Classic cars forum & vehicle restoration.
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Ray White
Joined: 02 Dec 2014 Posts: 6342 Location: Derby
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Posted: Sun Oct 18, 2020 9:32 pm Post subject: Fitting relays. |
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I have been reading about how in days gone by cars were not expected to be drawing much power from their electrical systems hence ignition/light switches were not designed to carry much more than 13 or 14 amps. Apparently, running too much power through these old fashioned switches can easily burn them out.
I have been advised that the answer is to fit relays. This is not something that I know anything about but I am willing to learn especially as I am fitting several extra gauges and halogen headlamps. How do relays work?
With the high cost of new ignition switches I had better learn...and fast.!
Please can someone give me a few tips on how and what I need to do. |
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Penguin45
Joined: 28 Jul 2014 Posts: 382 Location: Padiham
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Posted: Mon Oct 19, 2020 10:37 am Post subject: |
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Hi Ray.
They're really just a heavier duty switch. You can use the original circuit (say, headlight) to operate a coil in the relay, which operates a switch in the relay to power the light. Think starter solenoid, but a lot more compact. It allows you to use heavier wiring which is perhaps more suited to the continuous load.
Relay diagram by Penguin 45, on Flickr
Basic wiring diagrams for a circuit with and without relay. If you click the pic it'll blow up nicely for extra clarity.
Chris. _________________ '67 Wolseley MkI 18/85, '70 Austin MkII 1800 The Landcrab Forum. |
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Ray White
Joined: 02 Dec 2014 Posts: 6342 Location: Derby
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Posted: Mon Oct 19, 2020 12:44 pm Post subject: |
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That's helpful!
So each relay needs it's own earth wire?
I see how the relay fits into the circuit but how does it protect the switch? Will the fuse only protect the accessory?
(You see,,,I am really not very bright when it comes Ito sparks. ) |
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badhuis
Joined: 20 Aug 2008 Posts: 1392 Location: Netherlands
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Posted: Mon Oct 19, 2020 1:03 pm Post subject: |
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Excellent simple diagram.
Without a relay, all current needed for example for the headlamps would go through the switch. This will have an effect on the contact points within the switch, they will erode faster, get warm and ultimately melt. On a holiday in France I had this exact problem, the headlamp switch melted on my Citroen CX.
With a relay, only a small current is going through the original switch. No fear of melting. This then provides current to the relay where the "heavy" current wire from the battery / fuse is connected to the headlamps. _________________ a car stops being fun when it becomes an investment |
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Ray White
Joined: 02 Dec 2014 Posts: 6342 Location: Derby
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Posted: Mon Oct 19, 2020 1:22 pm Post subject: |
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I am amazed that happened on a CX. I always thought they were a marvel of electrical complexity! |
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Bitumen Boy
Joined: 26 Jan 2012 Posts: 1737 Location: Above the snow line in old Monmouthshire
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Posted: Mon Oct 19, 2020 1:50 pm Post subject: |
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Ray White wrote: | That's helpful!
So each relay needs it's own earth wire?
I see how the relay fits into the circuit but how does it protect the switch? Will the fuse only protect the accessory?
(You see,,,I am really not very bright when it comes Ito sparks. ) |
Each relay will need its own earth wire, but the mounting point for the relays is usually a convenient earth point so you're not necessarily looking at a great deal of extra wiring. |
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alastairq
Joined: 14 Oct 2016 Posts: 1956 Location: East Yorkshire
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Posted: Mon Oct 19, 2020 2:34 pm Post subject: |
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The current needs of the relay [solenoid switch]...will be quite low....your light switch [for example?] will simply operate the solenoid in the relay switch...instead of the lights directly. thus, the live wire goes into the relay , and the other connection goes to earth. You will have replaced the lights [power hungry] with a small electromagnet, inside the relay.
When operated, the contacts are worked in the relay..making the heavy duty circuit to the lights.
