Key Terms
ASD (automatic shutdown) relay- The circuit to this
relay is controlled by the computer to give power to the fuel system,
specifically the injectors, o2 sensors, coil, CPS and others. This relay
is located in the PDC but can be wired individually if you do not have the
PDC.
Battery Temperature Sensor- This sensor
will not allow overdrive equipped transmissions to shift into overdrive
until the battery temperature (and therefore battery voltage) is at a
certain level. This sensor is not required for start-up.
BCM (Body Control Module)- Grand Cherokees have a body control module that
operates many other features of that model. The absence of this module
will not affect the engine unless the PCM is equipped with a security
device. The PCM looks to the BCM for signs that the security system is
de-activated. There is no way to deactivate the security system without
the BCM because only the BCM has the required inputs to sense
deactivation.
CPS (crankshaft position
sensor)- This sensor is responsible for determining the timing of the
engine. It does this by picking up magnetic pulses from the flywheel or
flex plate and sending that information to the computer through
resistance. This is a major source of trouble in most 4.0L conversions.
Fuel Pump Relay-This relay controls power to the
fuel pump. Activation of this relay is controlled by the PCM. The ground
to the control circuit is controlled by the ASD relay for safety reasons.
This relay can be activated by a switch if desired (instead of
automatically). This relay is located in the PDC but can be wired
individually if you do not have the PDC.
PDC (Power Distribution Center)- This is responsible for housing many critical
fuses and relays including the ASD relay, Fuel Pump relay, and starter
relay. This is usually not included with the ?engine harness? that you
would normally recieve from a junk yard because parts of this harness run
to the rear of the vehicle as well as the body control module (Grand
Cherokees)
O2 (oxygen) sensor- This sensor
is responsible for determining the proper fuel-air ratio by measuring the
temperature of the exhaust and sends the information to the computer. The
primary O2 sensor is located after the collector (exhaust manifold) and
the secondary sensor is located after the catalytic converter. The
secondary O2 sensor only checks the effectiveness of the "cat",
and is not needed to run the engine (but IS needed for emissions). The
Primary O2 sensor is only monitored by the PCM during closed loop modes
(engine warm, normal operation), and therefore the lack of its presence
will not prevent the engine from running.
OBDII (On Board Diagnostics 2nd generation)- Term given to all fuel injected
vehicles produced on or after 1996. On jeeps, this means there are (2)
oxygen sensors and a required on board diagnostics hook-up junction
amongst other things.
PCM (Power-train
Control Module)- The Computer responsible for operating all fuel delivery
operations, emissions controls, and electronically controlled automatic
transmissions (Grand Cherokees). If the model that the PCM was taken from
was equipped with an anti-theft device, a new PCM will be needed that was
not equipped with such a device. This is the only module required to
operate the engine.
PN/PS (Park Neutral
Position Switch)- This switch senses if the transmission is in park or
neutral. The PCM adjusts the engines idle based on this input. The engine
WILL run without this input.
Starter relay-
This relay controls the starter solenoid (located on the starter). This
relay is located in the PDC but can be wired individually if you do not
have the PDC. VSS (Vehicle Speed Sensor)-
This sensor sends a signal to the PCM to determine speed and distance
traveled. The sensor produces 8 pulses per revolution. This sensor is not
required for the engine to run. This sensor should be installed or the
engine may die on rapid deceleration. The PCM adjusts idle partially based
on the VSS input.
Getting Started
There are several things to consider when doing a 4.0L swap. I will
attempt to cover everything needed in most kinds of swaps, but I cannot be
cumulative:
Fuel pumps, tanks, lines and regulation
 |
This is an example of a 98 TJ tank and pump. The pump is internal
and the regulator is on top.
|
MPI (Multi-port Injection) requires a fuel pressure of 49.5 PSI on OBD
II engines and 32-39 PSI on OBD I engines. This pressure must be regulated,
and there are two types of regulation:
(1) On the fuel rail itself, and
(2) in the fuel tank as a part of the fuel pump.
Rail regulation seems to be associated with OBD I systems and requires a
return line to return the excess fuel back to the gas tank. In-tank
regulation, also known as returnless regulation, is generally found on newer
OBD II systems. EFI rated rubber line is required to handle the extra fuel
pressure in conjunction with standard steel line. Rubber line should not be
used all the way to prevent vapor lock. A variety of tanks can be used as
long as the pumps that produce the fuel pressure can attain the appropriate
level of pressure. Newer OBD II returnless systems use unique gas tanks and
internal pumps and require a returnless fuel rail. If desired, an OBD II
engine can operated using a different tank and pump as long as a regulator
is plumbed inline that matches the pressure requirements for OBD II(49.5 PSI).
