How do ignition coils fail?

[raueda1]

I'm finally doing an ignition tuneup - wires, plugs, cap and rotor.  To be clear, in general the car runs well.  However, the idle is sometimes lumpy, other times not, sometimes missing a little, other times not.  My snap judgement is always to blame K-jet, WUR, etc etc but it's all well set up.  Meanwhile, I know I don't have exactly the right plugs.  At the time I got resistor plugs.  Wrong, I know, but there was some reason and they were an improvement over what was there so I put that issue on the back burner.  Probably not right wires either, so time to do it all properly. 

But what about the coil?  I've always assumed that coils are a kind of "binary" item, either good or no good.  But is this strictly true?  Can an old coil be compromised somehow, but not so much that car doesn't run at all?  Or, for example, be "weak" at idle but OK as engine revs?  If so then I'd replace on principal.  Thanks and cheers,

[Nabstud]

I had a coil that started failing when it started to warm up. Cold start and the first few minutes were smooth, then a slight miss progressively getting worse until it was undriveable. Same again on the next cold start.
But normally I agree that they either work or don't.

[daantjie]

Coil is cheap.  I would replace on principle if you're doing all the other components.

[raueda1]

Coil is cheap.  I would replace on principle if you're doing all the other components.

That's my thinking too.  I did a little searching.  Actually not so cheap from what I've seen, at least for a Bosch coil and not a generic Chinese copy. 

Anyway, in searching I assume that there's nothing special about the 6.9 coil and that they were shared over a number of MB models and maybe chassis, such as W123 &/or W126.  So, in searching, look for a Bosch coil for V8 models of the era?  The other question is about age.  It's not clear to me how an NOS unit would deteriorate in storage but who knows? Cheers,

[andrewk]

My coil failed but not in the way most do - the terminal plastic and threaded part actually disintegrated. The car would crank and fire briefly, but wouldn't stay running. The coil output would drop after a few ignition cycles.

Replaced coil with genuine, new Bosch unit and car fired into life. When undoing the old one, the terminal failed completely.

[raueda1]

My coil failed but not in the way most do - the terminal plastic and threaded part actually disintegrated. The car would crank and fire briefly, but wouldn't stay running. The coil output would drop after a few ignition cycles.

Replaced coil with genuine, new Bosch unit and car fired into life. When undoing the old one, the terminal failed completely.

OMG.  Good reason for others to replace these ancient parts "on principal."  But it's not so easy.  The OE part is 000-158-28-03 as far as I can tell.  It seems that this was superseded by 022 112 22001.  These coils were specific to "transistorized" ignitions in MB but also many other cars of the era.  Despite that, both seem to be very hard to find and turn up as out-of-stock or NLA on my usual car parts websites.  In fairness, I haven't tried Classic Center yet, so maybe they have the only ones left.  I did find a new one on ebay, but very expensive.  Probably there are generic equivalents but I prefer OE so I got it anyway.  Cheers,

[orientrot]

OMG.  Good reason for others to replace these ancient parts "on principal."  But it's not so easy.  The OE part is 000-158-28-03 as far as I can tell.  It seems that this was superseded by 022 112 22001.  These coils were specific to "transistorized" ignitions in MB but also many other cars of the era.  Despite that, both seem to be very hard to find and turn up as out-of-stock or NLA on my usual car parts websites.  In fairness, I haven't tried Classic Center yet, so maybe they have the only ones left.  I did find a new one on ebay, but very expensive.  Probably there are generic equivalents but I prefer OE so I got it anyway.  Cheers,

Deloreans use the same ignition coil afaik and Delorean Texas claims to have original Bosch ones in stock: https://store.delorean.com/101052-ignition-coil-w-bracket.html

[raueda1]

OMG.  Good reason for others to replace these ancient parts "on principal."  But it's not so easy.  The OE part is 000-158-28-03 as far as I can tell.  It seems that this was superseded by 022 112 22001.  These coils were specific to "transistorized" ignitions in MB but also many other cars of the era.  Despite that, both seem to be very hard to find and turn up as out-of-stock or NLA on my usual car parts websites.  In fairness, I haven't tried Classic Center yet, so maybe they have the only ones left.  I did find a new one on ebay, but very expensive.  Probably there are generic equivalents but I prefer OE so I got it anyway.  Cheers,

Deloreans use the same ignition coil afaik and Delorean Texas claims to have original Bosch ones in stock: https://store.delorean.com/101052-ignition-coil-w-bracket.html

Thanks.  Who knew?I my current attempt fails I'll give it  try.  Cheers,