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1991 300te 4matic
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146 Posts
Discussion Starter #1 (Edited)
The other day one of my headlights burnt out - so i replaced both with new bulbs. Shortly after my daytime running lights started to fail to come on at start up, if I switch off the climate control the daytime running lights come on immediately. Once the daytime running lights are on I can switch on the climate control with no issues. Also when the daytime running lights have not come on I can still switch on the headlights and they all work. After about 1-2 minutes of warming up the daytime running lights come on.
I have no warning lights on dash, I think my battery may be on the way out?
 

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85 300 TDT
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2,260 Posts
Can't say exactly but on my 89 the voltage regulator (location on the back of the alternator and a cheap,e asy fix from underneath) caused electrical gremlins on its way out and finally stopped the car on a busy highway. My two cents'.
 

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W124 400E, W126 500SE
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971 Posts
I think there is a bad ground wire somewhere aroung headlights or daylight running lights. DRLs usually get the "started" signal by observing voltage and when it raises it is assumed that engine is on, but it is not at all perfect or precise...
 

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1995 E320
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2,548 Posts
For me, the sure sign of a worn battery is how the vehicle starts.

Keeping the car in a good tune gives a good baseline. Since I start the car the same way everytime (turn key to ON, hold for a second, then turn to START,) I can tell when the car starts taking a little longer to start.

Since I basically use the same gas stations over and over, it's pretty easy to detect changes in starting.
 

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2013 E350 sedan, 2013 E350 cab, 1993 400E sold,1999 ML320
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2,437 Posts
With the daytime running lights and the blower motor/climate control which are the high draws on the system I would second the regulator and clean grounds on strut towers and by the bulb areas like in the trunk under the carpet.
 

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Registered
1991 300te 4matic
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146 Posts
Discussion Starter #6
I am leaning towards a bad ground somewhere... once or twice since I noticed the problem it has not occurred. I will try to track down the problem. I feel confident it is not the battery.
Thanks for the ideas.
 

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Registered
89 300E; 00 E320
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2,515 Posts
Circuits for headlamps and climate control do have a common ground point

W1 Main ground (behind instrument cluster) Behind left side of instrument cluster 235-5
124 ground w1.GIF

Wiring Diagrams
-Headlamps 124 124-1

ACC Grounds Distribution.....312 313
 
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