-section you got until you find the problem.
There are shortcuts and modifications to help us, though. For instance, if you know that the same fuse feeds more than one circuit and some or all those other circuits are working all right, you can ignore the common parts. In our example case, the same relay and fuse (35) is responsible for the passenger's side window. If that works, you don't have to consider the common circuit sections (from the battery to pin 5 of the relay on the diagram). Also, when you determine where to measure next, don't always go for exact halving. Some parts are much more readily and easily accessible than others. Fuse 35 and the relay (809) both are near the middle if you want to halve the uppert part of the circuit, still, the relay requires the removal of trim and swinging down the main board while fuse 35 is conveniently sitting behind a simple door. Remove the fuse and measure in its socket in this case. Only remove the trim and go for the mainboard when you're already positive that the fault is there.
Armed with this knowledge, how shall we proceed? The easiest troubleshooting session would be as follows (after checking obvious things like fuses involved):
1. Start with the window switch, it's easy to prise off its place. If it doesn't light up in the dark, you might suspect it's not getting any feed but even then, the bulb might be failing inside as well. Better remove it, measure on pin 3 as visible on the diagram (don't forget to have the ignition in contact, though). You should find 12 V on the pin.
1A. There was feed on switch pin 3: Although we could halve the bottom section, an obvious quick check would be to measure on pin 1 while pressing on the switch while we have it in our hands. If the feed doesn't appear there, the switch is at fault. If there is feed, we go on with the usual routine: halve, this time to the window lifter motor. Measure on pin 3.
2A. There was feed on motor pin 3: Motor might be failing? Could be. But because we reached the load, we can't go on halving; feed is only present upwards from the load. We should now check the opposite end of the circuit, the earthing. Because the earthing point is certainly not very well accessible in the footwell, we simply switch to resistance measuring this time. First, make sure there is no current in the circuit at all (under current, we cannot measure resistance). Although if you don't press on the window switch, the motor here will be without current, generally it's a good idea to remove all relevant fuses or disconnect the battery before we start resistance measurements on the car. Measure resistance between the motor pin 1 and any available earth (in our case, the door lock jam would be the simplest one).
6A. There is no resistance (zero Ohms) in that section: The earthing is OK, the motor is suspect. You can check that definitely by providing a temporary feed to motor pin 3: the window has to lift then. Look out for moving parts before you connect the temporary feed!
6B. There is an open circuit (infinite Ohms) in that section: The earthing is at fault. But it goes back to the car via pin 13 of the door connector. If you haven't checked and cleaned it yet, you can go on with out halving routine: measure between the connector pin 13 and the door lock jam. You'll know then if it's the connector or the earthing itself.
2B. There was no feed on motor pin 3: What we're left with is a short piece of wire between the switch and the motor. Check the connectors or the wiring, one of those is at fault.
1B. There was no feed on switch pin 3: Let's halve the upper circuit section. A convenient point would be the socket of fuse 35 because it's easily accessible. Remove the fuse, look for voltage in either socket pin. You should find it on one, it doesn't really make any difference for us to find out which one.
3A. There was feed in the fuse socket: the current section we examine is from this fuse socket down to the switch pin 3. This involves two places, the relay and connectors on the mainboard and the connector on the door pillar. It might depend on you which one you'd prefer to tackle first... Let's say the pillar connector. Disconnect, measure in the pillar side, pin 14. Do you have feed there?
4A. There was feed in the door connector pin 14: The connector or the wiring is at fault.
4B. There was no feed in the door connector pin 14: It's time to go to the main board now. Remove the lower shroud, undo the nut holding the main board and swing it out and down. The relay is at the bottom, first row, first relay. First observe whether the relay clicks when you make ignition contact. Remove the relay, clear the connectors, re-seat. If in doubt, replace the relay. If you don't have a replacement part at hand, you can use any similar five-pin standard switching relay temporarily from another circuit, the Xm is full of relays. Wiggle the 13-pin white connector as well, there might be a connection problem with the connector as well.*
3B. There was no feed in the fuse socket: Go up again, halve the upper section. This would be the shunt in the same fuse board. Measure again.
5A. There was feed in the shunt socket: Go up again, halve the upper section. But this is unlikely in our specific case, this shunt serves may other circuits in the car, not just the electric windows. Unless most everything else is dead as a dodo, this part should already be all right.
5B. There was no feed in the shunt socket: You definitely have some serious electrical problems near the battery but this means the electric windows are the least of your worries.
* Don't forget here that a relay always means two circuits: one that controls it and another one that it controls. If you couldn't find the fault in one, you have to repeat the process for the other as well.