Ask The Mechanic

Multimeter Mystery

Image by Steve Thompson

READER: I bought a multimeter the other day and was showing it off to the guys in the coffee shop. Of course, everything that is brought in there is subject to scrutiny. So, the discussion about my new meter dealt with the checking of ohms. We were able to determine that ohms is a measure of resistance using no voltage from the item being checked. We all drank coffee that morning confused. How does the meter work when it is checking something with no volts?

STEVE: This measurement is a little tricky. Your meter has a battery in it. It is in there to be used when you set the meter to ohms (ohmmeter). When you put the meter leads on an electrical source that is unplugged, the battery sends volts through the item being checked. The number of volts that the battery can cram through the item being checked for resistance is looked at by the ohmmeter, and it gives you a reading of ohms of resistance. So, an open wire in a harness checked at both ends will read exactly like the ohmmeter does when the leads are just held up in the air with the ohmmeter turned on. The ohmmeter allows the tech to take look at each wire inside that big wiring harness for problems. Caterpillar dealers call this diagnostic procedure a “CAT” scan.

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READER: I am the owner of a late model 3020 John Deere tractor, and I have a problem with the hydraulics. When I push down the clutch and hold the clutch down too long, it suddenly loses all hydraulic functions. Also, when I grind feed with it and want to run my drop feeder with hydraulic motor, the motor will go real fast at first, and then it slows down and almost stops. When it does keep going, it is usually slow and jerky. What could I do to solve this problem? Is there anything that could be wrong besides changing filters?

STEVE: What is going on with your tractor is a common problem when you are using hydraulics with the clutch depressed or using more hydraulic oil than the transmission pump can supply the main pump. Oil is supplied to the main radial piston pump (big pump in front of tractor) by a pump driven by the transmission. So, when the clutch is depressed, the transmission does not turn to run the supply pump. When the main pump runs out of oil, you lose hydraulic functions until you let out the clutch, and the transmission pump again begins to supply oil to the main pump, or you lessen the demand on the transmission pump. There is nothing wrong with the tractor if your transmission pump is up to specs; it was not designed for extended hydraulic demands with the clutch depressed or even with the clutch not depressed and running the transmission pump. When you are operating the hydraulic motor, you will need to place the transmission in park or neutral, and release the clutch; and if your main pump is still starving, then the hydraulic motor is simply using more oil than the transmission pump can supply.

SAFETY TIP OF THE MONTH:

“Never work on a baler-tying system when the machine is in motion.” We have heard that all of our lives, but working on a machine that is not in motion under power can also be a good place to get injured. A twine baler system is one of these dangerous places to work. When raising the knotter assembly for inspection or repair, the tying system is activated by raising the knotter assembly. The twine knife scrapes across the billhook at an amazing speed and will slice your finger to the bone. Raise the knotter assembly very slowly, and keep your fingers on top of the knotter system when raising it for service.

Write Steve Thompson at Ask The Mechanic, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email mechanic@progressivefarmer.com.

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