How do most manufacturer's "NORMALLY" wire relays

[Silvic]

The title kind of states the issue.
I understand rather well how relays work and why they are used and the functionality of them.

My question is I am trying to back trace the wiring on a Kubota V2403 diesel engine that is having a few problems with the starter circuit.
It seems that Kubota is applying a continuous 12 volt to the one energizing pole of the relay and is using a controlling/switched ground on the other energizing pole to activate the relay. I am confused as to why that would be. I would expect that the 12 volt to the relay would be the switched line and not the ground. Is this normal and why would they use a switched ground to control the relay instead of a switched +12 volt. I am not talking about the armature side of the relay only the energizing side. The relay is a V23234-B1001-X004 with terminal 85 (+12v)and terminal 86 (switched ground) if it helps answer the question.

 

[Luke'sScreenName]

I recently worked on an accessory citcuit and the 12v power switched by the ignition powered the relay. Not sure if that is of any help.

 

[banjodunn]

G'day you will prob find that the ground wire goes back through the safety switches and goes to ground at the last one in the chain saves them a few cents having to loop a wire back to the start

Jon

 

[LD1]

Not sure on tractors, but with autos, they all switch the ground relay, and do so for a number of reasons.

If doing conventional hardwiring, you can get by with less wire. Meaning if you were to switch the hot, you would have to have a hot coming into the switch, then a hot going back out to energize the relay. Thats 2 hot wires under the dash or inside the cab.

By switching the ground, you can run only 1 wire into the cab to the switch, then pick up a chassis ground instead of running all the way back to the relay. So less wire, and more important, fewer hot wires. If one of them shorts to ground, the worst that happens is whatever you are controlling turns on.

So...safer, fewer wires.

With computer/ECM controlled circuits, it is easier to switch a ground than apply power to a relay

 

[Gator6x4]

Most starter relays have two voltage sources coming to the relay. A wire from the ignition switch to the relay provides voltage when the switch is turned to start. This voltages collapses the relay causing it to go to ground which in turn picks up the 12 volt hot voltage coming from the battery being supplied to the relay and contains a fuse. A then true battery voltage surge is then supplied to the starter solenoid. Most starter relays have 4 or 5 attaching points. Most times the 5th attaching point on a starter relay is not used.

Most starter relays on agricultural equipment is wired; ( 5 prong or post relay)

Current wire from switch to starter solenoid to relay and attached to post 85

Ground wire is attached to post 86

Wire to starter solenoid attaches to post 30

A fused 12 volt battery power source attaches to post 87

Post number 87a is not used

 

[Soundguy]

From 1947 thru end of production of the 4cyl prior models in 1965, ford used a similar way to energize start relays. ( at least on the straight gear models )

Power to one side of the relay, and a thumb button on the trans cover that merely provided ground to the relay.

On the early models thru late 54, it was a 3 terminal relay... one big post hot to battery, one big post to starter, and one little post to the thumb button. relay coil was between little post and always hot bat post. ( yes, you could make the starter turn over any time the battery was installed and tractor not in gear ) ( starter button on trans top was interlocked with tranny neutral ). from late 54 on thru the end of that prior line, they changed to a 4 terminal relay that sourced power on the new 4th terminal from the ignition key switch.. thus the relay coil was across the 2 small terminals. ( and still an isolated case ) style relay as before. ).

so the idea is certaintly not new... ( switched ground anyway.. )

The title kind of states the issue.
I understand rather well how relays work and why they are used and the functionality of them.

My question is I am trying to back trace the wiring on a Kubota V2403 diesel engine that is having a few problems with the starter circuit.
It seems that Kubota is applying a continuous 12 volt to the one energizing pole of the relay and is using a controlling/switched ground on the other energizing pole to activate the relay. I am confused as to why that would be. I would expect that the 12 volt to the relay would be the switched line and not the ground. Is this normal and why would they use a switched ground to control the relay instead of a switched +12 volt. I am not talking about the armature side of the relay only the energizing side. The relay is a V23234-B1001-X004 with terminal 85 (+12v)and terminal 86 (switched ground) if it helps answer the question.

 

[bumperm]

Commonly done on aircraft as well. A primary reason is safety. No need to fuse a ground wire and if for some reason there's an abraded control wire that's shorted to ground, no big deal (relatively speaking) as it just activates the relay rather than doing the arcing sparking thing.

 

[Silvic]

Most starter relays have two voltage sources coming to the relay. A wire from the ignition switch to the relay provides voltage when the switch is turned to start. This voltages collapses the relay causing it to go to ground which in turn picks up the 12 volt hot voltage coming from the battery being supplied to the relay and contains a fuse. A then true battery voltage surge is then supplied to the starter solenoid. Most starter relays have 4 or 5 attaching points. Most times the 5th attaching point on a starter relay is not used.

Most starter relays on agricultural equipment is wired; ( 5 prong or post relay)

Current wire from switch to starter solenoid to relay and attached to post 85

Ground wire is attached to post 86

Wire to starter solenoid attaches to post 30

A fused 12 volt battery power source attaches to post 87

Post number 87a is not used

Post 30 is the starter solenoid
Post 87 has 12 volt battery power
Post 85 is hot 12 volts not switched as far as I can tell
Post 86 is a switched ground or so it seems

When you state:
"This voltages collapses the relay causing it to go to ground which in turn picks up the 12 volt hot voltage coming from the battery being supplied to the relay and contains a fuse."
do you mean the magnetic field collapses which would open the relay switch or do you mean the relay closes which would pass the 12 v battery voltage? The way I read/interpreted this sentence is opposite to the rest of my interpretation of your explanation.

Thanks

 

[LD1]

The way yours is wired is a normal "ground switching" circuit.

Think of a relay as a switch. This relay/switch is used to control a high current device (such as headlights, horn, starter, etc). Rather than using control switches with big contacts to handle the current, they use a relay. This relay switches the high current devices but is controlled by low current.

So there are 2 parts to a relay, the high current side, and the control side (low current).

The high current side is just a set of contacts. Post 30 is 12v high current coming in. Post 87 and 87a are the outputs. 87a is a NC post and 87 is a NO. Meaning that 87 will get juice when the relay is activated, and 87a only gets it when the relay is not activated. Which isnt normally used on most circuits.

85 and 86 are simply the control, which requires very little amperage to operate (and smaller wires). Simply put, apply 12v across those two posts and the relay energizes and switches the high current side. It dont care which one is + or -. And it dont care which one is switched either. You can leave 12v hooked up all the time to one of the terminals, and open/close the ground connection to activate/deactivate the relay. Or you can leave the ground hooked up all the time and open/close the 12v source voltage.

 

[k0ua]

But in actual common practice we do care which coil terminal is plus and which is minus for there is often a spike suppression diode across the coil. Sometimes external sometimes internal to the relay case. The diode suppresses the back EMF ( electromotive force) that flows out of the coil when the circuit is broken. This spike is a considerable amount of power,and must be dealt with to prevent damage to other sensitive circuit components. The collapse of the magnetic field in the coil winding is what generates this voltage. The diode shorts this voltage out .