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The limits I keep to with a manual-range car multimeter

#hardware #tools

A meter that reads wrongly is worse than no meter, because I'll believe it. If a number on my UT107+ doesn't make sense, my own setup is the first suspect, and the car waits until I've ruled it out. Below is the order I check things in, followed by the figures I treat as fixed and a short care routine.

When a reading makes no sense

My checks run in this order:

  • Is the rotary switch on the right function? With manual ranging, choosing it is my job.
  • Is the range sensible? With manual ranging, I'm the one choosing the range, so I check it before I trust the figure.
  • Is the black lead in COM? It should always be.
  • Is the red lead in the jack that matches the function? VΩ for voltage, resistance, frequency, temperature, RPM and dwell; mA for the 12 V battery test; 20 A only for high current.
  • Are the AAA batteries still healthy? Two of them sit in the rear compartment, and I'd rather swap them than trust a doubtful display.

RPM or dwell looks odd

For RPM I check that I short-pressed SEL/VFC and picked the right engine type, 2-stroke or 4-stroke. If I've picked the wrong one, the reading isn't one I can use. For dwell I confirm the cylinder count is within what the meter handles, which is 1–8 cylinders.

Hard limits

These are the figures I don't argue with.

  • High current: the 20 A jack is rated at 20 A for 10 seconds maximum. I keep high-current readings short and never leave the leads connected while I go and look at something else.
  • Category: CAT II 1000 V / CAT III 600 V. I treat that as the ceiling and stay well inside it.
  • Temperature: the meter's quoted range is −40 ℃ to 1000 ℃, and I measure with the supplied K-type probe.
  • Leads: if the insulation on a test lead is nicked or cracked, the lead is retired.

I also won't change the rotary switch with the probes still on a live circuit. I take them off, turn the switch, then reconnect.

Looking after it

  1. Switch it off before packing up.
  2. Unplug both leads from the jacks.
  3. Wipe off any oil or grime picked up in the engine bay.
  4. Coil the leads loosely and put everything in the carrying bag, the K-type probe included.
  5. Keep the English manual with it, since the test-specific lead positions are easy to forget.
  6. If the meter is going to sit unused for a long stretch, take out the two AAA cells.

None of this takes long, and it means I start each job from the same routine.

Anyone curious about the meter behind these notes can find it at autotrue.co.uk.

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