Here's my 'no doubt about it' glow plug test for vw diesel glow plugs, which
are in parallel, btw.
I remove the injector pipes and injectors, and injector heat shields. ( to
me this is a very easy 10 minute deal, or less ) .
Then the glow plugs are sitting right there,  where you can see them nicely.
You go turn on the key,
Then watch the 4 glow plugs in situ. . You can easily see if any are slower
to glow than the others, and of course a dead one is dead.
I've found the inner two generally seem to go out sooner then the end ones.
And one or two can get less that optimal in a year easily - i.e. sometimes
they don't last that long.  
( you're supposed to replace the injector heat shields  every time, and
injector torque is .....52 ft. lbs  I think  ) 
scott
www.turbovans.com 

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Allan Streib
Sent: Thursday, December 06, 2007 12:26 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Glows plugs

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