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Starter Relays - 3 terminal vs 4 terminal - SOI University

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I happened to pull out a like new 4 pole starter relay from a scrap carcass and thought I might whip together a small tutorial on confirming the operation of the relay.

In the pics below, I substituted a modest battery charger for a 12 volt power source.

In the first picture, an "overly simplified" multimeter is set to read the lowest OHMS range possible. The multimeter leads are then connected across the top "starter" studs of the relay. These studs will allow connection of the battery + cable to the starter + stud. The ohm meter will indicate if there is a conductive pathway from one top stud to the other.

Also note the + cable from the battery is NOT connected to the signal tabs of the relay assembly. Note that no current is flowing thru the battery charger since the + lead is not yet connected to the relay. If the relay is functioning properly in this mode, the ohmmeter should indicate no electrical pathway exists thru the starter studs and the ohmmeter should indicate an infinite resistance. This is usually displayed as a - - - - on most meter readouts. An analog meter will keep the pointer on the infinity symbol of the ohms scale.


The 2nd diagnostic step will connect the + lead from the battery charger of the relay.

Please note the negative battery lead is not connected to the relay frame but to the 2nd signal tab. A visual explanation of the 2nd tab will appear later in a x-ray view of the relay inners.

Current should now be flowing thru the solenoid relay inside the relay case. The battery charger should jump to about 2 amps of current if the relay coil is ELECTRICALLY intact. When the battery charger + cable is attached, a loud clack sound should also be heard / felt from the relay body. The clack sound would be the first clue the relay might be MECHANICALLY functional too.


Further, note the ohmmeter reading with the power applied to the relay assembly. If the "starter contacts" in the relay are ELECTRICALLY functional, the ohmmeter should change to a reading of near 0 ohms resistance both on digial and analog multimeters.


When near 0 ohms are confirmed, remove the + battery lead from the relay signal tab and another click should be heard, the ohmmeter reading should once again change to an infinite ohms reading (----).

So what goes on inside the relay body?

Here is a X-ray view of the inner workings. First picture is no power applied to the signal tabs and the starter relay studs are not bridged by the copper bar mounted to solenoid plunger. No electrical path should exist between the two top starter wiring studs.


Apply the 12 volts from the battery charger, 2 amps current should flow thru the relay coil and while the current flows, creates a magnet out of the relay coil.


The relay coil will now repel the iron rod plunger out of the center of the coil. The plunger has no where to go but up. Attached to the plunger is either a copper disc or a copper bar that will be forced up against the two top starter studs. This creates an electrical pathway thru the top starter studs and allows current to flow from the battery to the starter motor.

The only real difference between a 3 pole relay and a 4 pole relay is the electrical wiring to the negative signal tab. Instead of the negative wire from the relay coil going to a separate tab, the negative signal wire is spot welded to the metal frame of the relay body.


The ground connection for a 3 pole relay is picked up thru the mechanical bond of relay frame to tractor frame. In a 4 pole relay, the electrical connection is picked up thru ground wiring to the negative signal tab.

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Another great class on testing/checking electrical devices...Thank you!

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Thank you.relays I also work on a lot of lift gates for trucks and trailers. They all use the same relays. Most of the time they are the 4 post set up.

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Forgive the question from the electrically illiterate... but for a 3 pole relay, you would connect the negative lead from the battery charger to the frame where the relay is grounded, yes?

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exactly right


as an alternative, directly to the metal mounting frame of the relay if it is out of the tractor.

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