Doctor Diesel

Worn out brushes

Web05In issue 325 you featured a Passat with over 350,000 miles on the clock. I feel quite humble and I will now cease boasting about my V reg 2.0 Astra diesel with only 243,000 miles on the clock. I was interested in which items had been replaced on the Passat and in particular I noted that he did not mention replacing the alternator. I realise it could well have been changed before he got the car but, even so, it has done very well. I changed mine at 230,000 miles, but only because I got worried about European trips with it. Prior to owning the Astra, I used to inspect alternators at 80 to 90,000 miles, as I found from experience that they are usually just about to wear out around then. So why are alternator brushes not wearing out these days? Are they being made from a better material than carbon now, or have other things changed? Regards,
Peter Duffy

Peter, as compared with the DC generators or dynamos that preceded alternators, which required regular attention, the brushes of an alternator only carry the relatively low excitation current, not the full generated output. Alternators are supplied with their relatively low excitation current using continuous slip rings, rather than a slotted commutator used with DC dynamos, thus enjoying much reduced wear levels. The AC output is generated in the fixed stator, converted to DC by a diode and its voltage suitably regulated to charge the battery. So whilst we oldies remember replacing dynamo brushes, and also particularly starter motor brushes quite frequently, alternator brushes have a very long life on account of the low power that they are required to transmit. Not that you can easily source the brushes for most alternators these days anyway, because you are generally only offered a new or rebuilt complete unit. The so-called carbon brushes of alternators do now contain several other life-enhancing components, like metal powders, resin binders, and powdered lubricants, like molybdenum disulphide.

Ah! molybdenum disulphide, the magic lubricant. I remember it well; how many of us tipped a tube of molybdenum disulphide paste down the plughole of our engines (in my case, an ex-police LE Velocette 192cc water-cooled flat twin motorcycle) hoping that it was going to magically restore our knackered engines. I seem to remember that my water-cooled engine was not nearly hot enough to drive off the carrier solvent and “hydraulicked” such that it wouldn’t even turn over with the stuff in the bores; so I had to take the plugs out and turn the engine over, (with a hand-starter lever, set alongside the fuel tank) whereupon it spat out most of the contents of my £7.50 tube of magic engine restorer! Other uses of “moly” lubricating properties were in greases, and as a component of two-stroke oils that allowed you to reduce the oil to petrol ratio, and thus emit less smoke.

However, I digress. The items we are discussing are called “brushes” because they were originally made of compacted copper wire, not unlike those metal brushes that are sold to keep the gaps between the paving slabs of your patio free of weeds. But, being hard, they swiftly wore out the metal “commutator” – hence the switch to softer carbon brushes, which also suffered from less sparking. I have digressed, but I think I’ve explained why alternator brushes last a long time, and it’s usually something else that fails when an alternator suddenly dies! I did note in the report in issue 327 (September) on the 700,000 mile Peugeot 504 2.3D that the current owners changed the oil religiously (only on Sundays?) every 5,000 miles; a lesson to be learned there I feel!
Doc Diesel

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