Starting issues with 1980 300SD

[Freighttrain]

Hi everybody,

I'm new to this forum and to my car, but I'm really looking forward to learning as much as I can.

I recently bought a 1980 300SD with 165,000 miles. There were a slew of things replaced and it seemed to be working fine, but now it's giving me trouble starting. I turn on the glow plugs, they work fine (all the power instruments work fine when this happens so I highly doubt it's a battery issue), then I turn the key all the way and nothing. No chugging or coughing, nothing but a single, very faint click. This has happened to me twice now.

The first time, I took it to an independent Benz place and they replaced the neutral switch. Now, a few months later, it happened again. I pushed the car down the block (I needed to change parking spots) and an hour later it started up fine.

What is going on here? A loose electrical connection? A stretched starting chain? A bad ignition switch? I'm not sure if this is related, but both times this has happened, the ten or so times I've started it beforehand the car has started faster than usual and then has a few light shaking fits (as though I were tapping on the gas pedal.) A fuel injection issue?

Any help or insight you can provide would be greatly appreciated. Thanks!

 

[s class]

I'm not an expert on diesel specific issues, but some things that come to mind :

a) the starter lockout - this is the switch on the side of the tranny that allows the car to crank only in N or P - but I think this is the item you refer to as the neutral switch, so hopefully that's eliminated.

b) bad earth from engine to chassis - there should be a nice thick copper braid from one of the bellhousing bolts to the chassis - they do eventually corrode or fracture off

c) the solenoid in the starter is bad. When you turn the key to crank the motor, the solenoid has two jobs -
1 - it throws the armature of the starter back to engage the ring gear - this is the click you are hearing
2 - it closes the high current contacts that feed power to the starter itself. These contacts can burn over time resulting in insufficient current getting through to the starter

d) there is insufficient voltage getting through to actuate the solenoid. Even if your battery is good, the light current wiring from the battery to the ignitoin contacts at the ignition switch, and then to the solenoid deteriorate over time and build up resistance. There are two common solutions to this :
1 - replace all the wiring with new wiring of appropriate guage and heat resistance
2 - introduce a relay in the circuit.

The relay solution is common practice amongst back-yard type mechanics - it works, but brings with it the risk of it sticking 'on' and allowing the starter to run with the engine.

Right now my money is on c) or d) above. If you are the handy type, can talk you through some checks to verify which it is. If you are not the gung-ho confident type, then you need a good auto-leccy.

 

[Freighttrain]

Thank you s class for your very thorough response. That really helps me narrow down the possibilities.

I would describe myself as totally enthusiastic about doing this myself, but totally unschooled. If you think you can talk me through it, I will certainly take a shot at it. I'd like to eventually be able to do as much of my own work on the car as possible, so I gotta start somewhere...

 

[BAR]

Your playing around with a very nasty bit of the car electrics here.

The starter motor has an un-fused lead back to the positive battery terminal. So loose positive terminal here does some good welding!

Before you go flying around with screwdrivers and spanners, make sure the battery terminal is removed [at the battery end not the starter].

Oh, I'd be quite confident in saying that it's your starter that needs replacing, or if you can find a good auto electrician, they can supply a re-wound motor: just make sure all the internal componets are rebuilt as well. You should get a warranty on this.

 

[John S]

Try moving the gear lever backward and forward through the park and neutral positions and see if you get a start. The linkage may have moved or the switches may be altered, so this movement may pick up maladjustment. If it is no different check the battery voltage and the earth.