screwball electronic ignition concern on an older vehicle can be downright
1984 Jeep CJ
how many miles
4 Wheel Drive
Customer washed engine. Now
engine skips and sputters above 2500 rpm.
Something Old, Something New
From its humble beginnings as a
military vehicle designed by the Bantam car company to Tomb RaiderLara Croft’s shiny new Rubicon, Jeeps have always enjoyed
an intrinsic machismo, along with almost universal recognition and a longevity
very few other vehicles on the planet can match, either in the real world or on
the silver screen.
I “horse traded” my cousin Billy
Joe out of a rusted out ‘47 model Willys Jeep in 1983.It hadn’t run in years and I bought some
parts to rebuild the engine.The mill
was a little tight when I finally got it bolted back together, so I hooked the
old bomb to the trailer ball on my dad’s 1966 Chevy pickup, found a forward
gear, and dragged it across a pasture to get it started.It was in 4 wheel drive when it fired up and
it pushed the Chevy all over the place before I could get it stopped.After that it would crank with the key and I
had a lot of fun with it until my money got tight and I had to let it go.
Computer Controls, American [Motors] Style
With ever-tightening emissions
standards nipping at their heels in the early eighties, American Motors pieced
together a feedback fuel system for the 4.2L platform that included a microprocessor
control unit (MCU) and an oxygen sensor.This engine is outfitted with a Carter carburetor equipped with a
special stepper motor for feedback response and a two-stage throttle kicker for
limited idle speed control. Air management is handled via a vacuum solenoid
used by the MCU to operate the innards of plastic diverter valves that are
piped through a pair of hand grenade-size pulse air units to provide fresh air
for upstream and downstream exhaust processing.
For me, one of the most interesting
wrinkles on this system was way the AMC system handles computer-controlled
ignition timing.More about that later.
A computer programming contractor
named Chris walked into my department one day last week to tell me how his 84
Jeep CJ had been running just fine until he washed the engine.Immediately after the under hood cleaning, he
found that he couldn’t raise the engine rpm above 2500 rpm without it
sputtering and popping.He had already done
everything he could to dry everything out, but the problem remained.
We don’t usually work on vehicles
as old as this Jeep in my department. I get requests for cheap engine overhauls
on ancient pickup trucks and muscle cars, but I’m not running a hobby shop, so
I turn those away.But since we were
just finishing up a course on ignition systems, and the old Jeep was blessed
with an interesting ignition system problem, I told Chris to bring it in and the
students would have a look at it.
I started the Jeep when he brought
it in and raised the rpm up to 2500.He
had an aftermarket tachometer, and when the engine started misfiring, the tach
began bouncing toward zero, then back up to 2500.A downward bouncing tach generally indicates
a primary ignition fault, and an upward bouncing tach points to secondary
ignition faults, particularly high secondary resistance concerns.This problem almost sounded like the rev
limiter strategy on a fuel injected vehicle, only not quite so rhythmic.
Knowing how easy it is to drift off
track during the diag process, I kept reminding myself that the problem had manifested itself after the
engine compartment was doused with high pressure spray.
Checking for Moisture
Incidentally, teaching auto
mechanics in the shop/lab is a whole lot different than fixing cars, and
planting problems for students to troubleshoot isn’t always as easy as it would
For instance, I tried to ‘bug’ a ,91 Chevy S10 trainer vehicle for my
students by spraying water into the distributor cap, but after reinstalling the
wet part, the 4.3L fired up ran just fine.I was flabbergasted until I realized the water has to condense in
droplet form on the underside of the cap in order for a moisture-related
crossfire to occur. That’s why an engine might run just fine right after washing
but stall out on the way back home from the car wash.
If the problem had been an
intermittent concern, we would have connected the oscilloscope first, but since
the engine never failed to dump about half its horsepower and torque at 2.5
grand, the students opted to take some exploratory measures before connecting
I generally teach my trainees to
check the easy stuff first.What that
means is that even though the ignition switch might be the first component in
the problem circuit according to the schematic, it doesn’t make sense to drop
the knee bolster to check the ignition switch connections only to find later on
that the problem was in the easily accessible 42 pin inline connector on top of
With that in mind, the
student I had assigned to the Jeep removed the easily accessible distributor
cap right away just to make sure there were no beads of condensation hanging
around in there. The cap was dry, and
before connecting the o-scope he snatched the plugs out for a quick look.They were in fair shape, but since spark
plugs are cheap, the student screwed in a new set, but the sputtering and
popping remained unchanged, as we all figured it would.It was time to move to the next level.
showed us an interesting pattern; to begin with, with the Jeep idling, the
scope kept getting confused as to how many cylinders the Jeep had, and there
was nothing we could do to change that.The inductive pickup was properly clamped around number one plug wire,
and even moving it to a different wire made no difference.This particular scope doesn’t seem to like
some vehicles very much, but it works just fine on most units.
But when the rpm was increased, the scope gave
us our first relevant piece of data.Above
idle, it knew full well that there were only six sparks popping, and at 2500
rpm, we literally lost every other cylinder in the firing order, starting with
number five.The Jeep uses the ancient
153624 spark pattern common to straight six engines, but above the 2500 mark it
turned into 1—3— 2 —.
Normal Pattern below 2500 rpm - bottom illustration is above 2500
Clamping the inductive pickup of a
timing light around the plug wire leading to each of the six cylinder, we moved
from front to rear and found that cylinders 4, 5, and 6 were all going dark and
dead above 2500 rpm, an anomaly I either hadn’t seen before or had long since forgotten.
