The glow plugs do not fade out the are on or off. They do heat up and cool down however, but this is just a function of the on or off of the relay.

I just had a problem with the glow plugs on my 87 td, they were not glowing i traced it to a corosion problem in the small plug on the relay. The inside of the ground(brown wire) had a white flacky film in it that was creating an intermitting contact. I cleaned it out and the glow has been good sense.

There is still a problem with my glow light it was working intermitently a while back but has not worked at all for a couple of weeks. I have checked the dash bulb, it works, and the glow plugs are all good. The black wire at the small conector on the relay feeds the dash light and it will light my test when the key is on, so i must have a problem in the wire to the dash.

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82 Rabbit diesel (first d), 84 Jetta turbo d (300k when sold), 83 240d (305k when sold), 84 F250 6.9d w/Banks turbo(parts truck), 86 F250 6 cyl.gasser(waiting for 6.9d), 84 300d ( 347k Sold 8/04), Y2k New Bettle TDI (185k miles), 95 740IL (wifes), 87 300TD (206k Sold 7/05), 05 Passat TDI Wagon