I'm so stupid... Was: Head Cleaners

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Jul 1 17:09:01 2004

>
> My favorite: (on-topic, but not computer-related)
>
> Car starter motor is really weak, finally just won't turn
> over. Lights dim and all when I turn the key. OK, $65 new battery.
> Same symptom. Sigh. Well, battery was old anyway. Must be the
> starter motor. $75 new starter motor, and some time under the hood.

Starter motors can be stripped and rebuilt (I've done it -- replaced
brushes, skimmed the commutator (you don't undercut the insulation on a
starter motor commutator, BTW) -- to save the cost of a new one).

> Same symptom.

When are people going to learn that you do tests and make mesurements
before replaceing parts. In this case, voltage tests directly across the
battery terminals would have shown that the battery was OK, and a voltage
test at the starter motor terminal would have shown the voltage drop in
the cable/connections.

Another useful trick is to measure the voltage across the earth strap
using the 200mV range of a DMM. This will give an idea of the current
flow in/out of the battery (effectlvely using the earth strap as a
shunt), and can be used to ensure the alternator is actually charging the
battery. You can turn on loads of known current (like headlights) to
calibrate this measurement if you want to.


> Drat! Now who knows what's broken? Start tracing power
> wiring, to see what other boxes are in between....after a few minutes
> of this, the positive lead from the battery *comes off in my hand*,
> leaving the clamp still attached to the battery post.
> Doh.

A really nasty version of this happens when battery acid creeps inside
the insulation of the battery cables. It will then corrode the wire, and
leave a poor, high-resistance connection that will drive you insane.

Two other car electrical faults that have caught me...

Firstly, on a lot of UK cars the starter solenoid has 2 windings. One takes
about 17A and is used to pull the core in. The other takes about 4A and
is used to hold it in. Now the former is earthed via the starter motor
(the resistance of this is _very_ low -- remember the stall current is
often over 1000A). and is effectively shorted out by the solenoid
contacts to turn it off when the solenoid pulls in.

What this means is that the solenoid won't pull in if the starter is
open-circuit, e.g. due to worn or sticking brushes. This does not seem to
be well known -- certainly the so-called expert from the breakdown
company didn't know it. I only figured it out when I looked very
carefully at the schematic and started checking resistances of various
components.

The other one concerns the alternator. In a lot of cases, the startup
supply for the field winding comes via the idiot light. This has 2
implications. Firstly, if the bulb burns out, then often the alternator
doesn't charge. Secondly, if the field winding -- and remember this is on
the rotor of car alternators -- goes open circuit, or if the brush gear
goes open circuit, then the idiot light doesn't come on, even though
there's no chargeing. It's a really poor design (the idiot light doesn't
idictate a problem for a very common failure, namely worn-out brushes),
but every alternator I've worked on uses the same circuit.

-tony
Received on Thu Jul 01 2004 - 17:09:01 BST