300sl-24 (1991 R129) not turning over/starting but Battery ok

[Unsound Methods]

evenin'. Hope everyone is surviving the clement weather....

I'm assuming i have an electrickery issue so assuming i'm posting the right section... On turning the key in the ignition the dash lights all come on as usual, the fan and stereo all work fine but the car doesnt even turn over. This happens when the car is both hot or cold. It doesnt happen all the time, but when it does, if i keep trying it eventually turns over and fires up.

As I type i wonder if the starter motor is on the way out but the research i've done so far is suggesting a fuel relay issue in the MAS control unit. Does anyone have any other suggestions as to what might be stopping the car turning over?

Not sure if these 2 issues are related to the above but ....:
• my full beams have stopped working (no blue light on the dash also)
• on starting from cold and setting off the car will stall as i slow down - usually at the first junction i stop at, as the revs drop - but it starts up straight away on turning the key. It doesnt stall after a few more minutes driving.

cheers, Dani

 

[LostKiwi]

Could be the ignition switch (mine occasionally does the same as yours and it's definitely the ignition switch).
I'm pretty sure the fuel pump relay won't cause it not to turn over.
When you turn on the ignition the fuel pump should run for a few seconds. You should be able to hear it from the back of the car.
Could also be the starter relay.

 

[Unsound Methods]

Could be the ignition switch (mine occasionally does the same as yours and it's definitely the ignition switch).
I'm pretty sure the fuel pump relay won't cause it not to turn over.
When you turn on the ignition the fuel pump should run for a few seconds. You should be able to hear it from the back of the car.
Could also be the starter relay.

Thanks LK. Does the ignition switch start the fuel pump?

 

[rayhennig]

I've recently had a stalling issue under braking and at low speed. Often in the same place! Same engine as yours.

I've recently renewed all the HT components from the coil to the spark plugs, including cap, rotor and leads. All MB original parts.

So far, no stalling.

My message is for you to ensure that the HT side is 100% perfect. These engines seem to demand it. It won't be cheap but you have a 300SL-24 so money is no object!!!!

Your starting problem is a separate issue, I think and the others on here will comment further.

I have never had a starting problem but I have had other problems, notably blowing up ignition modules. We suspected a dodgy coil or an earthing issue but never fully identified the cause. 3 modules were blown in the space of a year; the last one in June 2020. Earths and other connections were cleaned and the starter motor and alternator were replaced with exchange units from Bosch. So far, all well, no blown module 2 years later. Fingers crossed.

I suggest that you have as many earth and other connections checked and cleaned. And it may be worth considering the starter motor and alternator. After all, they may be 30 years old.

And the ignition switch and barrel would be a good idea too.

Yes, you will spend some cash but, if you intend keeping this car, it should give it a new lease of life.

Oh, and on the subject of cash, I recommend finding a replacement ignition module. eBay cowboys are asking 700-1500 Euro for used or repaired ones. I've never heard of anyone who's had a repaired one. I got a replacement used one for 250 Euro but then I found that MB has a stock of New Old Stock modules. My NOS one was 1100 Euro from MB Bochum. I hear that the UK price was around 1000 quid. In France, they were asking for 2300 Euro.

You REALLY do not want to blow up an ignition module so remove yours and clean up the surfaces of it and the bulkhead on to which it is bolted. Then apply a thin, even layer of high quality heatsink compound (I got mine from RS Components) and reinstall the module. This measure ensures adequate dissipation of heat from the module.

Good hunting.

RayH

 

[Unsound Methods]

Thanks for your reply Ray. I replaced the HT componants a few years back so i hope they are ok. And youre right that money is no object- as long as i dont spend any....

cleaning some of the elctrical contacts on the ignition module seems like somthing i could do straight away, perhaps also the ignition switch - although theyre not quite as eye wateringly expensive as the module.!

 

[LostKiwi]

Thanks LK. Does the ignition switch start the fuel pump?

Yes. It's the source for all electrical components required to start the car. Fuel, ECU, starter are all controlled either directly or indirectly via the ignition switch.

 

[rayhennig]

cleaning some of the elctrical contacts on the ignition module

And elsewhere to ensure good earthing throughout the car.

My policy is to clean any and all contacts that I can access. Unplug, squirt squirt, plug, unplug, squirt again and reinstall.

In the case of bolted connections, uninstall, abrade with Dremel or similar, clean up, apply copper grease and reinstall.

R

 

[ajlsl600]

dirty spot on armature? Brushes goosed.? Check all connections to from starter.