Posted to Technical Tips Forum on 4/22/2014
An electrical ghost finally found
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 cable...zero 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
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
Dale from North Dakota
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16 Replies Received