Saves having a switch on the dash like a Dungeness circuit breaker?
Or, all that heavy duty[high amperage] wiring all over behind the dash?
Relays come in lots of shapes and sizes...[the size really depending on the heavier load it has to switch?]
Some relays are normally-closed [so your switch will open the relay ..turning it off, so to speak]......or, most are ''normally open..''
Some are single pole, some or double or triple [or more] pole....double throw.....
All this refers to what the relay operates. IE what it does to the main circuit[s] it switches.
For that is what a relay is.......an electrically operated, electrical switch.
My Dellow..being 'sorted out' for racing in the USA, whilst it has most of its original electrics in situ [where they were sound]....the owner had a complete self-contained box of tricks mounted [under the dash]....from Painless Performance Products, of Ft Worth, Tx...a couple of hundred dollars worth in fact...this below, or nearly so
https://www.painlessperformance.com/wc/30001
Everything built into the one compact package..all I have to do is connect the live feeds for whatever, to the marked terminals..hence for the Dellow, if I connected the live feed for the reversing spotlight, to the hazards terminal..the light flashes!!
Not that I left it thus, you understand...just trying the thing out really...gives me backache going under the dash. _________________ Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
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Ford Pop chassis, Ashley 1172 bodyshell, in pieces. |
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Ray White
Joined: 02 Dec 2014 Posts: 6342 Location: Derby
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Posted: Mon Oct 19, 2020 7:39 pm Post subject: |
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Thank you Alastair. A most fulsome explanation.
Electronics... it kind of scares me.
Actually, that looks pretty neat. Is it just a fuse block or are the relays built in? |
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Ray White
Joined: 02 Dec 2014 Posts: 6342 Location: Derby
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MVPeters
Joined: 28 Aug 2008 Posts: 822 Location: Northern MA, USA
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Posted: Tue Oct 20, 2020 3:04 am Post subject: |
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Ray
I think the only place you'd use a relay on the TC is for the headlight circuits.
A standard relay & socket from your local parts store is very few dollars; you can get dual-circuit ones (hi & lo beams) & ones with built-in fuses.
Wiring is simple - run 12V to the relay #30 from the battery, connect the h/l low beam switch wire to one side #85 of the relay & the low beam h/l wire to the other side #87. Earth is #86 & usually under the mounting screw. Same for the hi beam.
To put it another way, you simply cut the headlight wires & 'insert' the relay into the circuit. The only new wire is the 12V feed from the battery. _________________ Mike - MVPeters at comcast.net
2002 MINI Cooper 'S' |
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Ray White
Joined: 02 Dec 2014 Posts: 6342 Location: Derby
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Posted: Tue Oct 20, 2020 8:22 am Post subject: |
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Hello Mike. Your reply is appreciated. You are focusing on what I need to do for the TC which is what I need to do. Focus on the job or risk getting side tracked.
You rightly say the headlights need a relay. This would presumably protect the rather fragile ignition/light switch. What about the horn and the indicators? Then there is the fog lamp.
One unknown quantity is the draw from the (new) proposed hydrostatic fuel pump sender unit.
I am unsure which things need a fuse; a relay or both.? |
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alastairq
Joined: 14 Oct 2016 Posts: 1956 Location: East Yorkshire
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Posted: Tue Oct 20, 2020 12:42 pm Post subject: |
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I suggest, if concerned, fuse everything? {Separately if possible...but perhaps not always possible?]
It can do no harm...aside from introducing yet another set of contacts into the circuit....to become corroded or gungy?
A long time ago I had to 'invent' a wiring circuit for my old Cannon, when it was road-registered.
I tried to run everything through a fuse...I ran out of fuse positions very quickly...I only had a freebie old Lucas fuse box to play with..without actually going and spending real money on the thing?
