On-Line auto electrical info

The most important reason to use a relay is so that the path between a high current source (battery and alternator) to the load is as short as conveniently possible. Voltage drop along a cable is directly proportional to its length so if you double the length you need to double the cross sectional area to maintain the same voltage loss. Short fat cables are cheaper than long even fatter cables.

Secondly removing high current cables and connections from confined spaced reduces the heating effect of high current, also the possibility of fire if a connector fails on a high current wire.

Thirdly it takes the high current load off the switch gear, which in Land Rovers is often barely capable of carrying its rated current, e.g. head lamp switches.

Fourthly, if your relay is controlled by electronics, eg your ECU, it makes sense to keep high currents away from the ECU and manage them externally. A relay is simple to replace, an ECU that's fried is often very expensive and time consuming to replace.

Fifthly, if you have a number of high current devices at a distance from the main power source (e.g. at the rear of your vehicle), they can share a single large power feed cable and need only thin control cables for each function controlled by a relay. It's simply economical to save on heavy copper cabling which is now very expensive.

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Originally Posted by Ranga

Thanks Tim, can you explain to me and others why relays are used?

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Or more importantly why a company(such as Toyota!! on their '04 Rav models) don't use a relay to power the headlights!

The switch is one of those turn-ey type indicator stalks, and the contacts inside eventually burn out, which results in the extra expense of having to purchase an indicator stalk when one really only wants a headlight switch .. which should have been a relay anyhow!

May have to wire up a couple of relays into the lighting system to prevent future occurrence.

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Originally Posted by AK83

Or more importantly why a company(such as Toyota!! on their '04 Rav models) don't use a relay to power the headlights!

The switch is one of those turn-ey type indicator stalks, and the contacts inside eventually burn out, which results in the extra expense of having to purchase an indicator stalk when one really only wants a headlight switch .. which should have been a relay anyhow!

May have to wire up a couple of relays into the lighting system to prevent future occurrence.

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wow,i didnt know that

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Originally Posted by Eevo

wow,i didnt know that

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neither did we(brother in law and I)
Spent close to an hour looking for the relay for the headlights(as both not working) only to 'give up' and google it.
Found out that there are no relay(s) for headlights .. and so all power is to switch.

brother in law changed switch now, headlights OK(for now).

Quote:

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Originally Posted by AK83

Or more importantly why a company(such as Toyota!! on their '04 Rav models) don't use a relay to power the headlights!

The switch is one of those turn-ey type indicator stalks, and the contacts inside eventually burn out, which results in the extra expense of having to purchase an indicator stalk when one really only wants a headlight switch .. which should have been a relay anyhow!

May have to wire up a couple of relays into the lighting system to prevent future occurrence.

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Anyone old enough to remember the foot switch would know that's how it was in the "old days". Relays weren't common, most switches used to handle full device current.

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Originally Posted by mr_squiggle

Anyone old enough to remember the foot switch would know that's how it was in the "old days". Relays weren't common, most switches used to handle full device current.

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And driving barefoot on hot nights meant you burnt your big toe on the dip switch.[bawl]

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Originally Posted by mr_squiggle

Anyone old enough to remember the foot switch would know that's how it was in the "old days". Relays weren't common, most switches used to handle full device current.

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Love!!! the old days.
My dad's Caddies had those foot switches .. interesting idea they were.

I started with Rover cars before the RRC(and now D1).
Maybe 10 P6's early 2000's to a '76 V8, brother had a 3lt P5, and after my P6 epoc, I then got into SD1's('76 - '84).

I remember spending most of a day on my bro's P5 removing all the switches(the flip type IIRC ??) opening them up and using a small points file to clean up the green and black build up on the copper contacts.
Nice strong, solid copper. They made a strong, resounding click! sound when flipped.
Todays plasticky switches have no soul about them.

I quickly learned that to minimise the cost installing yet another bloody switch relays had to be installed(even before the roadworthy was booked in).
I reckon I could eventually wire up the relays to the headlights blindfolded, I'd done so many.

Can't remember if the ('79) RRC had relays for headlights or not. I was just an automatic process. Get another Rover and I just installed them, irrespective .. for my own piece of mind.
I was sold some relays with built in fuses(as I remember) and they gave me trouble in that the fuses(30A, on 30A relays) would blow for no reason.
I used to use one for low-beam and one for hi-beam(55/75's I think).
I eventually replaced them for the non fused types and had no trouble after that. To this day(30odd years later) I still avoid the relays with built in fuses from that experience.

So, yeah.. back in the day not having relays for heavy current devices/switches was normal. But nowadays it makes no sense.

Hi,
Early Moris Minors didn't even have a relay on the starter motor.
Pulling the starter switch cable drew the small gear on the starter into mesh with the teeth on the flywheel to full engagement, then put power onto the cranking circuit.
Cheers

OK folks, I’ll start a Digital Multi Meter ( DMM ) tutorial in the next week.


But first, make sure you have a DMM handy.


This is just a suggestion, if you need to buy one, there are some very cheap and useful DMMs but if you think you might get some use and want a little better than basic, Altronics have a DMM for $45 ( Q1134 ) and this one has a handy advantage over most.


Most DMMs have a maximum current limit of 10 amps, but this one can handle up to 20 amps and this is a more useful limit when measuring circuit currents around a vehicle.


Anyway, I will start putting something together and see how we go.

Quote:

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Originally Posted by drivesafe

OK folks, I’ll start a Digital Multi Meter ( DMM ) tutorial in the next week.


But first, make sure you have a DMM handy.


This is just a suggestion, if you need to buy one, there are some very cheap and useful DMMs but if you think you might get some use and want a little better than basic, Altronics have a DMM for $45 ( Q1134 ) and this one has a handy advantage over most.


Most DMMs have a maximum current limit of 10 amps, but this one can handle up to 20 amps and this is a more useful limit when measuring circuit currents around a vehicle.


Anyway, I will start putting something together and see how we go.

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Pic for those looking at this DMM [smilebigeye]

Q1134 Auto Ranging Digital Multimeter - Altronics

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