Do grounds really matter?

Hammermechanicman

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Had a tough one in the shop this week. Mid '90s White/MTD rider. Blade electric clutch won't engage. Zip tie failed and headlight wiring fell against muffler and burned it up. One of fhe ground connections for the clutch circuit goes through the headlight wiring. Clutch uses 2 relays for a hold in circuit. After fixing the headlight wiring the clutch would engage but not stay engaged when PTO switch was released to the on position using the other ground through the relays. Ground measured .2 ohms. The ground would carry current for the idiot lights, starter solenoid and carb solenoid but not the blade clutch. Ground was 14ga wire with a ring terminal attaced to the frame under the mower by the trans. Looked fine. Took it off and cleaned it, put it back on and problem solved. Couldn't find a decent wiring diagram. Spent a whole morning on this one.
 

Scrubcadet10

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ground wires can be tricky, one time on my mule whenever I pressed the brake, the headlights would come on and not the brake light. After 2 or 3 days i found a ground wire that broke underneath the seat. It was in a bad spot for limbs and weeds to grab it.
I moved it, but it fixed that problem.
 

Telesis

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Here's the thing to keep in mind. With a higher current load like a PTO that draws say 4 amps or so, having just 1 ohm of additional resistance due to corroded connections, switch contacts dirty etc, can keep it from pulling in. Here's the math. A typical PTO may have a resistance of 3 ohms. Hence, 12volts / 3ohms = 4 amps(normal conditions). What happens with the additional 1 ohm of resistance due to the bad connections. Current goes down... 12volts / 4ohms = 3 amps. Now 3 amps of current is flowing through the 1 ohm of extra resistance which causes a voltage drop of 3 volts of the available battery voltage and the PTO will only see 9 volts. This can cause the PTO to not be able to pull in.

Some folks have stated in other threads that if you measure the resistance it will still read low even with corroded wires or bad connections. However, the fact is the resistance has indeed gone up. I believe the problem is that folks don't appreciate what small a change(increase) in resistance can make in terms of the resulting voltage drop, particularly in PTO circuits and in starter solenoid trigger circuits that pull a few amps. You look at your digital meter and see it's "only" an ohm of resistance and think that is OK when it's likely not. It's easier to see when you measure the voltage at the battery, and then the voltage at the PTO or solenoid. The voltage drop number is bigger and appeals to your common sense more than the tiny increase in the circuit resistance!

Just some food for thought!
 

Hammermechanicman

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Normally i would do a voltage drop test but couldn't on this one. The clutch has 2 ground circuits. One is to engage it. That ground was good after i fixed the headlight wiring harness. The second ground and the positive voltage to the clutch passes through 2 sealed relays. When the switch is held in the engage position the first ground enabled the relays and engaged the clutch. As soon as the switch is released to the run position the good gound was lost and the current draw of the clutch dropped the voltage low enough in an instant for the relays to drop out thus dropping out the clutch. So couldn't do a voltage drop test. It is a goofy circuit that if the mower is put in reverse or the seat switch is deactivated it drops the hold in circuit to the clutch and you must move the PTO switch from on to engage to start the blades again. The ground ohmed out at .2 ohms but wouldn't carry the clutch load.
 

tom3

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Sort of like a bad battery cable on a car. Everything works, even headlights, hit the starter and get a click. Trick is to pull on the head lights, hit the starter and see if those lights go out and stay out. Bingo. That electricity stuff gets tricky sometimes.
 

Born2Mow

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Grounds are super important. And the lower the voltage, the more important great connections become imperative.

In my design work on military electronics I discovered a compound that can be applied to ground points that not only helps reject water, but chemically eliminates micro-corrosion that might already be present. This product is called No-Ox-Id by Sanchem. There may be many good commercial "No-Ox" products, this is just the one I know to be excellent and outdoor rated. Find it Here on Amazon
 

Born2Mow

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I've always used dielectric grease

A dielectric grease is great on new electrical connections where you simply want to exclude water from starting any corrosion. [And here you will need to open the dictionary and look up the definition of "dielectric".] After the corrosion has started, you need a compound that ACTIVELY promotes electrical connection. A dielectric doesn't do that ! In fact, dielectrics inhibit electrical connection. So if you're looking for Help, you really don't want to apply an inhibitor.

At least that's the way I see it.
 
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