How my MSD Failed Today! [WAS: The Commute From Hell]

[jlandry@domain.elided (John A. Landry)]

Well friends, I think we have the answer as to why my truck died on the
freeway the other day...

First I'll bring you up to date...  After the "Commute From Hell"
yesterday, the drive home was uneventful.  I stuck to surface streets for
awhile and stopped by an auto parts store to buy a bottle of water absorber
for the fuel (thinking the problem might have been a little water in the
gas).  Once getting on the freeway, the truck never ran better.  I was a
little paranoid all the way home, expecting a repeat engine stall
performance.  Thankfully it wasn't to be.  This morning the commute was
also uneventful and I actually started to relax a bit.

But at lunch today I took a long drive to the Boeing Surplus Store to hunt
for various hard to find items to repair my travel trailer.  On the return
trip, as I pulled off a freeway exit and slowed to a stop at the light,
GUESS WHAT HAPPENED?  The damn engine died just like the day before.  Here
we go again!  I was able to crank the engine almost normally, but not keep
it running.  It seemed that if I kept the engine rpms way up (maybe above
1500 - 2000 rpm) the engine would run more or less normally.  But let the
rpms drop below that point, and the engine would immediately die.  No rough
running... just dead!

I had to start and re-start many times in order to limp through two
intersections to a gas station... cars honking behind me all the way.  Man
that beast is hard to steer and bring to a stop with no power steering or
power brakes.  The first thing I checked was the fuel... carb seemed full
of fuel and the fuel supply hose was hard with pressure (normal).  So I
immediately turned my attention to the ignition system.

I keep a little laminated card under the hood with step by step
instructions on converting to breaker-point operation from the MSD 6A to
the Holley.  My memory is too unreliable to be trusted when I'm upset about
being stranded at an intersection with a dead engine, so I thought this
card would be a great idea.  Today it proved very helpful.  It took maybe 2
minutes to disconnect and reconnect the appropriate wires.  With the MSD
disconnected from the system, the engine fired right up.  The timing was
maybe 15 degrees retarded because I had been using the MSD Adjustable
Timing Control (which requires retarding the timing on the distributor).  I
whipped out my tach and timing light and reset the timing to TDC.  The
beast purred happily.  After that I drove the several miles back to work
without a problem.

Now I can't rule anything out, but it certainly appears the MSD computer
has a serious attitude problem!  I guess it could be something else besides
the computer, like the coil acting up, but after Steve Stegmann's MSD
failure, I'm suspecting the computer.  My unsubstantiated theory goes like
this... The computer fires each spark plug as many times as it can in 20
degrees of crank rotation up to a certain rpm (about 2000 rpm going from
memory), whereupon it fires each spark plug only once above that rpm.  I
suspect the computer is intermittently failing to fire the spark plugs in
the multi-spark mode, but is working fine in the single spark mode.  This
would explain why the engine will run if the engine rpms are kept high (as
in cruising down the freeway), but the engine will die when the truck (and
in turn the engine) slow down as when exiting the freeway or slowing for
heavy traffic.

So Steve, I get to join you as a sad member of the MSD failure club.  All I
can say is thank God I ditched the Mopar magnetic pickup I was using to
trigger the MSD a couple of months ago.  Had I still been using it,
converting back to breaker-points would have been considerably more
difficult on the side of the road.  My first order of business when I
arrived back at work was to place a call to Northern Auto Parts Warehouse
where I placed an order for a Pertronix Ignitor conversion for the Holley
distributor.  I've been wanting one for some time, but haven't had the push
enough to get off my tush and order one.  After this little bit of freeway
fun, I can't say enough for system redundancy!

Regards,

John
-------------------------------------------------------------------------- 
jlandry AT halcyon DOT com       |   I want to know the TRUTH about our
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