87-90 Jeeps – A Breed of Their Own

In 1989, after my employing dealership bought the Jeep franchise, I spent a lot of time in Jeep/Renault school.  I was most familiar with the Ford engine control systems, and it seemed to me that the Renault/Jeep systems were a horse of a totally different color, and so I grappled with the oddities of it. In time, however, my perspective changed as I became more familiar with the product.  The 87-90 Cherokee/Comanche isn’t like anything else out there, and the fuel system on these babies is a peculiar study, so let’s take a quick look at it.

The fuel system is fed through 5/16 rubber hoses connected with worm clamps to steel lines from a pump in the tank that should hold its familiar 30-40 psi for awhile with the engine shut down.  If it doesn’t, pull the pump/sender assembly (you don’t have to pull the tank to do this) and tighten the hose clamps inside the tank. Voltage is fed to the pump through a ballast resistor from a relay over near the ignition coil and diagnostic connector.  There’s also a familiar fuel pressure regulator on the fuel rail.

By  the way, there are four relays in a line over there – The Fuel Pump Relay, the Power Latch Relay (which is controlled by one Engine Controller pin to feed power to another Engine Controller pin), the O2 Heater Relay, and the A/C relay.

The Cherokee/Comanche carries a multipoint sequential fuel system with a bell housing-mounted two wire VRS Crank Sensor (mounted on driver side at about10 o’clock), a distributor-mounted Hall Effect Cam Sensor for injector timing (50 percent duty cycle), an old GM style Manifold Absolute Pressure sensor, and Engine Coolant Temp sensor (up top and in the front) and an Intake Air Temp sensor (in the intake plenum behind the Throttle Body).  There’s an adjustable Throttle Position sensor on the Throttle Body that is a dual function unit on Auto Trans equipped platforms, so it has two GM style weatherpack connectors, each with three wires, and the connectors are discretely designed so as not to transpose.  

The Heated Oxygen Sensor looks like any other O2 sensor but is actually a titania unit that measures the temperature of the exhaust (cool is rich, hot is lean) and reads from 0 to 5 volts, with 5 volts lean and 0 volts rich, an arrangement that is antithetical to what we’re used to, but if you set up a scope and plot it on a graph, the pattern looks exactly like what you’re used to seeing, but remember, high O2 voltage reading indicates LEAN mixture, NOT RICH like everybody else’s system.  More about that later.

Notice that this titania sensor switches from 0-5 volts and when the voltage is high, the system is lean – when it’s low the system is rich -this is the opposite of a regular, more conventional sensor.

The Bendix Engine Controller is mounted under the dash above the accelerator pedal at a peculiar angle and it is simple to remove (if you know where the screws are) but irritating to reinstall because the screw holes aren’t easy to find.  That unit doesn’t generally need attention anyway, but be aware of it.  The 16 ohm injectors are hard-grounded and fired with voltage pulses from the Engine Controller rather than ground pulses like we’re used to seeing on everything else.

Idle speed is controlled with a GM style four wire stepper motor, but I sure wouldn’t try a GM Idle Air Control on a Jeep. We had a special tool for checking the operation of the stepper motor at the dealer, but I have no idea where you could get one now. One set of windings in the IAC is connected to terminals A and B, and the other is connected to B and C.  Make sure you compare the cone-shaped pintle on the end of the old one and the new one if you have to replace it… there are a couple of different pintle shapes used and the aftermarket people tend to foul this up sometimes.

The Throttle Body has an idle stop screw, but don’t jack around with that one.  The idle air bleed screw is on the side of the Throttle Body behind an aluminum plug, and I haven’t seen one in recent years that hasn’t been tinkered with, but if it hasn’t been tinkered with, leave it alone.  Cleaning the Throttle Body and IAC bypass passage is very easy to do on this engine and should be done at every oil change.

The PCV system consists of a grommet in the rear of the valve cover with a ¼ inch vacuum line connected to it.  The grommet has a small hole that meters the vacuum.  With the closure hose disconnected from the air cleaner and capped with your thumb, you should feel a slight vacuum with the engine running.  If you feel pressure there and see crankcase steam, you’ll also have oil in the breather: Check the PCV grommet for clogging as well as the vacuum line and the hose nipple to which it connects.

 

The ignition system is beautiful in its design, with an e-core coil tidily mounted right on an ignition module that takes its orders from the Engine Controller.  To check the operation of the ignition module and coil, just disconnect the two-wire weatherpack unit, connect your test light to power, and touch the probe to terminal B of the ignition module. The coil should fire each time you tap the terminal.  The scope pattern it produces is unique, so get used to what a good one looks like.

