Classic cars forum & vehicle restoration.

[Kleftiwallah]

A seemingly simple question to which 1 do not have the answer.
When rewiring a vintage/classic vehicle,
which circuts need a relay and which do not?

Your instructon required peeps.

Cheers, Tony.


_________________
It may work in practice, but not necessarily in theory!

[MikeEdwards]

A relay is generally used to allow a "flimsy" (if you'll forgive the highly technical terminology) switch or wiring to control something that draws more of a load. So the specific choice will be down to the specific vehicle and the condition of the various items.

One of my cars requires relays to power all four headlamps because the switch isn't up to it, where the other one (which is very similar) has a better switch and does not. Anything you've upgraded that might require a higher current to operate would be the first candidates. Stuff like improved lighting, electric cooling fans, air horns (!) are all things that might well have one, perhaps also the heated rear screen might.

[Kleftiwallah]

Many thanks Mike for that informative answer. No major changes so dispense wth relays.

(The 'i ' key stll doesn't work on ths pad, i'm having to cut and paste)!

heers, Tony.
_________________
It may work in practice, but not necessarily in theory!

[petelang]

It's also wise to use relays where the length of cable is a long way from the item consuming the power, and for another example, the horn wiring where it goes up the steering column. If it shorts out your horn may be blaring away uncontrollably but you can isolate the relay switch circuit to stop it. If cables are long you will likely get volt drop especially if the item uses a few amps. Using a relay puts full voltage nearer the consumer.
Peter

[jp928]

I would recommend using a relay on horns in most cars, of any age. As said above, also where there are switches that are marginal, OR where the switches are buried inside something (eg Lucas CS6, CS12) thats difficult to replace, or to fix. My 80s Porsche has climate control and the controller has a built in relay for pulling in the AC compressor clutch, but its way under specified - when it fails its a big job to get out, and even worse to fix. Solution - before it fails, add a heavier duty relay out by the compressor.
jp 26 Rover 9