As has been pointed out already, intermittant starting problems are rarely
easy to diagnose, especially at long distances. ;-) Already mentioned have
been: starter solenoid; battery; connections. Also, however, the starter
itself can be the culprit. Here's how:
A typical starter will draw up to hundreds of amps when spinning over an
engine. Since starter motors are series wound, all of this current passes
through the armature as well as the field windings. There are typically
two sets of brushes, so ~100 amps needs to pass from the brushes to the
commutator while cranking. As the brushes and commutator wear, or if the
starter has been overheated from prolonged cranking, the resistance at the
brush/commutator junction can begin to rise. At this high current level,
just 1 ohm of resistance can generate ~100W, which works out to a power
density of about 1 kilowatt/square inch! This, of course, will burn the
face of the commutator and brush further, making the problem worse. It can
then become intermittent, depending on where the armature came to rest the
last time it spun down. A sharp rap on the starter/kick on the fender/bang
on the dash/etc. can sometimes get it to turn once in a while.
kent@parc.xerox.com (Chris Kent?) sez:
> The headlamps are not fused at all. Nor is the starter, for that matter.
And it's pretty interesting when the starter does manage to short!
That's what was wrong with Gretel, my white '62 TR4 when I bought it.
Well, actually, the coil was bad, and the butt-head that had it before me
sat on the starter key trying to get it started, which *melted* the
insulation off the starter field windings!! I didn't know this, of course,
so I put in a new battery, hit the starter, and >click!< the lights went
out, and the battery would start to *boil*! Luckily, it didn't take me long
to figure out what was wrong! I re-taped the field windings with PVC
electrical tape, and everything was cool (with a new coil, too, that is!).
As many of you may know, my company makes various DMMs. Recently, the DMM
group began pushing our products into the automotive market. As a
consequence, they have put together a bit of material on how to trouble-
shoot automotive electrical systems, including a really good poster full
of very good information on servicing the "new" breed of electronics on
todays cars - EFI, ECM's, etc. If anyone would be interested in getting
this stuff, send me your name & US mail address and I'll be glad to have a
package sent to you.
(I know the above sounds like a blatant commercial plug, but I do not mean
it that way. The information contained is very good, and you do *not*
have to use our products to take advantage of it. This is meant as a
public service only. Of course, the company wouldn't mind if somebody
actually decided to buy something ... and, No, I do not stand to gain
anything if anyone did decide to buy something, except as a shareholder,
of course.)
Somebody else (Jerry Kaidor?) asked about fuse ratings - fuses are actually
very application specific, so generalizations about ratings are somewhat
dangerous. But, I'll take a stab at it anyway ;-) The amperage rating on a
fuse usually means that the fuse will withstand that amout of current for a
minimum amount of time (typically 30 minutes) before opening. In addition,
they are rated for a "gross overload" condition - for instance, at 200% of
rating they should clear in 5 seconds maximum.
That's enough for now, this is getting quite long.
Pat Vilbrandt John Fluke Mfg. Co., Inc. Everett, Washington USA
UUCP: pwv@tc.fluke.COM or: { uunet, uw-beaver, sun, microsoft }!fluke!pwv
ARPA: fluke!pwv@uw-beaver.ARPA
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