NEWS
Ask The Mechanic
Mon Jul 6, 2015 01:42 PM CDT
(Page 1 of 2)
(Progressive Farmer photo by Photographer Jim Patrico)

Q: Both my newer diesel tractor and pickup have all the emission-control devices on them, including the DEF (Diesel Exhaust Fluid) tank. However, I have a friend who has a Volkswagen car that does not use DEF or have to go through all of that “regen” routine, with the light on the dash coming on and the area around the vehicle getting hot enough to fry an egg. How did VW and German engineering get by without using all of the emission-standards I have on my diesel truck and tractor?

A: On the small diesel engine that Volkswagen used in its cars until the last year or so, it was able to eliminate the regen cycle with a catalytic converter. It takes 600°F to burn the soot. With a catalytic converter placed right at the exhaust manifold, the temperature there got hot enough to burn out the soot under normal operating conditions without having to add excessive fuel to the exhaust system. Adding excessive fuel to the exhaust system is what is going on in your newer truck and tractor during the regen cycle in order to reach that 600°F threshold. The DEF is sprayed on the exhaust gas after the fuel is burned. Diesel exhaust today is clean. I have heard claims that the diesel exhaust is cleaner than the air entering the engine.


Editor’s Note: Diesel particulate filters, or traps, catch bits of soot in diesel engine exhaust. They must
be emptied regularly to maintain performance. This process is called regeneration, also known by the slang term “regen.” When the soot loading in the filter reaches a set limit, the vehicle’s engine-management unit initiates a post-combustion fuel injection to increase the exhaust temperature and trigger regeneration.

Q: I have a question about the tachometer on my 5103 John Deere tractor. Sometimes it works, sometimes it does not. The dealer told me the problem was probably with the sending unit. I took it out, and I found that it was actually a magnet. How in the world does a magnet make my tach work? I replaced the sending unit, and now my tach works all of the time.

A: The magnet on the tach sending unit that you replaced is smart. When metal is moved back and forth across it, it generates AC voltage, and the more AC voltage, the more your tachometer moves up. Since the magnet is placed just a small distance from a gear turning on the front gear drivetrain, it generates AC voltage according to how fast your engine is running. The key for this magnet to produce AC voltage is the metal has to cross back and forth across the magnet (like teeth on a gear). Do this little fun test: Put a voltmeter on AC volts, and put the leads on the two pin connectors in the tach sending unit. Now, move a screwdriver back and forth at different speeds across the magnet. You will see the AC voltage will increase as the speed of the screwdriver increases. By the way, if the magnet is good in the sender, when running, the sender should produce at least around 1 volt of AC. ⦁

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