1985 300d turbo won't start without jump
 

My 85 mercedes 300d turbo is having trouble starting. I put in the keys, dash lights come on, but as soon as I try to start there's a loud click sound and the dash lights go out again. If I immediately take the keys out and try again nothing happens, no dash lights or anything, but if I let it sit for a while then dash lights will come on again. Oddly, it starts fine with a jump, runs normally, but as soon as I turn it off it won't start again. Tested the battery at autozone and it seems to be fine. Any ideas?

Did you have the alternator tested also?

I had to replace both battery and alternator when that happened to me. I bought a 75 amp alternator for a gasser w126. I have had no more issues like that.

To explain the phenomenon - batteries will, to an extent, recharge themselves if left alone. They will NOT generate a full charge. So you're getting enough power to light the lamps but it takes a lot of amps to spin over any engine and especially a Diesel.

Sounds like the battery is OK or it probably wouldn't regenerate itself. I'd check the alternator for sure. Pull it off and take it to your local alternator shop. They'll spin it up on a test rig and evaluate. They can either rebuild yours or they may have a rebuilt sitting on the shelf. My local shop ALWAYS does a great job for me and charged me $100, which is a pretty good deal. They stand behind their work 100%.

Also take a REALLY good look at your cables and the state of your grounds. Remember that EVERYTHING (the chassis, body, and engine) have to get a complete ground loop back to the battery. Many electrical Gremlins can ultimately be traced back to grounding issues.

Dan

I'd check the battery first. Might be due for a change cause may not be holding a charge. If not that, then try looking at start/selinoid. You may want to pay to have it diag'd by a reputable mechanic, and replace the parts yourself. My bet is the battery. The new lithium batteries have a ways to go to be as reliable and long life as the old leaded ones. IMHO.

Also, those symptoms can also be caused by poor connection at the battery terminals. Have you pulled off the cables, cleaned them up?

If the battery light on the dash coming on when the key is turned? If it does, does it go out or stay lite?

CLASSIC symptoms of loose ground braid bolt... it will also do this if the large bolt on the starter solenoid is loose... or of course if the battery cables are bad or loose.

1. check the battery cables, at BOTH ends.

2. follow the large wire from the battery positive, to the starter and make sure it's tight on the solenoid.

3. elevate the car and climb under there and find the braided cable that connects the transmission bellhousing to the body of the car, and make sure the bolts are tight on both ends...

4. the fact that jumper cables connected to the battery get the car started lead me to focus on the battery terminals themselves. I'd pull the cables off the battery, and clean the battery terminals, and the inside of the battery cables...