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How do ignition coils fail?

Started by raueda1, 29 September 2023, 01:23 PM

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,

-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

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.
1975 280S - Australian delivered

daantjie

Coil is cheap.  I would replace on principle if you're doing all the other components.
Daniel
1977 450 SEL 6.9 - Astralsilber

raueda1

Quote from: daantjie on 30 September 2023, 09:50 AMCoil 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,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

andrewk

#4
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.


1979 450 SEL 6.9 (#5532) - silbergr√ľn metallic

raueda1

Quote from: andrewk on 03 October 2023, 01:06 AMMy 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,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

orientrot

Quote from: raueda1 on 03 October 2023, 07:54 AMOMG.  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
75 450SEL
80 300SD- cancelled project
83 300D- 5-speed OM617
84 300D- AMG 5.4L N/A M113 6 speed
92 300D- 722.6 OM603 3.5
93 190E 2.6 Sportline LE x2- 6-speed M111 turbo, 3.0 M103
02 C320 Wagon
03 Sprinter 2500
06 E500 4Matic Wagon- AMG 5.4L NA M113

raueda1

Quote from: orientrot on 03 October 2023, 01:16 PM
Quote from: raueda1 on 03 October 2023, 07:54 AMOMG.  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,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0