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The car has a single reversing light in the bumper. The bulb doesn't light at all (tested, also tried spares). Measuring the voltage at the socket I get around 9V with no bulb in the circuit (with just the ignition on; 10V with the engine running). The meter is high-impedance. All the other bulbs are fine.

With no other path to ground in the circuit that I know of (no reversing sensors powered off it) I'm puzzled as to what can cause this voltage drop. The voltage at the appropriate fuse is the normal 12-13V, higher with the engine running. So I've reached the conclusion that there must be a partial short in the switch, but this seems unlikely.

This is a 2002 Focus (UK spec) the 2 litre version with the MTX75 transmission. This means the switch plugs in to the top of the transmission housing, and is a little fiddly to get at but shouldn't be too bad once the air filter housing is out.

The ground at the socket is good, measured to the earthing stud inside the rear wing (where the wiring goes anyway; that looks good), and also to the exhaust pipe. In both cases I see the same resistance as just touching the probes together (0.6Ω).

Before replacing the switch (which will take a few days to arrive) is there anything else I should test?

  • In general try to avoid doing a voltage drop test on any circuit when one lead isn't connected to the battery. It can lead to false readings if there are bad paths back to the battery. – Ben Aug 8 '16 at 22:24
  • @Ben you're right in general. A couple of things in this case make a difference: only a multimeter with high impedance (to get the voltage I saw you'd need 100k--1M in the circuit; that's more like a broken wire closed by dirty water); multiple earths showed the same reading, including those which also serve working circuits. So I'd say "use judgement" rather than "avoid" but I'm more into electronics than cars – Chris H Aug 9 '16 at 7:10
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As you wrote, it is unlikely that the supply line of the bulb has a short, but not impossible.

Beneath that, it's possible that the return line has a bad connection to the chassis. If an other device uses the same line, the flowing current rises the voltage on that line, and you measure a lower voltage between supply and return line.

A bad contact in the supply line is not possible, as the high-impedance meter should show the full 12V then.

Here is what you can do, with the bulb removed:

  • Check for bad connection in return line
    Find a good conducting point at the chassis (not at a door!) This could be a screw or similar. Now, measure the voltage between this point and the terminals of the bulb. If one has 12V and the other 3V (-> 9V between terminals), you have a bad connection.

  • Check for short in supply line
    If the bulb has its own fuse, i.e. the fuse supplies no other devices, pull the fuse and measure the current across the terminals (meter set to highest DCA range). If there's a short, you should observe a current of a few amps, otherwise not.

  • Further checks of supply line
    Pull the connector of the switch. Is there still a high current through the fuse? Then, the problem is before the switch. Plug in the fuse again, and measure the voltage between chassis and the terminals of the cable of the switch. One should have 12V (from the fuse) and one should have 0V (to the bulb). Measure the resistance between that terminal and the chassis. If it is just a few ohms, you have a short in the line to the bulb. If it is above say... 1kOhm, the line is OK.

The main question with measuring the supply line is what other devices are connected to it. The ECU may be connected to this line to recognize the reverse gear, and most 3rd party reverse cameras are also clamped to this line and consume current / generate low resistance. This makes the diagnosis a little difficult.

  • Your second & third bullets look well worth a shot. I've got a 10A meter (from troubleshooting the camper 12V electrics on a transit conversion) and it's a 10A fuse. Also good point on the ECU (and/or possibly wiper controller if it auto-engages); there's no camera but I know what you mean as I've fitted one on the Transit. – Chris H Aug 8 '16 at 9:46
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While the switch is a possible culprit, what I'd suspect most is that the wires to the bulb are damaged or corroded, leading to a voltage drop along the wire.

You should be able to measure the resistance of the wires when the ignition is off.

I'm not that familiar with the Focus, but generally re-doing cable runs in Fords is pretty straightforward, so if you have to replace them it won't be a difficult job.

