Ask The Mechanic

Send It Home

Tractor rear lights, Image by Steve Thompson

READER 1: The rear work light on my tractor’s fender is driving me crazy. It makes no sense why it won’t burn. I thought it was the bulb, because when I stuck the hot wire going to the light, the test light would burn, but, the bulb would not burn. To my surprise, the new bulb did not make the light work. In all my wisdom, I removed the bulb and stuck the test light in the middle terminal of the pigtail--the test light came on. I put the bulb back in--the light still would not work. But, when I stuck the other wire leaving the light after the bulb, the test light would burn, but, the light bulb would not burn. How can this be? I then removed the complete light and stuck the wires on a battery. The light burned. I’m stumped, confused and glad to have a day job. I reinstalled the light on the fender, and, the light still won’t burn. Please tell me I’m not losing my mind. Both sides of the light bulb will burn, the bulb is new, the pigtail has voltage and will light the bulb, but, the bulb won’t burn.

STEVE: Your mind is OK. You’ve just never heard Pappy Thompson’s sermon on the fact that electricity will not leave unless it can come home. When you stuck the hot wire with the test light, you saw that the available voltage at the load was there. Next, you changed the bulb, which eliminated that possible problem. Then, you stuck the inside center of the pigtail terminal, and, voltage was there. But, you failed to realize that what you were doing with the test light was forming a ground with the clip, allowing the current to flow back home to the battery through your test light. Then, you stuck the ground side of the light; you had voltage there because the bulb was not using the voltage. In a DC circuit, the little bulb (or any load) will use all available voltage. So, since the bulb was not using any voltage, you did not use (drop) any voltage, so, the voltage was still there looking for a ground, and, your test light completed the circuit. You have a bad ground. Follow the ground wire, and, you will find your problem. If the light only has one wire, then the ground runs through the fender, through the axle housing and back to the negative post on the battery. The rear light ground is more than likely lost due to rust between the bottom of the fender and the axle housing if you are trying to ground through the fender. When you have completed the circuit, the light will burn. A connection or ground problem cause about 85% of all electrical problems. What goes around comes around.

READER 2: I tore into my John Deere 4440 engine--tech manual in hand--and found some shims underneath three of the cylinder liners after I removed them. What’s the deal with the shims being under only three of the cylinder liners? I figured each of the cylinder liners would be treated the same. What is the purpose of the shims?

STEVE: Wet cylinder liners (coolant all around them) require specific protrusion of each liner in order to keep the height between the top of the liner and the engine block as close to the same as possible so the sealing rings on the head gasket “crush” against the top of the liner as equally as possible. Sometimes, swapping around the liners will help align the protrusion, but, sometimes, shims must be added to the top of the counterbore (under the liner lip) to equalize the protrusion. If the protrusion across all cylinders is not within specs, then the head gasket can blow. The 466-cubic-inch engine in your 4440 is the same engine used in the 4WD tractors at that time, and, the increased power pulled out of the engine made it especially important to keep the protrusion in specs. The shims will raise the liner to the proper protrusion, if needed. However, some engines had a counterbore that placed the liner protrusion too tall. This was a real problem, because the block had to be counterbored to lower the liner height. Follow the tech manual closely on this part of the overhaul of your engine so your specifications for protrusion, head torque pounds and sequence are followed. Newer machining technology has really helped to keep liner protrusion problems to a minimum.

Safety Tip of the Month:

Be careful when unhooking a trailer that has what is called negative tongue weight. If a trailer is loaded heavily in the back, when the latch is released from the ball, the tongue of the trailer will fly up. That’s a dangerous situation, because you usually turn down the jack when you unhook rather than the jack turning on you. Even though the tongue can mess up the tailgate on your truck, it is best to get out of its way if it flies up. Now you know why you see so many tailgates with that deep scratch running up the center. It’s much easier for a body shop to fix your truck’s body than it is for a doctor to fix your body.

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


Past Issues