Observe section H in this section of this FSM.
https://www.box.com/s/d13jzf7prcr1udgpdtv9This is the glow plug relay, and essentially the whole system, for a
post 80 or so W123 with the OM617 5cyl or 616 4cyl (300D/240D)
This is a proven system that is generally quite trouble free and robust,
and nearly as important to me, is mechanically and electrically elegant
and exhibits classical German engineering and is the culmination of a number
of iterations, as you can read in the previous sections before H.
I've owned a number of these cars and never had a moments trouble other than a
single glow plug that died, which was easy enough to check since they are on separate
lines in a connector. The NTC sensor is integral to the relay, as is the fuse. The relay
has basic self diagnostic functions detailed in the FSM in that if a single GP fails
it will not illuminate the GP light as warning. There is an afterglow feature, and a time
based safety cut-off. Also there is a simple and common modification to the relay that will
illuminate the GP light at a lower level to show after glow, which is a true representation of the
actual current receiving state of the GPs.
They are pretty cheap used, and there are probably 10 of them for every VW around, and
aren't crazy expensive new, nor will they become unavailable in the foreseeable future.
Seems like a no brainer to me, unless I've overlooked some reason it can't be used on a
4cyl VW-D.
Objections to dicking with the stock setup would be, and I've voiced them quietly before, that the stock VW setup frequently works fine, and when
it does not there is generally an outside reason. I have had a 650K and have a 400K example, and numerous
250Kish VW-D's and every one of them had completely normal stock GP systems with all stock components for many years, since new in fact.
Maybe I'm just lucky. I also appreciate the huge gnarly wire with a ford starter relay with a manual switch approach, or any variation of
to get the current out of the fuse box. I wouldn't have designed it the way VW did, even though it usually works in my experience.
I'd have done just what Mercedes did, hence this thought.
The only less than ideal thing I can find here is wire size, and it's been my experience from owning a few of and working on a lot of, that
Mercedes back in that era did not cut corners on BS like that, if it needed a larger wire, it would have gotten one.
Nor do know exactly how the GP temp relates to voltage, as I've seen quoted with the stock system (but never measured) as less than
the ideal of battery voltage. If anyone has ever checked it with something able to measure north of 900F at varying voltages I'd be interested
to hear it. There is also some pertinent info in the FSM I linked above about target temps, for the Merc motors at least.
I've pondered this for many years, since I had spare Merc stuff around, but never got around to it largely since I've never
had a problem with the stock VW system.
Still looks like a neat project though and I hope to get to it shortly.
Thoughts?