So what on earth could cause the engine
to lose half its spark events at a certain speed?Well, since this Jeep ignition system
includes a microprocessor control unit (MCU) as a part of the ignition equation
and we didn’t have any spare parts to use for an A – B – A swap, it was time to
do a detailed analysis to determine whether the MCU or the Duraspark ignition
module was at fault.
Jeep’s Strange Blend
While the Jeep Grand Wagoneer
ignition system remained identical to a normal Duraspark setup until the end, the
83-90 4.2L Jeep ignition system engineered by AMC for the CJ/Wrangler series is
strange mix, with a familiar old Ford Duraspark module mounted low inside the
driver side front fender.The system
even uses a Ford style distributor and pickup coil, but with the distributor’s pickup
unit feeding its signal to the MCU instead of the ignition module.
This MCU basically looks like a 60 pin Ford
processor and is mounted between the passenger and the bulkhead, with the
dashboard in between.The MCU sorts out
the pickup coil signal, then sends a pulsed ground in on the orange ignition
module wire to trigger the Duraspark module, which in turn fires the ignition
coil. This setup is in place to give the MCU control over ignition timing, but
the system still uses a plain old vacuum advance.There were a couple of vacuum switches and a
temperature sensor that provided additional input to the MCU, but those had
been disconnected and removed on this unit (Chris had recently replaced the
engine), and they never seemed to make much difference anyway.The vacuum switches and the vacuum advance
have to be disconnected when checking or setting the ignition timing.
Unless the MCU and its hardware has been done away with, the engine rpm should be raised to
1600 when adjusting the ignition timing on one of these, and the sticker specs
under the hood call for 9 degrees plus or minus 2.
To strengthen the ignition system’s
foundation and on general principle, we adjusted the ignition timing to Jeep
specs.I’ve seen misadjusted ignition
timing cause odd problems I hadn’t expected, and I wasn’t about to be made a
fool of this time around.The timing
wasn’t set right, and we had to retard it several degrees and bring it into
spec, but to no avail.The sputtering
and misfiring remained.
Taking the MCU out of the Loop
have built a firm foundation in regard to the integrity of the coil, wires, and
spark plugs, I go to work on the ignition system’s superstructure to eliminate
different components and portions of the circuit.
I worked on a 98 Explorer a few
years back that was sputtering and popping rather famously at 4000 rpm, and I
determined that the problem was in the primary ignition signals to the coil
pack. Acquiring some connector terminals, I built a set of primary wires to
bypass the existing harness circuits between the PCM and the coil pack so as to
find the root of the problem. It was a grand and glorious troubleshooting
attempt, but the problem was still there.The Ford field service engineer suggested that I replace the 12A581
engine wire harness, and since I was out of ideas, and to make a long story
short, the Explorer cleaned up its act after I replaced the harness.In spite of the fact that the harness fixed
the problem, I get really annoyed by situations like that because they just
don’t make sense; the bypass I engineered should have cleared up the concern,
but it became obvious that another circuit was at fault, and I never could
determine exactly what the problem was with the original harness.Oh well…
Explorer ignition - the red arrow represents the part of the wire harness I overlaid...
As far as
our Jeep was concerned, we decided to remove the ignition module from its
mounting and use jumper wires to connect it directly to the distributor and
ignition coil.In this way, the MCU
would be out of the loop.
The ignition module has a two-wire
connector with a hot-in-start circuit (white wire) and a hot-in-run (red wire)
feed.The four-wire connector is the
business end of the module. The student quickly removed the ignition module and
laid it on the passenger side fender so it could be rewired to take its signal
directly from the distributor like an old Ford system. He attacked the job with
alligator clipped jumper wires from Radio Shack.
The black wire on the four-wire
connector had to be grounded to the engine block; the module uses this ground
to fire the coil (the harness anchor screw in the distributor bowl provides
this ground).The four-wire connector’s
green wire connects to the tach side of the ignition coil, and the remaining
two wires (orange and purple) connect to the pickup coil windings in the distributor.With the jumper wires connected this way, we
managed to fire up the Jeep and observe the scope pattern again using nothing
but the Duraspark module itself to operate the ignition system. The symptom and
the pattern were still there; the ignition module itself had to be at fault,
and when I had a replacement part sent from the local jobber, we connected it
to the jumper wires to be rewarded with victory.Mounting it in the original module’s place
and plugging it into the harness, we found all six spark lines.Chris was standing by, and he was measurably
impressed at how quickly the students located the problem.Why the problem appeared immediately after
Chris washed the engine remains a mystery.
Troubleshooting any system without
being able to understand the big picture is a frustrating exercise.And while it may have been a whole lot easier
to just plug in a new module, I’ve been in too many situations where a known
good part wasn’t available except across the parts counter, and we all know how
unpopular we can get with the jobber if we use his new stuff for
troubleshooting!That situation turns
even nastier when the part has to be ordered, particularly if it’s expensive!And if the problem is an intermittent,
somebody will wind up having to eat the cost part if the vehicle comes back
with the same concern.Can I get a
Personally, I’ve found that I learn
a whole lot more if I can gather enough hard data to determine (as nearly as
possible) exactly what the heck is going wrong.It’s a bit more time consuming on the front end but gaining insight and understanding
adds to our mental library and makes us more accurate troubleshooters.R.W.M.