Using a relay to protect the engine starting switch

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TradewindSailor

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Hi

I've just installed a relay, about a meter away from the starter, to take the starter solenoid's loads rather than the starter switch.

The boat is a St Francis 44 catamaran ..... so the cable length between the port engine and the start switch is probably more than 10 meters. The starter switch's points have been burning out and the voltage drop has been so great that the solenoid would not operate.

With this modification the engine starts first push every time.
 

fluffc

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[ QUOTE ]
The starter switch's points have been burning out and the voltage drop has been so great that the solenoid would not operate.

[/ QUOTE ]

You have a different problem to what you have solved - you've cured the symptom rather than the cause.

Since the starter motor has a solenoid in it anyway, not a huge current actually flows through the starter switch.

As to why your starter switch's points have been burning out; that's a mystery. Have you tried replacing the starter switch and also consider upgrading the cable so that the voltage drop is less.

IMHO another relay is just another thing to go wrong.
 

Birdseye

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upgrading the cable would increase the current through the switch and presumably make the problem worse.

Unless the solenoid (which is in effect a big relay) is partially shorted out and therefore taking more than normal currect, a simple starter switch should cope. So its likey the swicth is duff.

In effect, you are using a relay to operate a relay to operate the starter. Shouldnt be necessary
 

fluffc

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It is an inductive load! there fore a large voltage is present when the contacts open and cause an arc.

[/ QUOTE ]
Very true. But the amount of energy contained in this solenoid induced spark is VERY small.
 

srp

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[ QUOTE ]
upgrading the cable would increase the current through the switch and presumably make the problem worse.

[/ QUOTE ]
Surely upgrading the cable would result in a lower current? Which should mean the switch would last longer. Don't disagree with the rest of what you say though - particularly your last sentence. Maybe the original starter switch was under specified by the manufacturers in the first place.
 

pampas

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Are you talking about 1: the starter position on the ignition switch. 2: the starter solenoid contacts. 3: the main starter contacts. Knowing a bit about these things you have not made it clear which contacts are, or were being damaged,there fore I will reserve opinion until you have clarified the situation further.
 

William_H

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No birdseye is right better cable equals more current equals fast deteriation of the switch.

The relay fit is exactly what I did to my sons Toyota because of bad wiring or switch I never sorted out which.

Start switches are often barely adequate for what is quite a significant current to a starter solenoid. olewill
 

boatmike

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While I agree Olewill, there is another possible cause which I found recently on a standard wiring panel on a Volvo which was the preheater circuit. Due to the way it was wired with a common return to the switch the current from the heater solenoid was also passing through the starter contacts of the switch on starting. Removing the common strap between the two terminals of the switch stops this. If anyone has one of these panels (which will show the heater light still on when the starter motor is turning over) I suggest doing the same as, apart from the extra load on the contacts of the switch it puts extra load on the battery on starting.
 

billskip

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you say vote and see results ..sorry i cant vote because your question does not make sence.. ohms law..the current in a conductor is directly proportional to the ...etc etc please tell me the P.D across the original conductor "the thin cable"
that you are replacing and maybe I can make a guess /forums/images/graemlins/confused.gif
 

whipper_snapper

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It makes perfect sense if you assume 'all other things to be equal', especially that the thick cable is made of the same material as the thin cable!

Then the question could be rephrased as 'A load is fed via a resistor of value x ohms. The resistor is replaced by one of lower resistance.'

The answer is then obvious

(Although, I suspect, irrelevant to the damage to the switch contacts as the difference is likely to be insignificant. Probably just needed to clean the contacts or replace the switch!)
 

DaveS

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To me too the answer is obvious. But given some of the earlier posts and some of the votes cast, it would seem to be not obvious to all.

You are, of course right, for completeness I should have added a note to the effect that all other conditions stay the same, but it would be normal to assume that if not otherwise stated.
 

Bergman

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Quite right

The arc caused by the back emf from the solenoid will burn the contacts causing the resistance of the contacts when closed to increase - which in turn will cause the contacts to get hot when they close to try to start the engine which causes the contacts to burn some more until they are opened when a spark from the back emf causes the contacts to burn....................................................and so on.

Its a hard life being a contact switching an inductive load.

Point is that exactly the same thing will probably happen to the relay contacts over a period of time.

What you need is a spark quench circuit across the contacts to protect them.

Providing the cable is thick enough to carry the current to operate the solenoid making it thicker will have no effect.
 

TradewindSailor

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[ QUOTE ]
Are you talking about 1: the starter position on the ignition switch. 2: the starter solenoid contacts. 3: the main starter contacts. Knowing a bit about these things you have not made it clear which contacts are, or were being damaged,there fore I will reserve opinion until you have clarified the situation further.

[/ QUOTE ]

The points referred to are those of the start push switch ... i.e. on the engine control panel.

I've tried to install a new push-start switch but that too fails to operate the starter motor solenoid.

I will measure the current drawn by the solenoid ..... it has to operate the plunger so is bound to draw more current than the simple relay switch.
 

TradewindSailor

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Thanks all for the comments.

I haven't measured the potential difference across the push switch but I attempted to measure the current drawn by the solenoid on startup. I was using a quality clamp type DC ammeter ....... it showed 6 amps ..... but whether that is the true peak current I somewhat doubt. One thing I am sure of is that the relay draws less than an amp.
 

Stemar

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Is it the original switch? If not, it may be a switch designed for AC loads, where the spark is self limiting. DC switches are designed differently (bigger gap?), so they can cope better.

In any case, I would have thought that a small condenser across the switch would stop any sparking I'm sure that someone can tell you the right size, but without knowing how to do the sums, I'd use 100mfd, which is plenty big enough, and I don't think too big would matter, while too small wouldn't work.
 
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