350SL Starter woes

[Dyod76]

Hello All

So the joy of more than tinkering with my just purchased 350SL has commenced sooner than I expected or wanted.

My first planned job this weekend was to change the oil. My car had been warehoused for at least five years and the last known service was about five years and 10000 miles before that. Yes, the oil was very black. Changed the oil and and filter - all good so far.

Decided to crank the engine over for about ten seconds with the coil HT lead disconnected to get the oil circulating and some pressure up before actually starting the engine. New problem! When I released the ignition key the engine kept cranking until I disconnected the battery. This was in spite of turning the key completely off. I reconnected the battery - it didn't re-crank. So I cranked the engine over again - and yes, when I released the ignition key the starter kept cranking.

Decided the problem was more likely to be the starter/solenoid unit that the ignition switch. I'm frightened of masses of wires. So I removed the starter - and can confirm that this is an absolutely terrible job first time. Wasn't too sure what the right process was but disconnected some of the steering linkage and removed some heat shields to create just enough space. Left the exhaust intact.

With the starter on the bench I sprayed the solenoid piston with a light coat of silicone - and it moved back and forth freely. So I then bench tested the starter and everything appeared to work properly - solenoid activated, bendix drive was smooth/positive and the starter turned rapidly. All good I thought.

Refitted the starter and tested it with the HT lead still disconnected. For the first few attempts, the starter just spun without engaging the ring gear. On the 4th attempt the starter engaged but when I released the ignition key the starter kept cranking like before. It only stopped when I disconnected the battery.

I wanted to eliminate the ignition switch as a cause. So I connected a test wire to the ignition switch terminal on the starter and the positive on the battery. The battery was properly connected. The starter cranked immediately but when I disconnected my test wire it still kept cranking. Again it did so until I disconnected the battery completely.

So in spite of the starter testing well on my workbench, I believe that there's still something wrong with it. If I've got/done something wrong on my process/assumption, I'd be delighted to be told by someone with more expertise than me please. I'll take the starter this week to an auto electrical shop for proper testing analysis. If there's a recommended workshop please tell me. I live in Bowral and work in Chullora.

[Bartman4800]

Yes it does sound you did your fault finding correctly.

The only thing I can think off, it that the solenoid contacts fuse under heavy current.
If you take the starter back off and take the solenoid apart, have a good look at the contacts.

File them clean with a nail file. This should do the trick. The silicone spray sounds like a proper solution.

[CraigB]

Yep good analysis.... I was reading thinking 'I would do this next' ... and then read you did it!

If it helps describing this - and you probably know, the ignition switch energises the solenoid that pulls the big contacts in it that lets the current run from the battery to the starter and at the same time operates a lever that pushes the starter gear into the ring gear. So to me releasing the power from the solenoid, something is physically holding it in place, which is keeping those big contacts connected that are sending power to the starter. Sounds like the solenoid itself is free. Did the ring gear look ok? Put some lube on the gear perhaps. When a motor fires the kick would help release the contact pressure of the starter gear to the ring gear, which your not getting. I wonder if that's dry there is enough friction of it keeping to turn to keep it in place? What about pulling the starter to one side, away from the motor, so it gives a fraction of a mm more clearance? What about the clutch bit on the starter gear - can't see how that could cause the problem but something to check while out.

It sounds odd though and I am just chucking around some ideas. Agree with Bart re contacts too.

[Christo C]

My money is on a sticking solenoid.

[tsharkey]

I had one on the diesel that would keep running after the car started (but not be engaged on the flywheel) .... A thwack with the wheel brace on the solenoid would stop it.... Sticking solenoid

[Dyod76]

Thank you All.

I've had the starter checked. The diagnosis was a faulty solenoid - and the simplest solution was a complete reconditioned unit. Yes I could have had a go at servicing it myself but sometimes I just like easy.

The replacement starter came with the original Mercedes style solenoid that has screw-in connections for the two low-amperage wires. The faulty solenoid was non-genuine and had simple spade connectors. So I cut off the old spades and replaced them with new but original eyelet types.

However I just couldn't find a way to attach those eyelet connectors to the starter motor once it was installed. I couldn't see the screws for the eyelets - let alone get a screwdriver to secure them. So out came the starter again (I'm getting good at this) - and I made a two new short wires with eyelet connectors. And then joined those wires to the existing low-amperage wires.

And before I put the all the other parts back I removed to enable to get the starter out originally, I cranked the engine as a test. Everything worked perfectly - and the car started.

Victories are worth celebrating. Now just have to replace all the heat shields, steering bits and everything else, and then the job will be finished.

There's no doubt that the removal and replacement of my 350SL's starter has been the most difficult starter job on any car I've ever experienced. Thank goodness it appears that I've almost got the job done.

[CraigB]

Thanks for following up and that's good to know it matched what was thought. I think the difficulty of the job justifies a nice fully reconditioned unit - will last you for years now without going through that trauma again.

[Bartman4800]

Can someone enlighten me why this starter would have 2 low current wires?
Normally the starter has earth through the body and only needs 1 wire for power....

[CraigB]

Should look it up... but is one to do with the ignition and the resistors - under cranking that changes?