Write Up On Testing A Relay, Headlight Grounding For Low Beams Out

Oka · Post by **Oka** » 14 Jan 2019, 23:43

I am kinda deficient on most electrical work, including chemical. I can do auto test with my multimeter if I know where to test from.
Bot my low beams went out at the same time. Fuses are ok, don't know about the relay. The flat shunt resistor is ok, not burnt.

Any write up on 'exactly' on how to test for headlight grounding, that is, where to touch the red probe at the headlight?
The Low Beam relay has six pins, how to test this relay.

Thank

Post by **abscate** » 15 Jan 2019, 04:45

Have you tested the bulbs? Just before I went to dive into a full electrical troubleshooting on an XC70, a real mechanic figured out the alternator had spiked and burnt both bulbs out,

IslandV70 · Post by **IslandV70** » 15 Jan 2019, 10:24

Test the voltage at the light itself first and work your way back

Oka · Post by **Oka** » 16 Jan 2019, 06:44

abscate wrote: ↑15 Jan 2019, 04:45 Have you tested the bulbs? Just before I went to dive into a full electrical troubleshooting on an XC70, a real mechanic figured out the alternator had spiked and burnt both bulbs out,

Just curious from your question, I took one bulb out and tested for continuity and there was none.
I removed the other bulb, and found it burnt so bad that the glass melted at a spot.
So, it must have being a power surge from the alternator. Replaced the bulbs, and voila, there was light.

Now on this note, why would this happen? Would the alternator be faulty or what could have caused this?

Thanks you for your very simple suggestion. I did not go this route since both bulbs went off at the same time.
Well appreciated.
Cheers!

Oka · Post by **Oka** » 16 Jan 2019, 06:47

IslandV70 wrote: ↑15 Jan 2019, 10:24 Test the voltage at the light itself first and work your way back

Sure you saw my reply on the culprit.

However, I still would like to find some writeups on how to really do these electrical testing. At least, finding one or two would be enough to do most with a general knowledge. Seen lots of instructions but have not seen a very descriptive one. Will spend some time on Google.

Thanks for your suggestion.
Cheers!

Post by **abscate** » 16 Jan 2019, 10:17

Its a real skill and not something that is easily written up. You have to master the understanding of voltage, current, resistance and basic circuit theory - once you have done that, you don't need the internet!

The Cliffs Notes Version for simple things is:

Condition: Electrical gizmo A does not work.

Find owner manual, track down fuse for Gizmo A, check fuse by removing and replacing.
If that doesn't work, track down wiring diagram and look for voltage at GIZMO A, then check continuity of ground.

No voltage? Fault Trace voltage back from Gizmo A, following the wiring diagram until you find place where voltage is present on one side of wire but not the other.

Voltage present - replace Gizmo A

Here, looking for 12V at the lights would have led you to replacing the Gizmo - as needed.

**8 Where you start to get into trouble is the many things that get constant supply voltage, but are switched by grounding by the ECU - the purge valve is an example. You will always have 12 V present, but ground testing, for example, can put current into your ECU - not desirable.

IslandV70 · Post by **IslandV70** » 19 Jan 2019, 09:49

I know this stuff from years of being in the electrical-electronic business one way or another. This is not a Volvo specific skill or even a car specific skill. I would look online or buy a book and learn basic DC electricity, ohm's law, and how to use a meter.