Glow plugs not working

This is a first time post for me but I have been somewhat addicted to reading this great web site for the past few months! I have been a diesel mechanic for the past 25 years but always in the heavy duty field so my knowledge of the TDI leaves a little to be desired! My 99.5 Jetta TDI (114,000 mi.) has been starting hard. I do have the VAG-COM but have alot to learn to be able to use it to it's full potential. The glow plug indicator on the dashboard only comes on for a split second after the car has sat overnight in about 35 - 40 degree F temps. I replaced all 4 glow plugs with new Bosch (the originals were Beru) just for good measure. I did the VAG-COM test of the coolant temp sensor and it was off by about 2 degrees C. (my thermometer on top of the cold engine read 12C. and the sensor was at 14C.). I used a test light to probe the wiring harness (near the cyl. head where the two wires have what appears to be butt connectors) and check for current as my wife turned on the key. There was no current at all, not even a split second. I then unplugged the coolant temp. sensor and did the test again. This time there was current at the glow plug harness. After about 5 seconds I told my wife to go ahead and continue to start the engine and it started near instantly, and this was the first start up of the morning. After the engine started I kept on checking for current in the glow plug harness and it remained energized until I started to be concernerd that I might burn out all the glow plugs and after about 45 seconds had her turn the engine off. I am not sure if the coolant temp. sensor is the problem or if it might be the glow plug relay, or something else. Can anyone tell me if the glow plugs are even supposed to come on at these temps. The car does seem to start a little harder than it should even when warmed up to operating temp. Timing is good (center of the cloud is at 64, fuel temp at 108). No air in fuel. Runs great once started. No excessive smoke. Good fuel economy. Just starts harder than it should. Any advise is greatly appreciated!

The glow plugs don't come on unless it's below a certain temperature, about 9 degrees I think. Maybe they don't need to come on?

It is normal for the glow plugs to stay on for a little time after the car has fired up, on until it reaches operating temperature read this to learn more (it is for A3 but you will understand a lot of new things)
Well I think you will, if you don't, shame on me for douting

Yep, he's right. The glow plugs do not activate at all unless the coolant temp sensor reads below 9*C. So if it is colder than that and you do not get a glow, then consider changing your coolant temp sensor. They are only about $10 and a snap to replace.

The glow time is also directly proportional to how cold it is. the colder the temp, the longer the glow period. My car actually starts better below freezing then when it is above freezing for that reason. The glow period is longer. What you could do is run a pair of wires from the cab to the temp sensor wire. Put a switch in line with the voltage signal from the sensor. So when you flip the switch, its like you unplugged the sensor. This will give you the maximum glow time. Just don't forget to flip the switch back so that the ECM gets the proper signal for its engine management.

Thanks for the replies! I ordered a coolant temp sensor from the dealer today (they are going to mail it to me since I am about 50 miles away and work days). I will let you know how it turns out. Thanks again!