12v relays

[norsa]

Starting to make up the loom for the Mk1 Escort. Because we got it without any electrics im guessing slightly as im a body man not electrics. Apart from the obvious auxiliaries (spot lights fan, fuel pump etc) and of course flashers, should I run all the systems through relays or is this over the top?
Nick

 

[piman]

Hello Nick,

the things that benefit most from relays are halogen headlights as they are very sensitive to voltage drop, i.e. a volt less at the lamps gives a large drop in light output. You must make sure that you use heavier cables on the load side otherwise a lot of the benefit is lost.
Otherwise, unless your switches are not very robust electrically there is a lot to be said for keeping things simple, i.e. no relays. Ensure adequate sized cables, manufacturers original looms are always on the small side due to cost. Get a decent high quality crimping tool and use the correct sized terminals for the cable, both will ensure reliability, here is one company that can supply just about every thing you need :- http://www.vehicle-wiring-products.eu/VWP-onlinestore/home/homepage.php

Alec

 

[norsa]

Thanks Alec
Yes ive used vehicle wiring products for many years. So shall I assume that the standard factory wiring spec in 1975 would not have used relays at all?
Ive been in rallying for many years so know the benefits of heavy duty spot light wiring etc esp earth and terminal 87 out. On the rally car I have relays on the spot lights, starter button to starter sol and fuel pump. On a slightly different tack, should a fuse protect the switch, relay or load?
Nick

 

[AndyStobbs]

fuses should protect both. Any chaffing, be it on the control side, or the current side of a relay wants to blow a fuse before it melts wires. All lighting, with the exception of headlights can be direct (through fuses), doesn't really leave much you haven't covered.

 

[711jrp]

Never used a relay on the starter button myself but do on the wipers as well as the places you mentioned. You given a heated front screen a thought, I've just fitted one to my MK11 and love it but I don't have a heater as the YB is set right back into the bulkhead.

 

[norsa]

more thanks. Andy, are you saying I should relay main and dip, 4 relays, 2 relays??
711 no we have not given that any thought. We also have no room for a heater with the type 9 tunnel but it wont be going out in inclement weather. It will be a summer runabout and show car.

 

[Robotstar5]

more thanks. Andy, are you saying I should relay main and dip, 4 relays, 2 relays?.

2 relays should be OK, one for dip & one for mains.

 

[the snooper]

things that need relays are usually devices that draw to much power for the switch or loom to handle or where a heavy switch mechanism wont fit like switches built into the wiper and light stalks

 

[ajlelectronics]

The starter already has a "relay". That's the solenoid that engages the starter motor.

With a classic vehicle, I don't think that you can have too many relays on high power circuits. A design from scratch would have a high current busbar around the car which the relays connect to. Then all the control circuits can be low power and the switches will not burn out. Quite important when they may be irreplaceable. This would be vaguely similar to a modern system, except that the relays are modules, switched by data from the ECU.

 

[norsa]

interesting point.

 

[AndyStobbs]

switches also fail from other things beside burning out. Where do you draw the line? Adding secondary switches to avoid using the originals?

 

[dannyp]

I tend to wire automotive stuff up simale to some of the 2000- on vw's

small buzz bar in or on the battery box with several high current fusible links usally 120A takeing feed from alternator starter motor been the only unfused wire in the car .

some other links then feed relays for the high current switch stuff / fans / main head lights / light bars / heater plugs

then last 1 or two ,links would feed the fuse box for indicators, brake , tail, side lights, ecu, fuel pumps, radio ect ect

 

[Wedg1e]

With a classic vehicle, I don't think that you can have too many relays on high power circuits. A design from scratch would have a high current busbar around the car which the relays connect to. Then all the control circuits can be low power and the switches will not burn out.

I agree: TVR in their infinite wisdom fitted heater fans that took more current than the fan switch could cope with, now it's one of the most common faults on the 80s cars and one of the hardest parts to find.
Headlights were also directly switched by the column dip/main beam switch on a lot of cars until the 80s until quartz halogens started melting the switches...

 

[norsa]

Buz bar ???

 

[the snooper]

Buz bar ???

Usually a heavyish copper bar that all circuits connect to to draw power

 

[ajlelectronics]

switches also fail from other things beside burning out. Where do you draw the line? Adding secondary switches to avoid using the originals?

In a classic definitely. Like the TVR example above, the Scimitar has a hard to source headlight switch. So it makes sense to relay it to protect the switch and just as importantly, to get full Voltage to the bulbs. A Volt down will reduce the performance by a surprising amount.

 

[Fintray]

↑
Buz bar ???
Usually a heavyish copper bar that all circuits connect to to draw power

Busbar or bus bar normally but depending on the current flowing through them I have heard some buzzing.

 

[keithski122]

I'm just rewiring my beetle and I've fitted relays for the main/dip beam,front fog, rear fog, one in the back for all the engine electrics and one for the starter.Its common on the old vw's to use one on the starter as it helps with hot starting problems.

 

[the snooper]

switches also fail from other things beside burning out. Where do you draw the line? Adding secondary switches to avoid using the originals?

Obviously there is mechanical failure but if the car has to have a loom made up then why take a chance on it

 

[norsa]

So far, so good. Now indicators/hazards. Should I effectively have two circuits? i.e Circuit 1 ign switch- Indicator/haz relay- indicator switch- indicator lights....circuit 2 permanent live- indicator/haz relay- hazard switch- hazard lights??