I've got the same truck, and used to have a similar problem.
Your thunk probably means
1) Poor connection between battery or starter. Current is not flowing
between an otherwise ok battery and starter. Rust under the contact points
does not conduct electricity and the resistance it builds up creates heat,
which reduces conductivity even more. Start with cleaning the battery
terminals and making sure they're tight, then clean the starter and ground
strap connections with a little sandpaper or wire brush. Sometimes this
makes a big difference.
2) Dead battery. This may be because your battery has a bad cell, but more
likely means you really have a generator or voltage regulator problem;
3) Dead or almost dead starter (I'm assuming you have a foot starter, so
there's no solenoid, and that the starter switch is in good shape).
For specific tests, I usually pull the component and take it to my local
parts store, where they know me pretty well by now, and are happy to test
things. I think they're honest about it even though it's in their interest
to sell me stuff. For simpler tests, I follow the shop manual.
Other than recommending to you a thorough cleaning of battery terminals,
grounds and contacts, I want to address the general philosophy of dealing
with balky 6v systems. There are 2 approaches, or perhaps 2.5.
The first is to redo the stock ignition, charging and starting systems.
I've read lots of opinions to the effect that it is sacrilege to do anything
but restore the stock system, and I understand and sympathize with this
viewpoint. I had my starter, voltage regulator and generator rebuilt,
replaced the battery cables and ground straps, cleaned the attachment
points, got a new battery (a couple, actually) got a new coil, and a Mallory
dual point distributor. You know what? I still had the same problems. Not
all the time, but usually at the wrong time. Of these items, only the
Mallory dual point made a consistent improvement.
The second approach is what you come to after you get sick of the first
approach. Sacrilege or not, I wanted to drive my truck, so, I bought 12
volt components, including a new wiring harness, which I also recommend
strongly. Now it runs beautifully, starts every time, Vroom! and looks
absolutely stock, except that the brake and parking lamps now double as turn
signals thanks to the new wiring harness. I wish I'd done it straight away
instead of messing with the 6v system. Patrick's has a good, short "how to"
guide on the conversion.
The extra .5 solution is to get an 8v battery and use it in the 6v system.
I've heard that the extra 2 volts sometimes give a tired 6v system what it
needs. You don't have to change any electrical components, although some
people say the voltage regulator should be adjusted to charge a couple of
volts higher or you end up with an 8v battery putting out 6v. I believe the
voltage regulator charges at between 7.2v and 7.4v anyway, so adjustments
may be unnecessary. This might actually be worth trying before going to a
12 volt conversion. A lot of 8v batteries are made for boats.
Grant S. firstname.lastname@example.org
----- Original Message -----
From: tim <lloydt@Colorado.EDU>
To: Ole Truckers <email@example.com>
Sent: Wednesday, November 10, 1999 7:17 AM
Subject: [oletrucks] "thunk" is rarely a good noise
> I ran into a bit of a problem starting the old girl this morning. It
> occasionally takes me more than one try to get the truck started in the
> morning, especially when it's cold, but this morning, the second turn of
> the ignition key yielded only a dull "thunk," and then nothing.
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