Starter Relay?

[Simes]

What does the "Starter Relay" do?
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

[S/V_Surya]

Typically there are large loads or current draw on start circuits so the start relay isolates those loads. So your start circuit sees a 1 amp load while the solenoid sees 10 amps and you starter will draw hundreds. To eliminate the start relay you need to know the current draw and size the components to handle loads accordingly.

[Simes]

Hi S/V Surya,

So bigger wire and bigger switches. My old Ford Dorset (4.3 ltr) did not have any relays but did have big heavy wire and switches.
What do you good folks think for a 2.5 litre starter switch circuit, 10 Amp at the solenoid?, 20 Amp?
I would like to avoid the relay if possible, simplicity is the answer I hope.
Simes

[S/V_Surya]

Look on the solenoid to see what it draws. Rate wires and switches in circuit for double that.

[Simes]

Thanks for that, the simple is the best. I had not thought of finding the current draw written on the solenoid.

thanks,

Simes

[capn_billl]

The starter solenoid draws more than what you might think. Its not the size of the engine, its the size of the gear/spring it has to engage/disengage. Putting in a starter relay makes everything simple. When not if it fails you just replace the relay. If no relay what will you replace? starter switch? transmission switch? Wiring? The relay keeps the high currents to a small area. And provides one point of failure. There is a reason most boats have one.

[Mardav]

Short answer? Put in the relay. You do not want all that current going through the switch circuit.

[Simes]

Thanks Guys,

I will have to go and buy one I guess. The one that came with the engine has no indication of what terminal / connection does what or why.

These relays are spec'd by Amp I guess, How many Amps does a 2.5 litre diesel solenoid pull?

Thanks again,

Simes

[Sailmonkey]

Originally Posted by Simes

How many Amps does a 2.5 litre diesel solenoid pull?


Simes

Ummmm.......Yes.

[Simes]

Hi Sailmonkey, I think that you are commenting upon my spelling? It is always better if you can be clear when being sarcastic. What does "Ummmm.......Yes." mean?

I am moving this forward, the relay draws 40 Amps. I can buy one locally. The next question is what connections do what?

Thanks again folks,

Simes

[boden36]

I suggest that you buy the starter relay from a marine or auto electrician. Ask him to sketch out a rough diagram for you, and then pay him a little extra ( or a few beers).
Regards, Richard

[S/V_Surya]

If you can take a pic of relay maybe we can help. Most of the time relays have a little diagram of connections on them. Or use a volt meter. Read across pins. Small resistance like 63 ohms means that is the coil. Inifinte resistance mean open contacts no resistance means closed contacts.

[Sailmonkey]

Originally Posted by Simes

Hi Sailmonkey, I think that you are commenting upon my spelling? It is always better if you can be clear when being sarcastic. What does "Ummmm.......Yes." mean?

I am moving this forward, the relay draws 40 Amps. I can buy one locally. The next question is what connections do what?

Thanks again folks,

Simes

No comment on the spelling, just the general question. The world of solenoids is far and wide, as is the world of 2500CC engines. To be able to answer a question like that is nearly impossible.

[MSN-Travelers]

Originally Posted by Simes

...

I am moving this forward, the relay draws 40 Amps. I can buy one locally. The next question is what connections do what?

Thanks again folks,

Simes

Typical starter with starter solenoid built in -


As a general rule, system without a starter relay would be wired –

Heavy positive cable goes from battery/battery switch directly to the large terminal on the starter solenoid.

Hot wire comes from your power distribution panel to the starter switch/starter button. The wire then travels to the small terminal on the starter solenoid.

In this configuration, the full load of the current used to activate the starter solenoid and keep it engaged while the engine is being cranked passes through the starter switch/button.

Typical externally (4-terminal) grounded starter relay -



Typical internally (3-terminal) grounded starter relay -



Most starter relays have two heavy terminals and two small terminals. (Some may have only one small terminal if it is internally grounded.) As a general rule, system with a starter relay would be wired –

Heavy positive cable goes from battery/battery switch directly to the large terminal on the starter solenoid.

Hot wire comes from your power distribution panel to the starter switch/starter button. The wire then travels to one of the small terminals on the starter relay. A ground wire is run from the second small terminal to ground (unless your relay is internally grounded.)

A heavier gage wire (8 or 10) is run from a hot source (battery/battery switch) to one of the large terminals on the relay. A wire of the same gage is run from the other large terminal on the relay to the small terminal on the starter solenoid.

In this configuration, the full load of the current used to activate the starter solenoid and keep it engaged while the engine is being cranked passes through the starter relay direct to the starter solenoid. The current draw through the rest of the starter circuit is very small as it only has to energize the relay.

Personal experience – Our boat was wired without the starter relay. The loom supplied by Yanmar had wiring that, when new, was just able to carry the current required to activate the starter solenoid. As the years went by … If the battery wasn’t fully charged or there was corrosion at any of the connections in the circuit, the voltage drop when the starter motor engaged was just enough to cause the solenoid to disengage.

We added a starter relay to the system and haven’t had a starter/starting problem since.

You should be able to get a Ford starter relay in any automotive parts store.

[Simes]

Hello Folks,

Thanks for all of your help. I now have a relay (Bosch for a Ford Transit) 40 Amp 4 terminals and easy to wire. Thank you for all of the very valuable info.

System as will be this weekend:-
Press Button start switch on panel in cockpit.
Wire from Button to Relay, Relay connected to Battery (actually to Pos bus bar) cable capable of 80 Amps.
Wire from Relay to Starter Motor solenoid (80 Amp capable)
Solenoid connected direct to batt (well via an Isolation Switch, BEP Marine 775 Amps) Very heavy wire (96mm) from Batt to Starter Solenoid.

All looks good, Thanks folks,

Simes