Trouble Shooter
Engine management systems can compensate for a certain amount
of mechanical engine wear, but there’s a limit to that ability. If the
limit is exceeded, the PCM may not identify the underlying cause.
Kitchen Synch
Karl
Seyfert
[email protected]
I’m working on a 1996 Jeep Cherokee that has
about 150,000 miles on it. The 4.0L inline Six runs
pretty good, but a P1391 DTC (intermittent loss
of CMP or CKP signal) is stored in PCM memory
after the vehicle is driven for a relatively short distance. Even though it might pass a tailpipe test, I
don’t think it’s going to receive an overall passing
grade with the Check Engine light on and a hard
code stored in memory. Both the cam and crank
position sensors have previously been replaced.
The cam position sensor’s inside the distributor, so
a complete rebuilt distributor was installed. The
head and intake manifold gaskets were also replaced over a year ago because the head gasket
had burned through between cylinders 3 and 4.
Do you have any suggestions?
John Clay
Lakewood, OH
Photo: Karl Seyfert
Thanks for your question, John. There are several
possible causes for this problem. Let’s begin with
a brief description of the cam and crank position
A failed crank position (CKP) sensor (shown)
or a larger-than-normal sensor air gap may
cause erratic sensor output, engine performance problems and a stored DTC P1391 on
some 4.0L, 6-cylinder Jeep models.
sensors and explain what the PCM is expecting
from them. Both are Hall effect sensors that output square wave signals. As you mentioned, the
CMP is located in the distributor, while the CKP
is mounted at the top of the transmission. Specially positioned protrusions on the flexplate trigger
the CKP as they pass by. A pulse ring inside the
distributor serves the same function for the CMP.
The PCM uses these sensors to make decisions
about fuel delivery and ignition advance. It expects to see a relationship between the signals it
receives from them and will set a code if it believes they’ve gotten out-of-synch with each other.
If a synch problem develops, there are several
things that may be causing it.
First, a dual-trace oscilloscope can be used to
verify that both sensors are sending clean, strong
signals to the PCM. If you have any doubt about
the wiring between the sensors and the PCM, attach your scope probes by backprobing the PCM
harness connectors, rather than at the sensor connectors. That way you’ll be sure you’re seeing the
same thing the PCM is seeing.
A weak or erratic signal may be caused by a
failing sensor, or it may be due to the relationship
between the sensor and the signal notches.
There’s not much that can be done with the position of the CMP inside the distributor, but the
CKP may not be mounted as close to the flexplate as required to output a consistent signal.
Make sure it’s been mounted properly, using the
specially machined mounting bolts, but resist the
temptation to modify the mounting area or the
sensor to move the sensor closer. A properly positioned sensor should work just fine.
The flexplate itself may be damaged. Flexplates
on these vehicles have been known to break
around the mounting bolts. A flexplate with a broken center section may allow the outer section to
shift in and out of position, depending on engine
load. Accelerating the vehicle briskly may produce
a misfire caused by the broken flexplate and the
sensor synchronization problem it causes. You
should be able to see the damaged area by unbolting the torque converter and shifting it rearward.
continued on page 8
6
September 2010
Trouble Shooter
A borescope would come in handy here.
Ignition timing is not adjustable on
this engine. There’s a procedure for installing the distributor, and a locating
peg on the block keeps you from rotating the distributor housing once it’s been
properly installed. The procedure involves rotating the crankshaft until the
engine is at TDC on number 1. The position of the slot in the oil pump gear
should be just slightly before (counterclockwise of) the 11 o'clock position. If
not, a flat-blade screwdriver can be inserted into the oil pump gear to rotate it
into the proper position.
An indexing hole in the distributor is
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used in combination with a locater pin
to properly position the distributor shaft
before it’s installed. The same distributor
housing is used for both 4- and 6-cylinder engines, so indexing holes for both
are present. The distributor is correctly
installed when the rotor is pointed at the
5 o'clock position.
Some replacement distributors may
have the wrong distributor gear installed
(4-cylinder gear on a 6-cylinder distributor,
or vice versa). It’s a long shot but certainly
worth checking. Distributor gears are offset differently for the different engines.
Some advocate grinding the peg off
the block to allow a certain amount of
manual timing adjustment to bring the
sensors back into the synchronization
window, but I’m not one of them. If
there’s an ignition timing and sensor synchronization problem, the underlying
cause must be found, rather than attempting a Band-Aid fix.
If you have access to a factory-level
scan tool, you should be able to find a
menu to check sensor synchronization.
If they’re out-of-synch and you’ve already checked everything else, the most
likely cause is a stretched timing chain.
You can confirm this by manually turning the crankshaft with a breaker bar
while observing the distributor rotor. A
stretched chain may allow the crank to
turn through several degrees of rotation
before you see any corresponding movement at the rotor.
The stretched chain retards ignition
and valve timing. The PCM can accept a
certain amount of chain stretch, but it
can’t compensate once that tolerance
has been exceeded. Things get interesting when the engine goes through
changes in load and the stretched chain
allows the cam timing to retard, then return to near normal. The sensors may be
in synch part of the time and out-ofsynch the rest.
Grinding the peg off to adjust the
timing at the distributor tricks the
PCM, but it does nothing to address
the retarded valve timing. It may
sound boring, but the fix for an engine
with this many miles on it is probably
a new timing chain and gears (if necessary). This will put the sensors back
in synch and allow the PCM to properly adjust ignition timing.