Similarly, an OBD I engine can operate on an OBD II tank and pump; the fuel
pressure will just be regulated twice, and the newer style gas tank will
need a provision drilled for a return line.
For emissions legal swaps, be sure to use a legal gas cap. These caps
prevent the use of leaded fuel.
Swap's from 2.5l's; special considerations:
2.5L jeeps have shorter engines, different wiring, weaker transmissions and
poorer cooling capacities. To swap in a 4.0L, the engine brackets of the
2.5L jeep must be moved 4-4.5 inches forward to account for the length of
the new engine. The wiring needs to be integrated with the wiring from the
4.0L. A different transmission needs to be selected because the ax-5 will
not mate to the 4.0L and would not handle the power if it could be mated.
Also, the NP 231?s input gear from the 2.5L (21 spline) needs to be
swapped with one from a 4.0L (23 spline), which will match the new
transmission. Finally, I recommend that a new radiator be installed that
will handle the extra heat that the larger 4.0L engine produces.
 |  |
| The correct bracket location for the 4.0L is around 3.5 to 4
inches from the shock towers. | I made sure to keep at least 1.5 inches between the fan
clutch and the radiator to prevent damage should the body flex
towards the fan. |
Differences between OBD I and OBD II engines:
OBD I engines have very different harnesses than OBD II engines. Sensors
operate on a different number of wires and on a different voltage (OBD I= 8
bolts, OBD II = 5 volts). Nothing is interchangeable except for the block
itself and the various accessories. OBD II engines have (2) oxygen sensors;
more elaborate emissions systems, a required diagnostics terminal and are
much better able to determine a problem with the engine. They also
communicate to the instrument cluster via a computer-type communications bus
known as a CCD bus that makes it difficult to hook up gauges. This may
discourage you from choosing an OBD II engine for your swap due to engine
swap and emissions laws, or complicated gauge wiring. Both OBD II and OBD I
engines require a working ?check engine? light that can be difficult to
hook up.
Fuel pressure requirements are also different between the two types of
engines. This was discussed in the ?Fuel pumps, tanks, lines and
regulation? portion of this article.
CPS (Crankshaft
Position Sensor):
 |
The required hole and bracket for installing the CPS into an older
style transmission
|
The CPS is located on the transmission bell housing on the driver
side. It senses the flywheel position and thus determines timing for the
engine (along with the CMS or camshaft position sensor located in the
distributor). This sensor is the biggest problem for most 4.0L swaps. You'll
need the flywheel or flexplate from the donor(or another fuel injected 4.0)
because it has the necessary magnetic pickup required for hte CPS. Also,
newer transmissions have the provision for this sensor while older
transmissions need a hole drilled for this sensor along with a special
bracket. If this sensor is not installed correctly, it will prevent the
engine from firing. The PCM uses the input from this sensor to determine
spark and spark timing as well as injector timing and duration. Without this
input, you will not get spark, even at the coil. There is a CPS relocation
kit available from Hesco that locates the CPS on the front of the engine as
an alternative to drilling a hole in your transmission (although it is more
expensive).
The CPS has a very low tolerance for improper adjustment. A new CPS is
installed with a piece of paper that gets torn off on the first crank. This
is the proper distance that the CPS must be from the flywheel or flex plate.
I have another way to determine the proper gap. Turn the key to ?ON? and
loosen the CPS bolt. Move the CPS away from the flex plate. Slowly move the
CPS back towards the flex plate until you hear the ?click? of the ASD
and fuel pump relays (You may need to do this more than once to hear the
click because it occurs every other time). Tighten the CPS bolt in that
position.
The PCM and how it operates the engine:
There are two types of modes of operation: open loop and closed loop. During
open loop modes, the engine does not monitor the O2 sensors and mixes fuel
based on preset values. During closed loop modes, the engine monitors the O2
sensors and adjusts fuel mixture based on these inputs. Engine start-up,
engine warm-up and key-on modes are all open loop modes and are probably the
most important modes to understand because if these modes operate correctly,
the computer is hooked up correctly.
The following lists were from the Jeep service manual for a 1998 Grand
Cherokee:
Key-On mode
-The PCM pre-positions the idle air control motor (IAC)
-The PCM determines atmospheric air pressure from the MAP sensor input to
determine basic fuel strategy
-The PCM monitors the engine coolant temperature sensor input. The PCM
modifies fuel strategy based on this input.
-Intake manifold air temperature sensor input is monitored
-Throttle Position sensor (TPS) is monitored.
-The ASD relay is energized by the PCM for approximately three seconds.
-The fuel pump is energized through the fuel pump relay by the PCM. The fuel
Pump will operate for approximately three seconds unless the engine is
operating or the starter motor is engaged.
