Think I have been wrong with my ideas on the Glow Plug CEL. I went over to talk to the Guys at the local Dealership where I have been going to. I did not get much in the way of help.
One thing. There are not many VW TDIs on the road. I have a 2003 with 90k miles, so I am probably further down the road than most.
I have the Bentley Manual for the 2003. It tells me that I should see 6 amps through the Glow Plug. I measured over 20A and blew out the 15A fuse internal to my DVM.
I have been trying to check for specs on Glow Plugs from the different MFGs however I have not been able to get any site that will take questions. However it looks like there is a new Ceramic Glow Plug that is more efficient than the older ones. That these do take a lower Voltage, and Draw less Current. And my be pulse width modulated with a chopped DC voltage. Using a meter will read 6.5 volts or so. The Tip is reported to get very Hot Very Fast.
I would suspect that you just can't mix and match.
I am an old timer. I had a VW Rabbit. I had some minor problems, However, they were corrcted. What they were doing back then was to start to keep the local mechanic out of the engine. They were starting to use the Strech bolts. Torque them down and then go another 1/4 Turn or so. The service manager finally showed me a Service Bullitin where the Diesel still wanted the heads to be retourqued the old way, after 1000 miles after engine work, and a New Car as well.
Then there is the DexCool. But again, we are trying to keep people out of the engine.
There has been a lot of mis-understanding, and mixing and matching.
Putting 12 volts on a 7 volt Glow Plug will burn it out very quickly, but not right away.
How does the ECU measure a good Glow Plug? use a Shunt or a Hall device?
I have been known to make my oun Test equipment in the past. However it helps to know what you are looking for when you do this.
For example. Using a 12 Lamp to check the glow plug. If either type of glow plug is being tested by a Brake lite, the resistance of the GP will light the bulb pretty much to full brillyence. You need a better way to read the current.
Never said I can spell.
With the newer Ceramic GPs if they use a Pulse Width Scheme, you should be able to hear some sort of a tone, if you were to use a speaker and capacitor.
The local mechanic, what does he care? A GP is a GP.
BTW With my CEL, I have tried some NAPA Battery Post Cleaner on the Battery, Heavy metal Fuse links, and the GP Connectors them selves. And have been able to put the CEL out.
I do have kids in the Hot Zone, I have the blocker on tight. Glow Plugs will let you through.