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Discussion Starter #1
i have a 99 jetta tdi i recently had to put a new head on and t belt. The car runs great but on cold starts the glow plug light comes on when the key is turned on but the light goes out right away. I put a new temp sending unit on when i did the head. I unpluged the temp sending unit and left it unpluged and the glow plugs seem to work proper . I dont think its the temp sending unit cause the old one worked fine and when i just plug it in the glow plugs still dont work unplug it they work. Any idea thanks
 

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It sounds like you found the problem. The GP light works find with the temp sending unit unplugged so it's either the sensor or wiring. When you unplug the sensor the ECU defaults to max GP duration. It rules out a GP and relay problem.

What was the reason for the new sensor if the old one worked fine?
 

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I just replaced it while i was there had to have it out anyway . Ok so started the car with it unpluged like 3 times just on and off went to go to the store started the car and it acked like before let the glow plugs cycle again started fine but it triped the service engine soon light . Also the tsu i replaced was on the housing thats on the driver side of the head . what sensor is on the cupling like thing thats on the upper rad hose ? is there anything else that may cause this problem
 

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I pluged the temp sending unit back in and put around 70 miles on it the light has gone out and staying off as long as it is pluged in .Thanks for the help
 

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So im still at a loss myself im guessing its a wiring problem . Does anyone know which 2 wires on the temp sending unit controll the glow plugs ? Im guessing on its 2 wires i would think 2 go to the gauge and 2 go to the glow plugs . If you think im on the right track let me know to me it seems if the glow plugs work normaly with the temp sending unit unpluged and does not work with it pluged in that since they work that means the relay and fuse and glow plugs are good .
 

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So im still at a loss myself im guessing its a wiring problem . Does anyone know which 2 wires on the temp sending unit controll the glow plugs ? Im guessing on its 2 wires i would think 2 go to the gauge and 2 go to the glow plugs . If you think im on the right track let me know to me it seems if the glow plugs work normaly with the temp sending unit unpluged and does not work with it pluged in that since they work that means the relay and fuse and glow plugs are good .
I'd like to find this out as well as I just bought a 2000 Jetta TDI with the same issue (turn key, glow plug lamp is lit for about half a second then goes out, hard starting).

-David
 

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Have you checked each glow plug with a test light? There is a excellent article on this: search black smoke at startup. A great reply by keithuk. This discusses the complete system,codes,repair. Hit the link to tdi club.
 
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