Burning distrbutor rotor

Servus alle Freunde,

two years ago, my SL would not start ( right at the end of the MOT test, after it had passed). I found the problem to be a burned-out rotor arm. I put on a new one, and she fired up right away. I have just removed the distributor ( to do the timing chain and tensioners) and have noticed the 'new' rotor arm is showing scorching. it has only done 2000 miles. Is this a common problem, or have i just managed to buy a crap rotor arm at the motor factors ( it was Intermotor, made in italy, i was desperate, and it was the only place nearby that had one).
I have another new one as a spare, but I think i might try to find a Bosch or Beru.

Have any of you had this trouble?

Hi,
my advice : Buy Distributor Cap and Rotor only from Bosch...

Greets
Obelix

Hi Myros,

I would also advice to buy Bosch distributor cap and rotor. However 2000 mls sounds very low and I advice you to have a check of secondary ignition voltage on an ignition oscilloscope. Check for overvoltage and asymmetrical peaks.

BTW Myros: I have moved this topic to the English forum and hope you don't mind. It is new and I want to promote it.

Thanks gentlemen,

the cap and 1st rotor arm are Beru, as I put new leads, cap and rotor and plugs on the car when i bought it. The cap is in A1 condition.

Volker, not having an oscilloscope ( I have a boroscope), would it be easier if I just replaced the ignition coil, as that has to be the culprit for over voltage and spiking,doesn't it? Or is there something else hiding in the mix that could cause this kind of trouble.

Regards,

Myros

Hi,
even Beru is not a proper replacement for the original Bosch Cap.

Greets
Obelix

Hi Myros,

I again agree with Obelix. Beru has good ignition cables. But I would also choose Bosch for distributor components.

What you can do: Check the presence of correct ignition coil and pre-resistors. Swapping components without ptoper measurement is no solution. It will take you time to see whether that solves your problem.

Thanks again Gentlemen,

I will test my coil with my multimeter, probably tomorrow evening. I have the figures somewhere for resistance between the various terminals.

Thanks,

Myros

Okay,

so I have ordered a Bosch Distributor cap, and two Bosch rotor arms ( it never hurts to have a spare), and have just tested the coil in situ.
The resistance between 1 and 15 varied slightly between 0.6 and 0.7 ohms ( specification says between 0.3 and 0.6 ohms), so the primary winding looks ok.
resistance between 15 and 4 is 8.73 k-ohms ( spec says between 6-15 k-ohms). ( all tests ignition off)
So is that a coil in good shape ? It is at least 12 years old, as i have never changed it, and it is probably much older than that.

I have also examined the Beru ( my mistake, it wasn't an Intermotor) EV11 rotor arm. It is burning the end where the spark jumps to the distributor cap. The epoxy is blackening before the brass tip, on the top, and the resin is burning under the brass tip. I haven't tested it for resistance. Do any of you know what the figure should be for a 'good' rotor?

Thanks in advance

Myros

Hi Myros,

the rotor should have 5 kOhm resistance. A picture of the old one would explain better, how it burnt. Resistance values of coild are okay but it might still e.g. have a shortcut when it warms up.

M117 as well as M119 are well known with this Problem

use an other sparepart than bosch Problems easyly accure

a friend of mine driving the same M119 had real problems
we changed the distributer fingers and cabs (dont forget the rubber ring) his beru parts have been 2 Jears old
on both blocks and the Problem moved from his car to mine
he bought 2 new fingers and caps as well as the rubber ring and all was ok

Greetings
Peter

maybe a reason is we just change the front of the cab and not as well the rear maybe front and rear should be the same manufacturer

Thanks again gentlemen,

Wudi, when you say 'rubber ring' do you mean the big black platic moisture cap which fits over the brown distributor cap, or is there another thing I've missed in there . i haven't paid the distributor that much attention, apart from noticing the rotor burning.

here are some photos. The cap is not burned, but you can see the deposits on it from the rotor. the rotor is scorching very nicely. Th eone which burned out two years ago was so 'roasted', it fell apart in my hands when i tried to remove it ( gently).

Hi Myros,
looked at the pictures...
In my opinion the cap was mounted false, turned by 180°.
There is no wear to be seen on rim of the cap directly to the lower right screw - but at the upper left screw.
The notch (near upper screw) has to fit into the ward of the distributor.

