sratner

A Blue Coil Mystery

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I could use a little help figuring this one out-

I have a bosch blue coil that's been in the car for at least 30 years ('74 2002, owned the car since '79) so instead of getting dumped on the side of the road i decided to pick up a spare.  i bought a new bosch blue (the correct coil) as a replacement and then i replaced the old blue coil which works perfectly with the new one.  the car drives fine until it gets hot and then i experience bucking, backfiring and hesitation. the whole experience doesn't last very long and then the car drives perfectly. since the car was driving fine before i changed the coil i decided to reinstall the original blue coil.  the car ran perfect.  i figured i got a bad coil so i returned and exchanged that coil for a new one.  installed it today and the bucking, backfiring and hesitation was back. what's going on?  anyone have a theory on what's going on? Thanks.  

 

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For starters it looks like the resistor wire is still there the clearish wire that looks kinda like a speaker wire and the blue coil has a internal resistor. Then theres the fact that Bosch has gone from one of the best manufacturers anywhere to their products now are mostly made off shore and are absolute shit.

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This is interesting. I’m experiencing the same issue. I also have a blue Bosch coil. Keep us updated, I’d like to know how you fixed it

 

Also is that a spark plug wire case? That’s nice where’d you find that?

 

- Reyes

 

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It does look like speaker wire and from the research I’ve done I think it might be a resistor wire.  But why would this set up work fine with the old blue coil? Did the blue coil from 35 years ago have a resistor built in? 

 

Yes Reyes, that’s a spark plug tube.  I’ve had it for years, not sure where I got it. It really helps clean up the engine compartment.  

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Based on results it sounds like too much resistance. Can you test the 2 coils - even though they're both blue - and see if they're different.

 

And don't - whatever you do - buy a Bosch/Thermador dishwasher. +1 on Bosch makes crap.

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(edited)

Kingsborne wires offers the wiring loom option.  

 

http://www.kingsborne.com/

 

Those look just like my wires from them, except I do not have the tube.

 

You can test/compare the resistance in the two coils, if you have an ohm meter.

This'd be a good excuse to get one if you don't.

 

Does your rotor say R5 on the underside?  (the stock ones typically do)

Are you running plugs with built in resistance, by chance?  (R in the number, BPR6ES, for example)

As mentioned, you have the resistor wire going to an internally resisted coil...

 

I don't know the resistance of the Kingsborne wires, but stacking up components with built in resistance can be a problem.  (Toby says so)

 

I see that your old coil says MADE IN GERMANY... they do not say that anymore.

I would keep the old one installed.  (bypassing the resistor wire, of course)

 

EDIT:  it looks like you are using the vacuum retard pod on the distributor.  I think of vacuum retard as an attempt at cleaning up emissions... with no benefit to performance... although I do not know the curve for that distributor, I'd try running it without using the pod.

Edited by '76mintgrün'02

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Thanks. All good stuff. I guess I don’t understand why the old coil works fine with my current set up. FYI- The rotor is R5, the plugs are BP6ES and the wires are Kingsborne 12-7405. I’ll check the coil resistance tomorrow and report back. The vacuum runs to the Weber and that’s been like that since 1980 when we re-laced the soles and removed the emissions. I’ll try running without and see if it matters.  Thanks for your help.  

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Jumper 12v direct to the new coil, leaving the resistor wire off, and see if it still breaks up. 

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6 hours ago, dlacey said:

Bad condensor

This could easily be the issue, those symptoms definitely fit a failing condenser and if the new coil is actually putting out a higher voltage than the old coil (plausible), then a weak/failing condenser would not be able to suppress arcing, where as on the old coil it might still be 'just enough.'  Not saying for sure this is what's happening, but I can certainly see it fitting the behavior.  Plus condensers are cheap, so probably worth popping in a new one if you haven't replaced it in a while!

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Don't rule out the actual wire terminal connection to the condensor.  A loose connection will cause intermittent running issues.  Had another 73 owner with this same problem not too long ago. 

 

Also compare the numbers stamped into the bottom of the coils. While they may both be "Blue", they could be different designs.

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20 hours ago, sratner said:

 

I have a bosch blue coil that's been in the car for at least 30 years ('74 2002, owned the car since '79) so instead of getting dumped on the side of the road i decided to pick up a spare.  i bought a new bosch blue (the correct coil) as a replacement and then i replaced the old blue coil which works perfectly with the new one.

 

 

Other posters may be correct that there is a loose connection or that the condenser is weak, but how do you know that you have "the correct coil" ?  Have you compared the stampings on the bottom of the new coil with the old?

 

Despite what appears to be a common misconception, that the blue coil is an upgrade over the OEM coil, not all blue coils are the same.  While most Blue coils do not require an external ballast resistor, some do and some are even for 6 volt systems,  https://www.bmw2002faq.com/forums/topic/75974-boschs-bleu-blue-coil-marketing/

 

Quote

Can you buy the "wrong" 12V Bosch Blue coil? Well apparently, you can.   .  .  .  .  . 

Bosch number for the blue coil WITHOUT the ballast resistor could be 00012A

http://www.ratwell.com/technical/BlueCoil.html

 

 

 

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ok here's what i've discovered. the new coil has the same markings as the old one.  both coils measure 3.0 resistance. should i measure voltage output?  there are no loose connections and the condenser is new (but i'll try another new one this weekend). i did learn that the rotor reads R1 instead of R5, does that matter?  thanks. 

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I believe the R1 rotor is fine.  I am using that style with no noticeable ill effects.

 

My initial concern with the R5 rotor was that I have read that stacking up a series of parts with built in resistance can cause problems.  That's all.

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