The crank sensor that provides engine speed and crank position should read at least 480 millivolts  AC while the engine is spinning with the starter, and if it doesn’t the Engine Controller might not respond to the signal. The connector is at the rear of the intake plenum, usually with a red wire and a white one. Measure this voltage with:  A. The crank sensor disconnected,  B. A cold engine, C. Your Digital Meter on 2000 mv, and D. Somebody spinning the engine.  If the voltage is nonexistent, replace the sensor.  If it’s a bit low, use a long screwdriver and gently bump it closer to the pulse ring. The crank pulse ring is a study in and of itself.

On Auto Trans 4.0L Units, it has a lot of small windows separated from each other by very narrow metal vanes.  At 120 degree intervals throughout the circle, it has a wide window and a wide vane.  When it sees a wide window and vane, it knows that a cylinder is 90 degrees before TDC, and it uses the sync signal from the Cam Sensor in the distributor to determine which cylinder is coming up.  If the sync signal fails (and sometimes it does), then the Engine Controller will simply pick a cylinder at random on startup, and on some starts it might run better than others.  If something bends just one of the thin vanes inward, the Engine Controller will be confused and won’t fire the coil in time.

This is the cam sensor pattern

The spark plugs are easily accessible with the exception of #1, and the firing order is the familiar old straight six 1-5-3-6-2-4. A distributor sorts out the spark the old fashioned way, but the timing is not adjustable, primarily because the Hall Effect unit in the distributor has nothing to do with ignition.  Put the distributor in with the rotor just passing the #1 firing post when the timing mark is on zero.  Don’t, and you may have a road speed surge due to rotor alignment/timing advance issues.

With a cutout cap and the crank on Zero TDC (compression stroke), you can use a cutout cap like this one to turn the distributor so that the rotor tip is just past the #1 post – that puts the post in the center of the firing window where it should be.

One looming difficulty is the fact that the Bendix system’s datastream is totally inaccessible if you don’t have a Chrysler DRB II with a special adapter and cartridge, and to my knowledge, the aftermarket folks have never addressed the need.  There is no Check Engine (MIL) light, and the orange Emission warning light that comes on after a certain amount of run time is tied to a special clock box under the dash that is dedicated to that purpose.  The clock is resettable to kill the light if you know how to do it, but I won’t go into that here for lack of space.

There is no way to retrieve any trouble codes (which are worthless on this one anyway), and determining fuel trim numbers is a faraway dream without a scan tool.   The diagnostic connector on the Cherokee/Comanche platform consists of two separate plugs mounted in a stamped steel plate over on the passenger side of the engine compartment near the Ignition Coil/Module assembly.

The old MS1700 Renault tester we initially used would work, but it was so slow, squirrelly and clunky as to be non-usable. Be that as it may, there are multiplied thousands of these old vehicles (the giant majority are 4.0L) still running the road, and there are some ways you can track down problems on this generation, so if you’ve been buffaloed by some old Cherokees (pun intended, but it’s a stretch), maybe this article will be a good addition to your files.

If you have a transmission that won’t shift, check this fuse – it feeds the Transmission Control Unit, which is mounted right under the glove box. A Cherokee will shift manually if this fuse is blown but it won’t shift automatically.

First off, measure all the voltages on a properly running Cherokee or Comanche as soon as you can. This takes some creativity, but the sensors aren’t hard to find, and as technicians we have to be creative by our very nature, right?.  You need to know what the sensors are reading with the engine running and not running, cold and hot. Those numbers will be your guide.  Connect your o-scope to the ignition system on a good one and note the peculiar shape of the waveform – take a digital photo of it or store it if you can for future comparison.

One of the most common driveability problems is stalling, and if cleaning the throttle body and IAC passage doesn’t fix it, try having a look at the O2 sensor:

The one wrinkle here is that if the Oxygen sensor intermittently fails at low voltage (and it generally does), then an ECU in closed loop mode will attempt to correct to the lean side and cause the engine to stall. Remember, on these, low voltage is interpreted as RICH, not lean. The O2 sensor heater is fired by a relay in a small bank of four over near the ignition coil and the diagnostic connector. So how do you find it if the sensor reads normally in the shop and you don’t have a DRBIII, connect a Digital Multimeter and drive it until the stalling occurs.  Ideally, the customer may have noticed a pattern that you can follow to duplicate it.  If the O2 voltage plummeted to the basement and stayed there shortly before the stall it generally means the O2 sensor is at fault, and sometimes the threads come out with the sensor, so be ready with your 18mm spark plug tap.

As with any system, the key to fixing this one is to get to know it.                           R.W.M.

 

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