-The O2 sensor heater element is energized via the ASD relay. The O2S sensor
input is not used by the PCM to calibrate air-fuel ratio during this mode of
operation.
Start?up Mode
The following actions occur when the starter motor is engaged.
The PCM receives inputs from:
-Battery voltage
-Engine coolant temperature sensor
-CPS
-Intake manifold air temperature sensor
-MAP sensor
-TPS sensor
-Starter motor relay
-CMS position sensor
The PCM monitors the CPS. If the PCM does not receive a CPS signal within
three seconds of cranking the engine, it will shut down the fuel injection
system.
The fuel pump is activated by the PCM through the fuel pump relay
Voltage is applied to the fuel injectors with the ASD relay via the PCM. The
PCM will then control the injection sequence and injector pulse width by
turning the ground circuit to each individual injector on and off.
The PCM determines the proper ignition timing according to input received
from the CPS.
Warm-up mode
There are two key things to learn from this mode. The first is that the
park/neutral position switch is monitored. The engine adjusts idle based on
this input (auto trans only).
Second, once the engine warms up, the PCM starts to monitor the O2S sensors
and goes into closed loop mode. The engine adjusts its fuel mixture based
only on the first O2 sensor. If no sensor is equipped, the engine will enter
into ?limp mode? and will run like crap, but it will still run.
Choosing
the right PCM:
When choosing a computer that will operate the fuel delivery system of your
engine, almost any Jeep OBD II computer that operated a distributor will
work. This means that 96-98 Grand Cherokees and 97-99 TJ?s are
interchangeable (Sorry I don?t know about XJ?s. I think they range from
96-99). Just make sure that they don?t have a security module. Call a
dealership with the VIN from the PCM donor to determine if it was equipped
with one (Note, all TJ Sports were NOT available with security systems, so
any TJ with the sport package is a good donor).
If you need the computer to operate a transmission like the 42RE or the
lockup on the 32RH, then be sure to pick a computer with the appropriate
accommodations, meaning that the PCM donor should be from the same model(and
equipped with the same components such as the transmission.)
Gauges:
There are three options available: Using aftermarket gauges, using gauges
from the donor, using your existing gauges.
Speedometer:
Using:
Aftermarket gauges: Use the VSS for your engines year and use an
electronic gauge. The signal from the VSS travels through the speedometer
and then to the PCM. On OBD II engines, the speedometer uses 8 pulses per
revolution. Autometer gauges use 16 pulses per revolution but are
programmable, so this difference is of no consequence.
Donor Gauges: Use the VSS from your engines year and wire in the
gauge cluster via the CCD bus (easy! Just two wires!). A few other wires
from the gauge cluster need to be connected for power, back lighting etc...
Your existing Gauges: Use the VSS and adapter from a 91 wrangler.
These have the VSS needed for the computer and also a throughput for the
mechanical speedometer cable.
 | The sensor pictured was gathered from Pirate4x4.com in a post made
by ?gripguru?
He provided the following part numbers:
5/8"x7/8" speedo apapter #9400012
vehicle speed sensor #56027015
|
Temp
gauge:
Using:
Aftermarket gauges: Use a T-fitting that should be available from
any part store and T?s into the heater hose. My part store guy gave me a
car quest part number: HTR 278909. This fitting has two push on style ports
for the heater hose with a 3/8? NPT fitting that is perfect for an
aftermarket sensor. *Important! The engine uses the stock temp sensor to
determine fuel mixture and timing. DON?T replace it.
Donor gauges: See previous ?donor gauges? section.
Your existing Gauges: See ?aftermarket gauges? above but use
the sensor off your old engine instead.
Fuel
Gauge:
Using:
Donor Tank:
Aftermarket Gauges: Use a fuel gauge with an Ohm resistance of 240
OHM empty/33OHM full. This is very close to the existing fuel level sender
built onto the pump, which is 220 OHM empty/ 20 OHM full. Keep in mind, your
fuel tank will be empty slightly before the gauge reads empty.
Donor Gauges: Same as previous ?donor gauges? section.
Existing gauges: If your sending gauge has the same ohm resistance
as the donor sender, you?re good. Otherwise, buy an adjustable aftermarket
fuel level sender and adapt it to the fuel tank (requires drilling a hole).
Other Tank:
Aftermarket Gauges: Buy a gauge that matches the Ohm reading of
your tank?s sender.
Donor Gauges: Buy a sender for your tank that matches the ohm
resistance of the donor (220E/20F) or close to that.
Existing gauges: No modifications necessary(should already be
hooked up).
Posted Jul 24 2007 05:44PM