But the cap looks quite o.k - I would just turn it interchanging the leads correctly, clean it from the white 'falldown' of the burning rotor and reinstall it.
You keep the new Bosch one as a spare...

Regards
Norbert

Hi Myros,

that rotor looks really bad! It looks like the spark did not come from the metal but from inside. Very strange.

Thanks Gentlemen,,

I'm pretty sure the cap fits properly on the distributor, I did pay a lot of attention back then 12 years ago when I swapped it for the old Bosch one. Plug lead 1 is where the notch on the distributor base for TDC ( OTP) is, and when I rotated the crank to TDC, I pulled off the cap, and the rotor was at plug lead 1.
I have just measured the resistance of the rotor, it is 4.89 k-ohm across the clean bits of brass from centre to edge. If I measure it against the leading portion of the rotor, the edge which is not 'clean', it varies from 4.89 right up to 18k-ohm. the highest resistance is at the the very leading edge, where the spark would naturally jump from 1st. So is it burning out because it is faulty, or is it burning out because it is being overloaded?
The '1st' burned rotor arm lasted nearly 15,000 miles, before finally giving up. I check under there about once a year, so the fault must have developed in early 2014, and taken about 2-3000 miles to self-destruct in Sept 2015 ( looking at my mileage from 2013 to 2015). The 'new' rotor is already burning up at 2,000 miles, so either all Beru are faulty to some degree, or something else is funny in the ignition system. or both..
The answer will have to wait until I have done the chain and tensioners and reassembled everything.

Hi Myros,

well current alwys searches the path of lowest resistance. So if you face 18 kOhm, it is no wonder that the spark finds a side path.

Myros i just know it about an 1992 R129 500sl
there has been an o ring seal between the 2 cabs one is behind and the other is the one of the picture you made

check it whether there is such a ring or not
if than think about to change it
wish you sucsess
Peter

MyrosII wrote: Thanks Gentlemen,,

I'm pretty sure the cap fits properly on the distributor, I did pay a lot of attention back then 12 years ago when I swapped it for the old Bosch one.
...

... and I am pretty sure your cap wasn't mounted properly.
I took your picture and marked the indicators.

You have a chance of 50% to mount the new cap properly when you don't take care about the notch I marked.

Regards
Norbert

Norbert, you are makng me paranioac now.

This how it was set up when I dismantled it. Looking at the distributor, standing in front of the car, when the crank is TDC ( OTP), the rotor arm was pointing to approximately 1400hrs, if you think of the distributor as a clock face, It was right near the small TDC mark on the distributor body casing.
The black cap has a No1 marking, and that corresponds to the leadat 1400hrs that goes to the back cylinder ( nearest the UK driver) on the left cylinder bank ( still looking at the car from the front). I'm pretty sure the cap would only sit on the distributor in one orientation.
Is it possible to mismount it by 180 degrees without using force? ( which I never do, especially not on a distributor cap costing 70E). The Beru cap did not come with a black moisture cap, so i dismounted the one from the old Bosch cap, and that fitted fine.
When I reassemble the distributor, I will use the new cap i have ordered, and dry-fit everything and make sure it is correctly oriented. I have all the leads marked and numbered. I will even take comparison photos of the new and old cap side by side, so you can all stop me before i get it arse about face a second time.

Myros

Hi Myros,

calm down,no need to become paranoiac now.

You can mount such a cap only in 1 correct position. If you twist it, one of the hooks will not lock. In that case, it can move during operation. You should always check that it cannnot move and that both hooks sit tight.

Dr Volker,

I have cancelled my psychiatrist's appointment...

Anyone can make a mistake, I know, I make lots of them all the time, but when it comes to 8 ignition leads, I rely on empirical methods and double-checking everything and marking everything. Not only do i have tape on the plug end of the leads, with written lead numbers ( which have now faded, does nothing anymore), but i also have them colour coded from front to back, with little cable ties. Red, blue, green, yellow, on one bank, and double red, blue, green, yellow on the other bank, and chased through to the cap end and marked there as well. I did all that off the car, with my brother-in-law making sure I got them right. Schwieger-power.

The cap eases into position nicely, and I never have to force it on, The screws always catch nicely. Nice of Bosch to put numbers on the black cap.

