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An electrical ghost finally found
Posted to Technical Tips Forum on 4/22/2014 16 Replies

I don't normally do Ag machinery service calls but when it's one of your best "Just fix it, I don't care what it costs" customers, well, you make exceptions.

The machine was a John Deere 8960 articulated 4wd tractor that the customer had bought brand new in about 1990. The reason I was there fixing it was because a wire harness that runs from the cab to the rear of the tractor had gotten tangled up in the center drive shaft. The customer had spliced the wires but couldn't make the hazard and turn signals at the implement connector work. That fix was easy enough as he had a few wires flipped around and didn't see two more wires that were damaged. Sorted out the wiring and re-secured the harness to keep it away from the spinning drive shaft and all was well.

As I was finishing up the owner asked me if I would check the charging system since they had added some more field lights and wanted to be sure the alternator and the 3 new batteries were up to the task....So I hook my voltmeter up to the center [handiest] of the three 12v batteries and with the tractor running I find only 12.30 volts. Hmm, not good. I go to the alternator stud and find 14.44 volts. I move to the starter lug and still find 14.44 volts. Go back to the battery and find only 12.30 volts. What the??? This thing has BIG battery cables that are in great shape and connect the 3 batteries in parallel so it's like one giant 12v battery. Next I start checking for voltage drop...thinking maybe the positive cable has a bad spot hidden somewhere under the cab but get essentially zero voltage drop...but still the battery is reading only 12.30 volts. I do a voltage drop on the ground again. Now I'm really perplexed and the customer is telling me that since this tractor was new they've had lots of problems with slow cranking in cold weather and battery failures. While it was under warranty the dealer tried different alternators, batteries, and a starter. In the years since it became one of those tractors that you might have to jump to get up enough cranking speed for it to start if it was cold out.

So I check the actual cable to post connection and find nothing but perfection. I load test each battery by itself...all good...except the middle one needs charging. Finally with everything hooked up and the tractor running I was rechecking voltage and happened to check the first battery and find...14.40 volts. Check the second battery again...12.30 volts. Check the third battery....14.4 volts. Aha! I do a voltage drop between all the positive post cable clamps and find zero. Do the same on the negative side and find a drop of about 2.10 volts at the middle battery cable clamp.

Turns out that when the cable was produced and installed when the tractor was NEW there was almost no contact between the copper wires and the center of the three cable clamps that attaches to the center batteries negative post. This meant that the center battery...which I randomly hooked onto while testing...wasn't being recharged and wasn't contributing to the cranking power during cold weather starts. There was just enough of a connection, about 180 ohms of resistance, that when the tractor sat long enough the discharged center battery could slowly siphon off power from the two fully charged batteries.

To avoid having to dig out the entire big cable to repair the problem we installed a short jumper cable between the center batteries negative cable clamp pinch bolt and the ground lug. Bingo! No more voltage drop and an instant and noticeable increase in engine cranking speed.

I might have had to scratch my head longer on this one but I had run into a similar situation years ago where an older 6 volt tractor of my own had developed a slow cranking problem. To fix it I had converted the system to 12v negative ground, changed over the ignition system, replaced all the battery cables only to find it only cranked over just a wee bit faster after all that work. The battery tested out good as did the starter and all the connections. It charged at 14+ volts, but was a slow cranker. If it was really cold you'd have to add a jumper [and we'd jump it at the easy-to-get-at starter terminal] to get it to crank fast. One day when it was in the shop I had the battery cover off and it was cranking slow as usual so I grabbed the jumper pack to jump it at the battery and for some reason accidentally touched the positive battery cable clamp with my thumb and immediately discovered it was hot enough to instantly blister the skin. What???. I Dissected the "new" cable clamp and found that despite being and looking brand new there were only about three strands of wire actually fused into the cable clamp. Enough connection for the battery to stay charged but not enough to fully power the starter. A new cable fixed the cranking problem instantly and left a good lesson in it's wake.

Long story short, voltage drop tests are worth their weight in gold and remember to test EVERYTHING even when it looks or is new. You might be surprised at where you find the problem.

Dale from North Dakota

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