Starter relay JD6068

[Wdeertz]

I recently replaced the starter relay on my John Deere 6068 engine. The newly installed starter relay was a JD OEM version and was significantly smaller than the old one removed. Also the old relay had what appeared to be a resistor (see photo) across the control switch lugs. I installed the new starter relay without the resistor and the engine started right up.

Anyone know what the purpose of the resistor across the control lugs is for?

[C lectric]

I doubt that thing is a resistor. Likely a capacitor which was intended to absorb the backflash voltage of the coil. Coils, when de-energized can produce a high voltage reverse voltage. THe cap will absorb it and feed it into the coil slowly.
I say slowly but all of this goes on in less than an eyeblink for small devices like this..

The red and black leads are an indicator. Resistors do not care about polarity, many caps. do.

JMO

[Wdeertz]

JMO, thanks for the reply. On another forum someone suggested the device was a diode to absorb the voltage spike when de-energized, similar to your reply. I’m familiar with capacitors being round cylinders like what you see on hvac systems. In any event I didn’t replace as the original OEM schematic didn’t show this device.

[C lectric]

Diodes and capacitors can be used depending upon what you are trying to protect.

[diver dave]

Diodes are very common across dc coils to reduce back emf as you say. Thus the red/blk polarized marking. I would verify that with a continuity meter and install it. If a mechanical switch is used, the diode will prolong its contact life.

[Jeff F]

My JD has a strange two-step process for the start that I really don't understand. I only discovered it doing a deep dive on the schematics and computer operations. This could be related.

[Jeff F]

Thinking about this, it could be a test for closed circuit to disable if running.

OTOH computer already knows when running.

Dunno.

[diver dave]

If a computer is driving the cranking command, even more important to squash the inductive kickback with the diode.

[Jeff F]

They would be listed as separate parts, so no surprise the new one didn't have it.

Sounds like you should put the old whatever-it-is on the new solenoid.

[Jeff F]

Make sure it goes on the right way.

[twistedtree]

I agree it’s a diode to clamp the reverse EMF from the coil when it’s released. That limits the pulse going back to whatever is controlling the solenoid, possibly the ECU.

That said, some solenoids have the clamping diode built in. Try measuring the resistance across the diode in both directions. It should be high in one direction and low in the other. Then try the same across the coil terminals in the solenoid. Both directions will be low because of the coil windings, but one lower than the other. I’d be interested to hear what you find.

[Wdeertz]

Originally Posted by twistedtree

I agree it’s a diode to clamp the reverse EMF from the coil when it’s released. That limits the pulse going back to whatever is controlling the solenoid, possibly the ECU.

That said, some solenoids have the clamping diode built in. Try measuring the resistance across the diode in both directions. It should be high in one direction and low in the other. Then try the same across the coil terminals in the solenoid. Both directions will be low because of the coil windings, but one lower than the other. I’d be interested to hear what you find.

TT, thanks for info. Using my multimeter red to red and black to black on the diode measures OL and in reverse 2.2 ohms (or .58v). I’m not on the boat so can’t measure across the solenoid terminals. On the new relay the resistance is 27.7 ohms in both directions. With the diode measuring high in only one direction which terminal the red and black diode wires connect to matter but there is no + or - markings on the relay. I’m inclined to use the relay without the diode, if it craps out prematurely I have a spare at which time I”lol know to add the diode.

[sunchaser]

WD
That relay looks pretty rough. Did you have a raw water leak on top of it? Did it possibly give out due to corrosion or bad contacts? Whatever the reason it should be addressed to prevent a further occurrence.

[diver dave]

The diode is not there to protect that contactor. Its there to prolong/protect what is driving it. I will say that i have not seen them used to protect “simple” key start setups.

[Wdeertz]

Originally Posted by sunchaser

WD
That relay looks pretty rough. Did you have a raw water leak on top of it? Did it possibly give out due to corrosion or bad contacts? Whatever the reason it should be addressed to prevent a further occurrence.

Below the coolant petcock valve and got covered during draining.

[Wdeertz]

Originally Posted by diver dave

The diode is not there to protect that contactor. Its there to prolong/protect what is driving it. I will say that i have not seen them used to protect “simple” key start setups.

According to the schematic I have it’s a simple button start.

[Lepke]

The original relay is common as dirt. It's available at any auto parts store. But not by a JD number. John Deere makes most of their engines but not all. It doesn't make relays. Nor do other engine makers. It is made by a specialty company. Probably in China now.
This relay was used on cars and equipment from the 1940s. Cars, tanks, tractors, even aircraft, anything needing a 12v starter relay. Some versions have fewer posts. Some have a button in the end to bypass the key. Handy in an engineroom to jog the engine.
Auto parts stores have a catalog with pictures. You pick the one with the right number of posts and amp rating. $20-40 on ebay.

[diver dave]

Good info Lep. Didnt know about the button option.

[Jeff F]

Wait, doesn't a pair of pliers do that?

[diver dave]

Sure, but dont miss that second post