# VD Test



## Bob Driver (Nov 1, 2017)

No not that kind…. *V*oltage *D*rop Test. It takes a long time to get fast/good at diagnosing electrical problems. It seems that about 40% of the work I see coming through the shop is electrical and about ½ of that the machine has "VD". I really appreciate working on 12v DC systems because it won’t kill you. It might scare you, when you touch a wrench to the frame when tightening down a connection on the hot side of the battery, but nobody’s going to find you laying on the shop floor looking like a dead crow below a high voltage tower. I started working in a shop in the early 60’s. The old guys that first taught me about electrical systems had worked on 6v DC systems and really understood the havoc VD can cause in a DC electrical circuit and passed that paranoia along to me.

Old lawnmowers and old tractors have lived in a harsh environment. You can usually see/hear mechanical wear, or fluid leaks. Unless a brass electrical connector is the same shade of green as the 1960’s GE refrigerator you keep the beer in for the shop, you can’t usually see the “wear” in electrical circuits. That’s where I get to tell the customer I’m going to test their machine for VD and get the “look”….

It’s quick, easy, and the first thing I was taught to test for when trouble-shooting an electrical problem. 1) Power on. 2) VOM on 12vdc. 3) Red lead on one end of the circuit. 4) Black lead on the other. The voltage displayed is the level of the VD infection. Anything more than .5v is usually a problem. The longer the wire and the smaller the wire (18ga vs 12ga) compounds the problem. Bear in mind you can also have VD on the NEGATIVE side of a circuit and it’s pretty common on lawnmowers and tractors (the elusive “bad ground”). You can use VD to test for a bad switch, multi-pin electrical connector, light socket, or even a starter. Putting the VOM leads across the connections of a component tells you how much voltage the component is drawing out of the circuit under a load. A component, or wire, that's "consuming" .5v is usually a lot (except a starter -- 1.0V -- before you throw a new starter on, you need to do an amp draw test also)

Here’s a nice PDF that illustrates how to do a VD test on both the + and – side of battery cables, but the principal applies to any circuit, or component. Next time you’re headed down to the shop, tell the wife you just read a post on the Tractor Forum and you’re going to test the lawnmower/tractor for VD and you’ll see what I mean by the “look”. Mine told me to wear rubber gloves, wash my hands before I came back in the house, and stop parking the lawnmowers so close together at night...

https://fme-cat.com/docs/1519.pdf


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