Glow Plug question: Delay in GP light. - PeachParts Mercedes-Benz Forum

zill · #1 02-03-2007, 07:37 AM

The 240D was not starting and the GP light was not coming on with key on. GP # 1 and 2 were recently replaced. I replaced two glow plugs, #3 and 4, that I had suspected failed. It was cold and in parking lot and I did not have my multi tester. Anyway with the new GPs the 240D started fine.

The question: Now when I turn the key there is noticeable delay from when the key is turned on to when the GP light comes on. However, once it has lit and if I turn the key off and then on again it lights instantly. The car starts fine. You guys have any ideas for that delay in the GP light?

Thanks very much,

Zill.

yellit · #2 02-03-2007, 08:04 AM

The glow plug light can be very misleading in these cars....If you check the voltage on the glow plugs you will find that voltage is still on the plugs after the light goes out and I wait about 15 seconds of total glow time before cranking ...longer if it is very cold outside...I did not see a year model in the post... I am assuming you have the parallel glow system...The voltage remains on the plugs up to a minute before the relay cuts off the voltage/current to the plugs....It also cuts off when you turn key to (crank) position......
The glow light can be too unreliable at telling you when to crank or not...It is controlled by a temperature sensor ....a timer circuit and a current sense circuit and it gets whacky after time ....I put in a seperate voltmeter gauge to let me know when voltage is present on the plugs before cranking.....This light drove me nuts trying to sort it out...I finally took my relay apart to see what was happening.....
Another thing to check is that the solder pads inside the relay may get cracks in them and should be re-soldered....this is a common failure....
The light ends up doing whatever it does because of the multiple circuit inputs to it changing over time....but generally the relay itself passes voltage to plugs like it should.....whatever the light is doing...If the current path is intact....Just count the seconds before cranking to find your best glow time for your car....There are a lot of posts on this subject so you have plenty of reading material....

diesel don · #3 02-03-2007, 08:04 AM

I have noticed a number of inconsistencies over the years with the glo light. On nearly all occasions, it has been necessary to replace a glo plug. If your multi-tester reads that all are fine, I wouldn't worry too much about it.....especially if the car is functioning well now.

After all, the light indicators and guages on these beasts are not the impressive part of the car.

diesel don

yellit · #4 02-03-2007, 02:20 PM

If you want a real life test to make sure the plugs are all getting hot like they should and you have a good relay path and ground system to the head....Pull all the plugs and reattach the feed wires to them with heavy alligator jumpers from the threaded portion of the plugs back to the cylinder head to support them while they are just suspended where you can see them......
Then opearate your glow system and see how quick and how hot by just looking at them...see if they all compare.....do not crank the engine just get the glow.....

Just do this long enough for a test because the plugs are not in that big heat sink cylinder head......I had two that checked good with an ohmmeter but would quit glowing very quickly when they started heating.....the other two got white/orange hot fast...good eyeball test....