[W11x] Eating Coils - 66 250SE

[justfalcon]

I have a 66 250SE that I've started to do some intensive overhaul work on. <50k original miles, 3 owner, original purchased in Hamburg, Germany, much paperwork included. Sunroof even works perfectly

Long story short - sat it an airplane hanger for ~10yrs but prior to that has been maintained well and had a lot of large ticket items tended to. I drove the car all the way from Wisconsin to Indianapolis on pickup - and have put ~2500mi since I got it back in March this year.  Haven't started to have issues until I swapped out wires and coils (well - this specific issue).

I keep running into the same issue though - I'm eating through coils.

On the initial drive back it turns out the alternator was shot and I replaced it with a 50a one w/ new relay. The engine harness and grounds have been completely replaced w/ newer wires. Optima Yellowtop installed. New spark plug wires, plugs, cap, rotor have been installed. 1.4 ohm ballast replaced to support the "red" bosch high coil.  Cables have been twice replaced. Have swapped plugs twice as well.

Car runs like a hot damn otherwise though - I've done a lot of other maintenance on it and am planning on pulling the engine to go for a full rebuild/paint over winter.

It seems to happen only when hot/warm, but the car will randomly just die. Now - I know this isn't necessarily a coil/spark issue and I've narrowed this particular problem to I think the fuel damper gasket needing replaced. What I will say is that I'm also somewhat determined that the injection pump isn't set 100% correctly, though I don't think that would relate to the coil issue I'm having.

But I've completely melted 3 different coils now.  I've checked dwell and timing, etc and all seems to be within spec.

Am I missing something here? I can provide any specific info and pics if needed. I'm kind of at a loss at this point...

Edit: To clarify - I was driving it last Saturday ~3mi into the drive and the car just flat-out died. It hasn't done this in awhile and the temp outside was not high at all and it wasn't being driven hard so I don't think the fuel damper issue popped up here - I've only noticed that when it's realllly hot and I've been driving 2+ hrs straight. I've driven this from Michigan to Cincinnati and back multiple times this year @ 90F+ temps and that's where it'll die from the fuel damper issue I believe.

[lpeterssen]

Dear Friend

I also own many w111, but none presents that failure.

As you say your coils are operating too hot, make a measurement of the voltage coming to the coil after the 1.4 ohms resistance.  Look on the specs especially for that coil.  Read the electrical technical manual at this forum for MB pagodas, the section related to ignition and check whether the voltage your coil is receiving is on that specific range.

On the other side you also gave a clue on your explanation. You changed your alternator. Have you measured the voltage at your battery with the car running at Iddle and at 3,000 RPM. It should not exceed 14.1 volts. If value is too low  (Less than 13V) or too high (More than 14.1V) that may explain your coils burning out.

Also as you changed your alternator. Did you increase the cable square section in the segment alternator/starter motor T30 terminal? Originally is 4mm2. Should be risen to at lest 6mm2 to handle increased alternator capacity.  You can keep the cable section going to main light switch at the cabin, but cable going to starter T30 which communicates directly to battery should be changed to allow the new overflow current to go somewhere (battery). Are you sure that your voltage regulator (I guess is external) it’s doing is job properly?

These were the ideas that came to my mind.

Best regards
L.peterssen

[Cees Klumper]

For what it's worth - my car last summer was erratically dying one second to the next. Tried several causes and in the end it turned out to be a bad contact of one of the two wires that connect to the coil - a spade connector that I had not crimped on properly onto its wire. It seemed ok, but was not and although the car would run fine most of the time, sometimes the connection would just not manage and the car would instantly die. Wait a couple of minutes to cool down a bit, and it would start right up. I don't know if there could be a loose connection someplace in your ignition wiring like I had, but the symptoms sure are similar. It might even influence the health of your coil, that I don't know. My coil seemed ok after my issue.

[justfalcon]

Dear Friend

I also own many w111, but none. Presents that failure.

As you say your coils are operating too hot, make a measurement of the voltage coming to the coil after the 1.4 ohms resistance.  Look on the specs especially for that coil.  Read the electrical technical manual at this forum for MB pagodas, the section related to ignition and check whether the voltage your coil is receiving is on that specific range.

On the other side you also gave a clue on your explanation. You changed your alternator. Have you measured the voltage at your battery with the car running at Iddle and at 3,000 RPM. It should not exceed 14.1 volts. If value is too low  (Less than 13V) or too high (More than 14.1V) that may explain your coils burning out.

Also as you changed your alternator. Did you increase the cable square section in the segment alternator/starter motor T30 terminal? Originally is 4mm2. Should be risen to at lest 6mm2 to handle increased alternator capacity.  You can keep the cable section going to main light switch at the cabin, but cable going to starter T30 which communicates directly to battery should be changed to allow the new overflow current to go somewhere (battery). Are you sure that your voltage regulator (I guess is external) it’s doing is job properly?

These were the ideas that came to my mind.

Best regards
L.peterssen

I'll double check to see if it's not going over 14 - the wires have been replaced with much beefier wire and a new voltage regulator - albeit not a bosch one or a "model-specific" one - so I'll double check that too.

[Benz Dr.]

That coil uses a 1.8 ohm ballast resistor with a red strap which is what I see in your picture. Overvoltage is a good clue.

[justfalcon]

That coil uses a 1.8 ohm ballast resistor with a red strap which is what I see in your picture. Overvoltage is a good clue.

Sorry - that was a typo in the original post. It is a brand new Bosch 1.8ohm ballast.