[Vwdiesel] No glow ....

[Roger Brown]

Derek wrote:
> Yup, that's the motor and I think I may have solved the problem.  I'll try to confirm tomorrow.  I am sure its the glow plug temperature sender.  It makes perfect sense.  With my switch hooked up ( a wire literally is wound on the terminal that SHOULD be grounding to make the whole thing go like its supposed to  ... ) it works fine.  So, I am simulating the ground that the relay should be seeing from the temp sender, but isn't.  I guess the light on the dash is also somehow connected to this sender ?  The great thing about all this, is the fact that now that I know how it is all wired and supposed to work.  I can hook up an intercept switch to prevent the GP's from turning on in the summer when they are hardly needed ....

The temp. sender provides an input to the relay.  That input does two things.  One is it sets a timeout for the GP light on the 
dash whose on-time varies with coolant temperature; longer when cold, shorter (or not at all) when hot.  It also sets a second 
time out for the glow plugs themselves (via the heavy relay contacts in the GP module).  That on-time is longer than the GP 
light.  On some models, the sender is a variable resistance and that provides a wide range of on-times.  Other models use a 
temperature switch, on when colder the X degrees and off when warmer than X degrees.  There are a variety GP module circuit 
designs out there.  SOme are very robust and "do the right thing" given flakey temp sender inputs.  Other circuit designs are 
very finiky and and follow the GIGO rule (garbage in, garbage out).  I built a GP module tester a few years ago and tested a 
drawer full of good and "bad" modules.  Funny thing is given good inputs, all the modules (good and "bad") worked fine. 
However, when given bad inputs, the bad modules "failed" and the good modules worked.

-- 
    Roger