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87-90 Cherokees - An Odd Breed
Contributed by Richard McCuistian Friday, 06 February 2009 Last Updated Friday, 06 February 2009

Those four years produced a solid vehicle with an interesting fuel system, and there are thousands of these babies still in the wind.by Richard McCuistian

In 1989, after my employing dealership bought the Jeep franchise, I spent a lot of time inJeep/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.

Note: This is a little advanced for a DIY person, but it'll be an interesting read if you're aCherokee buff. 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, pullthe 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, thePower 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 (50percent 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 withthree 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 andplot 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. 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 irritatingto 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 surewouldn’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...