  • Where the wires are accessible they look good (and clean). I also wonder how they could cause a drop in the open circuit voltage. But it's quite easy to test from the fuse box to the socket with my extension cables. – Chris H Aug 8 '16 at 9:42
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Yes. You should determine whether or not there is a good ground for the light – especially if you were reading the voltage across the terminals of the light socket. When you read between the terminals you see the voltage difference between them, and if the ground is poor (high resistance) then it will act as a voltage divider and there will not be enough voltage at the light to power it.

Something is causing a voltage drop… As @sweber mentioned with a high-impedance meter and no other loads on the circuit you should be seeing something closer to 12 V (since very little current is flowing due to the meter alone, high-resistance connections will only produce a tiny voltage drop). This makes me think something may be feeding current back into the circuit or that your actual path to ground is through the filament of another bulb.

  • Good point about the earth; I've edited the Q as that was the first thing I tried. Cleaned contacts/new socket didn't help either (which didn't surprise me with the open-circuit voltage being low) – Chris H Aug 8 '16 at 9:41
  • Unless the socket has a solid connection to ground at its attachment, check /clean the point where it does connect to ground. – dlu Aug 8 '16 at 9:44
  • The connection is in the rear wing, shared with the main rear left light cluster which works. It tests good and there's no rust. – Chris H Aug 8 '16 at 9:47
  • So when you measure the voltage between the supply (+12 V) to the reversing lamp and the ground point you see ~9-10 V? Which in itself is interesting, because you'd expect the voltage rise to be about the same as the difference between the battery voltage and the alternator output (~1.6 V). – dlu Aug 8 '16 at 9:58
  • I wouldn't read too much into my measurements due to rounding error (I assume you mean I should get 9-10.6). Plus the battery had just done a couple of starts when I measured. – Chris H Aug 8 '16 at 10:05
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This sounds like a perfect job for a voltage drop test. You'll need a digital voltmeter and (for this specific job) a pair of long leads for the meter. It also helps, but isn't strictly required, to have a rough idea of what connects to what in the circuit path.

Put the car in whatever state you would need to get the bulb to light (it may not visibly light up, but if it's not burned out it should pass current through). Touch the positive lead of the voltmeter to the positive battery terminal, and touch the negative lead of the voltmeter to the input side of the first thing in the circuit. For the sake of this example, I'll guess it's a fuse in the fuse box. Don't disconnect or switch off anything while you're doing this test; you want the circuit complete the whole time.

In an ideal world, the connection between the battery and the fuse is a solid conductor, and since electricity follows the path of least resistance most of it should go through the wire and not the voltmeter, resulting in a reading of damn near zero volts. In practice, the conductor is not perfect, and readings in the 0.2 V neighborhood are expected and fine.

If the connection were bad, the resistance would be higher than it should be and more current would pass through the meter looking for a path through. In this case the voltage reading would be higher -- 0.5 V or more -- indicating that something flaky is happening between the two leads.

Assuming the first segment tests good, move down the circuit to the next component. Test the voltage drop across the two fuse terminals. Then test between the fuse and the backup switch, then test across the switch, then test between the switch and the bulb socket. If you can find any connectors or test points along the circuit, use them to help narrow down the suspect area.

If the positive side doesn't reveal anything, start testing again from the negative battery terminal to the chassis, chassis to wiring harness ground bolt, ground bolt to bulb socket, and so on.

It's not really useful to do the voltage drop test directly across the bulb because the bulb is expected to have resistance and a large voltage drop (that's how it works, after all). But I'm guessing the fault is an area of high resistance in the circuit, and if that's the case you'll certainly find it this way.

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I had a similar problem today, I ended up shelling out £20 for 2 LED bulbs and they work good. I think they will operate on a lower V. No matter what regular bulb I tried it just didn’t work until I tested on a 11v spotlight that worked. So I popped to Wilco and bought the LED bulbs for rear. Might work for the next person to get stuck in a situation.

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