ShopTalk Forums | Aircooled VW Tech & Restoration Help

Speedy..

What's a Ground Loop?

From this thread

http://www.shoptalkforums.com/viewtopic.php?t=110906

Hmmm Dig up some nice, moist ground and form it into a loop shape.

Oooops! Wrong forum.

In our case, a ground loop exists where more than one current or signal
tries to exist on a single wire. Here, this would be the ground strap
from engine to chassis.

The tiny signal from the temp sender travels over this ground path
since the sender is screwed into the block. Then along comes the
mammoth current from the alternator as it makes its way to the battery
via that same ground strap.

If the alternator is putting out 50 Amps and the strap has a resistance of
only 0.01 Ohms, the result is a voltage drop across the strap of 1/2 V!
This 1/2V gets added to (or subtracted from) the sender signal.

If the sender signal is already tiny, 1/2V can make a huge difference
in the reading.

One way around this kind of problem is to run a separate ground wire
all the way from the dash gauge to the engine case. It can be a
very thin wire. The gauge housing must be insulated from ground
if the gauge is designed so that its Ground terminal is connected to the case.

Thanks..

Never heard of this.. is it common or just something we dont' recognize.

It's not common on a VW because we don't usually have temp gauges to
deal with.