1974 450sl won't turn over (repeat from tech help)

[phango13]

My apologies if some of this sounds daft, I'm only a few months into owning this baby, and my mechanical DIY skills are fair to midling have worked primarily on motorcycles in the past...

So here's the deal--the starter motor runs and runs, but no sign of life from the engine...Engine was cold at the time, has been running swell otherwise...First time this has happened...

Been having some electrical difficulties (tail lites out, no dash lites, no clock) and have a broken antenna that runs the battery dead if I put a fuse in the #2 slot (so i've been leaving thet fuse pulled until i can fix the antenna)...but all other fuses are new as is the battery...again, starter motor runs and runs so i'm guessing there's plenty of juice...

One other wierd thing--over the past few days, I think always when the engine is warm, the starter motor would come on only after the passenger seatbelt was engaged--even had to dis-engage and re-engage it once to get a start (again, battery's been fine thru-out)

But I digress--what I'd like to do is some basic troubleshooting before I haul off and have her towed to my mechanic...Unfortuntely, she's stuck downtown so I'm trying to compile as much info as I can befire I head down there tomorrow and give it a go...

So far, I've asssembled the following plan of attack:

1. neutral switch--try starting in netral, shift from neutral to park

2. fire--check pulgs/cap/wires, pull wire to check for spark

3. fuel--spray starter fluid into intake manifold and if that works, work backwards up the fule line from there...

Again, I'm guessing that the solenid is okay because starter motor runs??

Any other suggestions??

[300SDog]

Just a wild guess since I've never owned an electronic fuel injected Benz, but take some electrical parts cleaner plus some steel wool with you tomorrow... then unplug and clean various sensor and brainbox connections in the engine bay.

[Tomguy]

First thing you want to do is check for spark - forget the neutral safety switch idea, that'd prevent it from cranking at all.

If it's getting a spark TO the cylinders (I usually check the #5 plug, that's the front driver's cyl) then it's a gas issue, if not, check to see if spark is going into the distributor. If not, check to see if the coil is recieving voltage at all. It's possibly a loose/corroded connection at the coil or ballast resistors.

Clean the points out with a piece of notecard or other stiff PAPER, NOT sandpaper. Make sure they open and close properly.

If it's a fuel issue, check the fuel pump relay - you should hear the pump hum when you first turn the key to the run position (prior to cranking) - if not, there's your problem. If it does hum, check for pressure at the fuel lines. If that checks out good, pulse 12v across the cold start valve in the engine compartment (1/2 sec is more than enough) - then crank it and see if any sign of life comes forth.

Edit: NEVER leave the car sitting with the key in the run position but not running - if the points happen to be closed, this is a sure-fire way to burn out a ballast resistor or the transistor box.

[Strife]

I really don't think using steel wool is a good idea. Pieces come off and can develop shorts which might be hard to see. If you need to clean contacts, using a PENCIL eraser is fairly safe on silver or gold plated contacts. If you are desparate or if you are dealing with unplated copper, a pen eraser (more abrasive) is OK.

[phango13]

went back with a buddy so i could listen better and i think its the starter gear not engaging...bummer...getting underneath the car is not an option for me at the moment so it looks like its a tow to the shop...

but i'm filing these checklists for further reference!!

thanx so much fro the help, beth

[300SDog]

Starter gear not engaging?? That sounds like the solenoid sticking.... just smack the solenoid with a 2x4 or rubber hammer and then it will grab.