Working out the fuse sizes was the hardest par..ther ewas always one circuit that wanted a slightly heavier fuse...So I also ended up with some 'inline' fuses... Relays for the headlamps...and, when it eventually got one, the wiper motor [one wiper, ex-polo rear window item] _________________ Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
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Ford Pop chassis, Ashley 1172 bodyshell, in pieces. |
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ukdave2002
Joined: 23 Nov 2007 Posts: 4117 Location: South Cheshire
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Posted: Tue Oct 20, 2020 2:56 pm Post subject: |
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alastairq wrote: | I suggest, if concerned, fuse everything? {Separately if possible...but perhaps not always possible?]
It can do no harm...aside from introducing yet another set of contacts into the circuit....to become corroded or gungy?
A long time ago I had to 'invent' a wiring circuit for my old Cannon, when it was road-registered.
I tried to run everything through a fuse...I ran out of fuse positions very quickly...I only had a freebie old Lucas fuse box to play with..without actually going and spending real money on the thing?
Working out the fuse sizes was the hardest par..ther ewas always one circuit that wanted a slightly heavier fuse...So I also ended up with some 'inline' fuses... Relays for the headlamps...and, when it eventually got one, the wiper motor [one wiper, ex-polo rear window item] | The fuse is there to protect the wiring, so its not too complicated to figure out what size to use.
It gets messy when folk start mixing cable sizes up a case in point:
I moved house not so long ago, the garage was fed from the consumer unit by some 4mm twin and earth, protected by a 32A breaker in the consumer unit dedicated to the garage spur, in the garage the 4mm T&E splits in to 2 separate circuits, one providing power to the sockets using 2.5mm T&E and the other for lighting using 1.5mm T&E. Now whilst the cable for the lighting and sockets is the correct size, the only protection is the 32A breaker in the consumer unit; 1.5mm T&E is rated at 15A, meaning in the case of a fault in the lighting cable , the cable could overhead before the breaker activated, a genuine fire hazard.
Sadly its not an uncommon arrangement when garage or shed has been DIY wired.
Dave
Last edited by ukdave2002 on Tue Oct 20, 2020 3:19 pm; edited 1 time in total |
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MVPeters
Joined: 28 Aug 2008 Posts: 822 Location: Northern MA, USA
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Posted: Tue Oct 20, 2020 3:07 pm Post subject: |
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Ray White wrote: | Hello Mike. Your reply is appreciated. You are focusing on what I need to do for the TC which is what I need to do. Focus on the job or risk getting sidetracked.
You rightly say the headlights need a relay. This would presumably protect the rather fragile ignition/light switch. What about the horn and the indicators? Then there is the fog lamp.
One unknown quantity is the draw from the (new) proposed hydrostatic fuel pump sender unit.
I am unsure which things need a fuse; a relay or both.? |
Ray
Just for reference, early cars generally only had 2 fuses on the control box - one for ignition-switched devices & one for not. Lights & horns were not fused. Switches & wiring were designed to carry the necessary currents of the time.
The primary purpose of relays is to make sure the end device gets the full 12V & adequate amps.
For your car, I think I'd fit 3 or 4 relays, 2 for headlights, one for fog & maybe one for the horn, although that particular horn shouldn't draw much current.
Run a heavy wire from the battery to all 4 relays & continue the original wiring from there.
You could add a 4-position fuse box on the output side of the relays if you wish.
(Also for reference, my '02 MINI has 50 or more fuses!). _________________ Mike - MVPeters at comcast.net
2002 MINI Cooper 'S' |
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Ray White
Joined: 02 Dec 2014 Posts: 6342 Location: Derby
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Posted: Tue Oct 20, 2020 4:43 pm Post subject: |
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Mike you say run a heavy wire from the battery to all four relays. There is no wiring in the car at all at present but I have bought an expensive new wiring loom. Is this insufficient?
I had also forgotten the wiper motor. Originally this would have been operated by the switch on the unit itself but I will have it operated from a switch on the new dash. When I decided to move the ignition warning lamp I had a space available and that seemed like a sensible idea. |
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