Thanks to Eric… and wiring diagram theory

[Steve Webb]

Hi guys,

First of all I’d like to extend a big thanks to ETCG. I passed my level 2 City & Guilds in auto maintenance and repair…
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It was cos of ETCG I decided auto repair was for me so I went part time and went back to school. I start level 3 in September and honestly, I can’t wait. Thanks ETCG B) (PS, City & Guilds is England…From watching vids the US certifying board is ASE???)

Anyway, on to car stuff B)

The level 2 I’ve completed was mainly around servicing with small ”visits” to areas like transmission, suspension etc. Level 3 is going to be a big step up. it will include all sorts beyond servicing including diagnostics, electrical diagnostics, clutch jobs, transmission and engine remove/replace etc

I’ve of course got lots of info to go on in the ETCG vids. it’s the electric side that I need to learn most about though. I have a basic understanding and i’d like to give an example here of a wiring diagram, i’ll explain how the circuit works and you guys tell me if i’m right or wrong?

Here’s the best image I could find of a horn circuit:
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ok, item 1 is obviously the battery and it showing where it earths (sadly I haven’t got a key code but that’s ok… it’ll make it more challenging 🙂 )
power then goes through fuse F176. Through to item 2 (unsure as to whether this is a switch or not) and through to item 3 which is again another fuse. item 5 (location 76) being a relay which appears to give low current power to a switch (top right) and high current power to item 78 being the horns themselves (bottom right).

SO. say our horn doesn’t work. using logic and the diagram we have BASICALLY two fuses and a relay we can test. if theyre good we can use our DVOM to look for resistance between each wire start and end (e.g. between fuse F176 to F40)to look for shorts (im thinking any resistance above 1/2 ohm from the DVOM leads would indicate a break down in the wire or a short). We can test the resistance of the horn assembly itself with our DVOM. We can check our ground connections are good. i’m also thinking that as the horns are supposed to work all the time whether ignition is on or not then we can unplug connector at the horn and do a voltage test… i’m assuming we should see battery voltage?

As said this is just an example but i’d love it if someone could read my mammoth post (sorry) and let me know if i’m at least on the right lines. really want to get in to diagnostics 🙂

cheers

steve

Topic

[twiggy]

You understand the circuit correctly.

I recommend reading voltages at various points to isolate the problem then checking resistance after you’ve isolated where its failing.

Start by using the neg battery post to connect your neg test lead of you DMM. Then check for voltage on the horn pos terminal while pressing the horn button. Also verify you measure 0v on the ground terminal of the horn while pressing the horn switch.

If no voltage there, check for voltage at the coil of the relay, if missing then work your way back to the battery to see where the voltage is finally showing up.

[Steve Webb]

Thanks Twiggy. Glad to know it’s going in the right direction. 🙂

[twiggy]

Let me know what you end up finding 🙂

[Author]

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