I have now received my replacement Bosch rotor arms and cap, and I have to say the difference in product from the Beru is quite signifcant. The Bosch parts have a glossy finish/coating, and the rotor looks much more substantial. the mounting shaft is much thicker than the Beru. So much so, that when I compared the Bosch side by side with the Beru, I thought the mounting aperture on the Bosch was too small. I checked it with a ruler, and it is exactly the same as the Beru, but the thickness of the shaft makes it look smaller. Like one of those psycholgist's optical illusion drawings.
I will photgraph it when I come to remounting the distributor. Yesterday evening was spent replacing the cylinder block-thermostat coolant hose, and then swearing at the thermostat and housing and trying not to bend it when remounting. ( which I didn't). and then doing the same swearing at the air-slide which I had to remove to get at the coolant hose.
I would recommend an inspection of these hard to get at hoses to all of you. As I already have all the big hoses off and the coolant drained, it would be daft for me not to. I was shocked to see how much oxidation there was on the cylinder block flange . I used my dremel and a brass brush attachment to clean it up. I also reassembled with a smear of silicone grease on the flanges and just inside the hose. and of course new hose clips, mounted so i can undo them easily if needed. It all helps when reassembling., and will hopefully prevent any more oxidation or build up of deposits there. I also used a silicon hose for the replacement. I love new technology...

Guten Feiertagabned freunde,

the quest for truth goes on and on.. I have managed to replace the timing chain and upper guides okay, in fact I am very pleased with the work. My wife saved me a fortune after I manged to let the end of the old chain slip back down the timing cover, she helped me fish it out. Unfortunately, everything else is going to the devil since I recommissioned the moror on Saturday.

on the topic of rotor arms, i fitted my 'spare' Beru cap and rotor arm, just to static time the ignition and in case anything went wrong. The engine has been run for less than an hour, long enough to warm it up for an oil change, and to try the dynamic timing with a strobe lamp.

the picture of this brand new rotor arm says it all. less than an hour's work, less than 100,000 revs, less than a 50km drive. It's burning out.
I have taken it and the Beru cap off, and fitted my new Bosch ones. Ignition timing and rough running are preventing me from driving the car anywhere, but this can't be good. Any ideas gentlemen?

Hi Myros,

when a new rotor arm and distributor cap burns that quickly, I do not believe in just bad quality of Beru parts. I would check pre power resistors and their wiring before ignition module and ignition coil.

Dr Volker, I think I agree with you completely. The 1st beru cap and rotor I fitted lasted 10 years without trouble. I have always liked their spark plugs. I will continue the detective work as much as i can and keep you posted. the engine is running like a schwein and i can't set the timing. something must be out of whack from the chain job ( the chain installation was eventually perfect)

Dr Volker und freunde, here is a picture of the top of the rotor arm. There is a distinct pattern of 8 marks, presumably corresponding to the position of the rotor when firing for each sparking plug. I haven't seen anything like this before.

Hi Myros,

it looks as if you are missing the carbon in the centre of the distributor cap. There is a small spring and pen in the centre pressing on the rotor arm.

Check please that this is present.

it was certainly there when I took it out of the box on Saturday. I will look again later on. Thanks.

Dr Volker, the ( new) distributor cap carbon electrode seems to have a bit of a hotspot. You can make it out in this photo if you zoom in.

Hi Myros,

to be honest, I do not recognize something on that photo. However, it seems you still have higher voltages than normal. What could be the reason?

1. Missing or wrongly connected power resistor. They should be 0.6 and 0.4 Ohm to coil.
2. Wrong ignition coil
3. Wrong spark plugs
4. Gap in secondary cable (4) from ignition coil to distributor.

Why could a gap create such a problem? It would take higher voltage to make the spark jump over. And for sure your burnt rotor arm looks like too high voltage.

Dr Volker, thanks for the reply. It looks like I will have to throw some parts at the problem. i will start with the vacuum hoses and pipes, as they might have reacted badly to all the dismantling and remantling going on when I did the timing chain and guides. the car is running really really rough, and I can't get the dynamic timing to set. So vacuum first, then electrics, although it is probable that the overvoltage would have to be from the failure of the ballast resistor. If the coil is meant to run on 9volts most of the time, but is running on the full start-up 12 volts, then that's a lot more volts than the rotor arm is meant to have continuously. It has to be unlikely that the ignition module causes over voltage, all it does is give signals to fire.