Check your Distributor Cap Brush!                               
2/21  R. Kwas  [Comments added]


Reference Information


This failed Distributor Cap was first shown on the VW Samba site, and since we use the same Bosch Distributors in the vintage Volvo world, the information is quite applicable for vintage Volvo owners also!  

Picture by mbwesty of forum, and used with his kind permission.

Distributor Cap with failed center Brush. 

My comments to a thread Death by Distributor Cap at thesamba: 

The center contact in the Dist Cap is a carbon brush, little different than the brushes in the Starter, Alternator, Blower, or any other DC motor...its job is to make a good elec connection between a moving and a stationary component...and that is precisely what it is also doing in the contacts the Rotor...or at least is supposed to...and that contact is supposed to be under a little spring preload, provided by a spring behind the brush, so that no arcing occurs!

From the picture above, it looks like the brush is not protruding and wanting to push down onto the Rotor...brush or spring are somehow jammed, so with no contact, this presents yet a third place where the high voltage must arc across (in addition to at spark-plug and rotor-to-individual plug wire station)...and the carbonizing damage this arcing has left is unmistakable...I expect the condition was quite possibly accompanied by difficult starting, misfires and rough running...or any combination of those...

You could try to repair the carbonized area (lightly grind away the carbonized cap material [and seal with some fingernail polish] ) and try to repair brush and its spring, because the 4 individual Plug wire stations look to be in almost-new condition!... (then again, I'm an I-can-fix-that-kinda-guy)...otherwise, I suggest you replace the Cap, and before installation, have a look inside, and check that brush is obviously present, protruding from its hole a little (about 1/4"), with some spring preload if you lightly push on it.

...and here's a Tip: Put the tiniest dab of synthgrease [like Mobil 28] on that brush before installation. Yes, I realize this might be an unusual suggestions (and you may have heard it here first!), but this will lightly lube the Brush to rotating Copper of Rotor interface surface and significantly decrease wear of the brush (the resulting carbon dust inside the Distr Cap is not such a good thing as it can cause HV tracking and discharge, especially when combined with condensation droplets on a cold morning startup). Remember, just the tiniest dab of lube on that brush will do it...I will not be held responsible for giving bad advice if lube is applied from ten feet away, with a shovel!


Reference Information: 

This is what a (near) new Dist Cap looks like: 

Dist Cap as removed from a spare Distributor.  A spotless Ignition System is a happy, reliable Ign System!


Close-up of the Brush, showing very little wear.  When this carbon brush wears, the dust particles settle out on all surfaces under the Cap...not so good!  Periodic cleaning of this dust from the inner Cap surface should be part of regular maintenance. and when installing a Cap, the tiniest dab of Mobil 28 Synthgrease is recommended on the Brush to minimize wear, and resulting carbon dust.  Also a quick check of the Rotor contact area to assure it is nice and smooth and not abrasive can't hurt... 


Brush with its Spring removed for inspection.  Brush is captured by the last two turns of the Spring.  


External Links:

Link to thread Death by Distributor Cap  


External material sources are attributed.  Otherwise, this article is Copyright © 2021.  Ronald Kwas.   The terms Volvo and Bosch are used for reference only.  I have no affiliation with either of these companies other than to try to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at around the watercooler, or worshipped, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future!

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you donít, youíre just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the unique metaphor and possibly wise-a** comment. 

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