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gnolo 05-29-2002 03:50 PM

preheat light
Just a question ...

I have read on the technical book for the diesel W123s that when the engine is already hot (above 70C or 160F or something like that) the preheat light on the dashboard should not be lighted and you can start the engine directly, without preheating.

But on my 240D, the preheat light is always on, even when the engine temperature is about 90C (or 195F). And if I remember well, I believe that when my 240D was youger, the preheat light didn't light above 70C.

So, how can I fix that ? Do I have a sensor problem anywhere?

JimSmith 05-29-2002 04:04 PM


I think the duration of the light being on changes with engine and ambient temperature, but it should always go on when you turn the key. I also think there is a minimum time the glow system will cycle the plugs on for, and the manual note is to tell you if you are in a hurry, and the car is at the temperature it normally operates then go ahead and start it. I do not think there is a way to make the light not come on at all if the car is warmed up. Jim

gnolo 05-29-2002 04:19 PM

And by the way, I have notice that when the engine is cold, it start better when I wait a few seconds after the extinction of the preheat light. My glow plug have been changed last year and they have today about 30 000 km.

If i don't wait and start the engine just after the extinction of the preheat light, the engine run jerky during about 15s.

I know that the glow plug are still operating during a while after the extinction of the preheat light on the dashboard, but why need I to preheat my engine more than it is required ?

JimSmith 05-29-2002 05:07 PM


That sounds like you have a single or maybe two glow plugs that are not performing correctly, or are on their way out. If they are burned out the system usually causes the glow plug indicator lamp on the dashboard to light up oddly. If they are getting poor electrical connection to the system, they will just not get hot enough in the allotted time. I do not think the timer circuit is "smart" enough to notice whether or not the electrical connections are good or bad as long as they are not open or short. I would check the connections, clean them and put them back carefully, then check the performance again. If it is still bad, maybe you have some bad plugs, in which case, take them out and check them with an Ohmmeter. Jim

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