What does the "Starter Relay" do?

[Simes]

And do I really need one?

This is a Ford FSD 2500 diesel engine. The engine is now on its bearers and the prop' shaft is alined and connected. The fuel system is connected.

All I have to do now is connect the 12v system. There is a short wiring loom supplied with the engine but no diagram showing the components or their placement. I have identified all but the Starter relay circuit.
So my question is . . . Do I actually need the starter relay? Is there any reason why I can't connect the Engine isolation switch to the start switch and connect the start switch to the starter motor solenoid?
Where does the "Starter Relay" go and what does it do?

Simes

 

[Ross D]

It depends on the engine and how much current your started motor solenoid takes and how much current capacity your ignition switch has. For small engines most ignition switches act directly on the solenoid.

I hope this helps.

Ross

 

[halcyon]

Where does the "Starter Relay" go and what does it do?

Depends on switch rating, and starter motor solenoid amperage.

The relay is to take the load of the switch, by remotly switching starter solenoid.

Brian

 

[monkfish24]

I couldn't find a diagram simple enough so i drew one:

Basically as said before, it is a remote switch.

It uses a small amount of current to turn on a coil. This coil creates a magnetic attraction to the switch contact on the high current side on allowing a supply to the starter.

I'd fully expect to see one on something like a 2.5 diesel engine as the current load on the starter motor to be high. You could just by pass this by having the switch set to the run position and connecting a decent sized wire up to the contact on the starter motor. ensuring the engine block is earthed to the battery.

P.S I accept no liability for this action but it is how I used to start my Golf when the ignition barrel packed in!

 

[VicS]

Monkfish's diagram is over simplified.

The coil of the solenoid mounted on the starter motor takes a smaller but still substantial current.

It can be controlled by using an additional relay which requires a much smaller current to operate it.

One would typically use the relay if the distance from starter switch to solenoid and motor is great of if the starter switch or its local wiring was not heavy enough to supply the solenoid current

(Note the solenoid also engages the pinion gear with the ring gear on the flywheel)

 

[Simes]

Hi Vic and Monkfish,

Thats exactly what I thought. So given that there is no indication on the relay as to what connector does what, Is it possible to walk into Halfords and buy a relay? If so which relay do I want?
Or maybe I just bump the size of the basic wires by a factor of 2 or so. By this I mean go from 3mm dia to 6 mm dia, that is a big jump in wire size. The switch I have in mind is rated at 25 Amp.
Thanks for your help so far,

Simes

 

[VicS]

There is a diagram here http://www.tb-training.co.uk/MarineE06.html#STARTERS & CIRCUITS that shows the standard connections to a relay... be simple enough to work it out with a meter anyway.

Halfords? Possibly, but you would have to know what you wanted .. they'd not be my first choice.

A Lucas agent would be better

Lucas relays here http://www.autoelectricsupplies.co.uk/product_list/113

 

[snooks]

Hi Vic and Monkfish,

Thats exactly what I thought. So given that there is no indication on the relay as to what connector does what, Is it possible to walk into Halfords and buy a relay? If so which relay do I want?
Or maybe I just bump the size of the basic wires by a factor of 2 or so. By this I mean go from 3mm dia to 6 mm dia, that is a big jump in wire size. The switch I have in mind is rated at 25 Amp.
Thanks for your help so far,

Simes

Maplin have a whole host of relays I got a 60A one for about £6

 

[Playtime]

The switch I have in mind is rated at 25 Amp.

If you have a 25A starter switch (i.e. push to make with a strong spring) IMHO that should be sufficient to wire straight to the solenoid without an additional relay. The associated cable for the switch (from battery through to solenoid coil (but not the main starter feed)) should be capable of carrying at least 25A. The actual thickness (gauge) depends on the total distance the current has to travel.

VicS is good on wire gauges for a given distance and current.

 

[VicS]

VicS is

in the doghouse for being on the forums and forgetting to fetch the washing in before the sun went off it!

 

[monkfish24]

sorry Vic S and Simes, I didn't RTFQ properly

 

[VicS]

sorry Vic S and Simes, I didn't RTFQ properly

before anyone asks RTFQ stands for "read the full question" at least that's what one of the deputy heads at school said it meant when he used it. What did you think it meant

 

[Ubergeekian]

It can be controlled by using an additional relay which requires a much smaller current to operate it.

One would typically use the relay if the distance from starter switch to solenoid and motor is great of if the starter switch or its local wiring was not heavy enough to supply the solenoid current

It is also possible to wire the operating side of the secondary relay in parallel with the ignition indicator (normally that's between the appropriate alternator terminal and +12V) so that the engine can only ever be started at times when the ignition warning light is on. This is a simple way of avoiding accidentally trying to start a running engine, which may be an issue in some control setups.

It's how semi-automatic Citroen DSes are wired - you start the engine on them by pulling the gear selector lever outwards, so it's rather easy to do by mistake when the engine is running. Cruuuuuuuunch.

 

[monkfish24]

before anyone asks RTFQ stands for "read the full question" at least that's what one of the deputy heads at school said it meant when he used it. What did you think it meant

In the masculine environment I work in, expletives are common language. The F can easily be replaced by another word

 

[snowleopard]

The reason for the relay is voltage drop in the starter circuit. Because of the usually long length of cable from the starter to the control panel the voltage drop in the cable can be enough to prevent the solenoid from engaging fully. The result is a click and nothing else when you press the starter.

A simple headlamp relay from a motor factor or Halfords will do the job. Most of them have the circuit diagram embossed into the base of the plastic case. Connect the wire from the start button to one side of the coil and the other side to earth/engine block. The switched terminals go to the solenoid and the battery positive.

 

[Simes]

Thanks Guys, you are Awesome,

I have just sketched out the simple circuit using Monkfish and Vic's diagrams. Pretty much as I suspected, just nice to have it confirmed. I have spent the evening sat at the saloon table with bits of wire a multimeter a handful of books and the laptop open.

I now have a working plan for the Starter circuit and for the charging circuit.
Vic, the link you gave to Tony Brooks' site is terrific. Thanks you.

Simes

 

[Deleted member 36384]

Consider A Second Starter Near The Engine

I worked on a yacht which I think had both systems. The engine was started conventionally with a key via a starter relay but a second push button starter was located in the engine space. This push button starter was used if the fuel system required bleeding. I don’t know for sure how it was wired up but from the descriptions in this thread I suspect that it was direct: heavy duty push button, quite a bit of force to overcome the spring, large wires. Anyway, the point is if you are wiring in a starter you may wish to consider having a second near the engine, it was certainly a useful feature.

 

[VicS]

A local starter button could be connected in parallel with the relay contacts or between the the main battery connection on the solenoid and the connection to the solenoid coil.

Usual considerations about fuses of course.


The solenoids for old inertial type starter motors sometimes had a button built into them. (Mechanical rather than electrical.)

 

[Ubergeekian]

The solenoids for old inertial type starter motors sometimes had a button built into them. (Mechanical rather than electrical.)

Much sought after in the Triumph Herald world, those old ones. They make setting the rocker clearances much easier!

 

[charles_reed]

VicS is

in the doghouse for being on the forums and forgetting to fetch the washing in before the sun went off it!

I invested in a tumble drier - IMHO an essential bit of kit in the weather experienced in the UK - mine rates B on efficiency and works on cheap-rate electricity.