Fitting an Ignition Switch Relay

[oli8925]

I know some people recently have fitted a relay to the ignition circuit for seemingly two reasons. One being to boost the voltage voltage to coil especially when cranking (along with taking out the resistance wire) and the other to save some strain on the ignition switch.

I had a look through this with my dad a couple of months ago and neither of us could see how it would boost the voltage (other than replacing the resistance wire) or save the ignition switch, due to the way it is wired through the bulkhead connector and solenoid.

If someone could explain in detail how the addition of the relay in this situation gives an advantage, and specifically where in the circuit to add this relay, I'd be very grateful.

I'm doing the final mock up of the wiring today before it all comes out to be taken apart, modified and stuck back together.

[oli8925]

Can anyone give some advice on this. It's the one thing I'm still unsure about before I start my soldering.

[russlima]

I'm no expert, but the whole point of fitting relays is, I believe, to be able to control circuits which draw a high current (head/fog lamps etc) with a switched circuit which draws a small current. The original switches on your car are now 30+ years old and will be old and tired. Equally the wiring to the switches will have been sized to the minimum during manufacture and are equally old and tired. However, those switches and wiring can be used to control a relay - which is a switch - which draws a small current, but can be used to control circuits drawing larger currents. If you fit headlights with larger wattage bulbs, or fog lights, for instance, you can fit larger cables between the relays and the lights which can cope with the larger currents. Also, there is a voltage drop experienced in the smaller cable which is reduced in the larger cable - this is the boost you mention - it is not 'boosted', but the drop is diminished.

Russ

[oli8925]

I understand that and have fitted relays to the headlight circuits for that exact reason. However when following the wiring for the +ve feed to the coil, on cranking the power comes from the battery via only the solenoid, thus 'protecting' the ign switch and I may be wrong but I remember the running +ve supply to the coil coming from the top row of the fuse box, which is 'powered', i.e the top row acts as a distributor box taking a power feed from the battery and splitting it across the 3 or 4 pins in that row. Therefore again the ign switch is 'protected'. So all that considered I don't see how adding a relay between the switch and the bulkhead connector is going to further protect the switch or provide any kind of 'boost' to cranking or constant voltage across the coil.

In theory I agree it's a great idea, but from what I can see the wiring setup in the HC doesn't actually require it.



Edit: On second thoughts the running feed for the coil can't come from a constant power source, otherwise when you turn the key to switch the engine off it wouldn't do anything. So it must go through the switch, and now the addition of the relay does make sense. Thanks for making me type that, it's enabled me to see the error in my thinking

Although it still stands that on a standard ignition circuit with the cranking voltage to the coil coming via the solenoid as I explained, adding a relay shouldn't do anything to further protect the switch nor increase voltage/'boost'/decrease losses to the coil during cranking. Something I've seen several people write.

At least I can get on with my wiring now

[oli8925]

So if someone could please confirm, it would be ok to run a single white from the ign switch to the relay as the trigger, and two thick whites from the output of the relay to the relevent pins on the bulkhead connector? Thus protecting the switch and meaning 12v to the coil at all times instead of 9v?

[vivaha]

Hi, If you wire in a 12volt feed to your coil to replace the ballasted feed you will burn out your points and damage the condenser, if you want to go down the relay route you will have to change your coil for a straight 12volt one.

The point of a ballasted system in the first place is so the coil can receive a boost of 12volt on cranking which comes direct from the starter solenoid and by-passes the ballast resistor, this improves the spark strength on start up. The primary resistance of a ballasted coil is lower than the resistance of a straight coil.

On my HA i have left the coil feed as it is but put a Boyer Branston ignition unit between the coil - and the points, this ensures the coil always gets a good earth when the points switch to earth.

Hope this is of some help to you Tim

[oli8925]

I have an electronic ignition unit that needs a constant 12v feed and have changed the coil to a straight 12v one. It has always seemed to read 12v at the coil when running rather than the 9v it should have been so the leccy ignition has worked, but it's become clear now that the resistive wire in the loom has been overheating and burning out. Lucky I'm doing the wiring when I am really.

I just want to be sure adding a relay into the circuit so that it powers both ignition feeds is going to be ok before I take the plunge and cut the white wire. I don't see why it wouldn't be ok though.