Engine Won't Turn Over When It Sits Overnight

Do not assume it is just a bad battery! Diagnose first...

Customer writes: "I have a MB 1984 300 TD. The problem is the engine won't turn over. The battery is full, glow plug light on, blade fuse ok. Basically when engine sits for half day or so - it tries to start but doesn't (turn key and it does a weak rev of engine - not enough to start car). The only way to start is with tow truck jumpstart (after jump start - runs ok until I park it and let it sit for as little as half a day - then fails to start). Can you let me know what is wrong."

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Problem & Solution

Common among these chassis:

Classic Sedans
W108 W109
W110 W111 W112 Sedan
W111 W112 Coupe

It is almost impossible to trouble shoot electrical problems via email. If you are not familiar with basic automotive electrical testing then we highly recommend you watch Kent's training video on automotive electrical troubleshooting. Starting issues like the one described about could be due to a number of problems

  • Poor charging circuit (alternator or voltage regulator)
  • Weak or dying battery
  • Starter motor weak
  • Bad ground connection from battery to car or engine to car
  • Some electrical component in your car draining the battery overnight

Just because a solid jump causes the car to start it does not always mean it is a bad battery. Poor connections could also be the fault only to be masked when extra voltage is supplied.  Here was Kent's recommendation of what to do first.

"First you want to make sure the major components of your charging / starting system are good -alternator, battery and starter motor. Take you car to a local auto parts store that can test this for you in the parking lot. Call around first. Many will do that for you for free. In most cases they will be able to isolate the problem with their portable test equipment."