Tabe Johnson <greentabe-vanagon@YAHOO.COM> writes:

> A glow plug is just an electrical resistor.  Imagine a very heavy
> duty light bulb without the glass globe.  When you pass electricity
> through it, it heats up.  To check, remove the bolts holding the
> electric wire to the glow plugs.  Put an ohmmeter between the
> terminal (where the wire was) and the engine block.  If you get an
> open circuit (infinite resistance), the glow plug's filament (like a
> light bulb) is broken and the plug needs to be replaced.  If you get
> a number like 10 or 30 ohms, (in that range- don't know exactly what
> it should be) the glow plug is OK.

I don't know about the glow plugs for volkswagen diesels, whether they
are series or parallel, etc.  But by comparison, on the Mercedes
"pencil" style glow plugs, the cold resistance of a good glow plug is
very low, about 0.8 Ohms.  If its much above that at all it is bad; be
aware that many bargain meters do not read very accurately this low in
the range.

However, resistance tests are more for identifying bad glow plugs than
confirming good ones.  A better test to confirm proper operation is
measuring the current draw as the plug heats up, it will start out
very high and fall as the plug heats up.  Exact specs are hopefully in
the Bentley or maybe you can google the brand and type of glow plug to
see what the current draw should be.

Allan