Jeep Technical How-to: Electrical Troubleshooting and Quick Fixes

    Troubleshooting skills play a large role in four-wheel drive survival. Many mechanical problems can be repaired in the field.

     Although late model Jeep 4WD powertrains and chassis are dependent on modules and electronic components, most of these devices have long service lives. Fortunately, items like the electronic control module, powertrain control module, sealed electrical switches, an air compressor or non-rebuildable hydraulic cylinders usually display symptoms before they fail. Some electronic devices even have a "limp-home" mode.

Making Friends with Your "Electronic" Jeep 4WD

  "Luddites" may argue that current Jeep models, and every other high-tech auto or truck built today, are not suitable for remote backcountry use. I would assert that a 4WD Jeep Wrangler or XJ Cherokee is far more dependable than the so-called “simpler” technology of bygone days.

  Consider the OEM electric fuel pump in the 1987 YJ Wrangler 2.5L model that now has 203,300 miles on the odometer. That original, permanent magnet motor is still operating! By contrast, imagine the lifespan of a dual-diaphragm mechanical fuel pump found on the F-134 four-cylinder Jeep engine in a classic (yes, my favorite flat fender) CJ-3B?

  Or how about the original ECU, MAP sensor, coil and TBI assembly that still function precisely and reliably on that YJ? Would a Carter YF carburetor or a mechanical fuel pump go 203,300 miles without a rebuild? “Minor tune-ups,” consisting of a new breaker point set, condenser, spark plugs and rotor change, were every 6,000-12,000 miles! And periodically, vintage models required a new distributor cap, spark plug wires and a carburetor “overhaul”—and carburetors did cause trouble in the backcountry...

     The electronic modules and electrical components of the modern Wrangler or XJ Cherokee are rugged, proven and reliable. Solid-state electronics and longer duty-cycle parts withstand severe shock loads, temperature variations and vibration. And that’s surely what a trail use Wrangler 4x4 will provide!

  Become familiar with these components, their function and ways to diagnose problems. Gain confidence in Space Age technology that powers up and operates your Jeep 4WD's powertrain functions, the contemporary automatic transmission's shift patterns, speed sensors and anti-lock braking systems. Make friends with the electronic era in Jeep 4WD technology!

  In the backcountry, I have never encountered the total failure of a key electronic component on a Jeep vehicle. Fail-safes, backups, limp-home modes and mechanical overrides take care of all but the most complete system failures. Systems interrogate themselves, and with a data code retriever or scan tool, the failed component will even identify itself through the powertrain control module’s stored messages. The later the vehicle model, the more sophisticated its onboard diagnostics.

    So, is there no such thing as a mechanical breakdown in the backcountry? Hardly. One truth, however, is that failure to perform routine service often underlies a Jeep 4WD's poor performance. For example, an electric fuel pump module can be tested for its amperage draw and cycling functions. The findings help predict how long the pump will perform properly. By comparison, a fuel filter is far more unpredictable. A virtually new, paper-type fuel filter, doing its job, can clog completely from one gasoline fill-up with water-contaminated gas. For a remote overland venture, carry a spare fuel filter.

Common Trouble Spots

     In my experience, the engine performance problems that plague 4x4s in the backcountry are apt to be simple: clogged fuel filters, dirty air cleaners, worn spark plugs, a worn and fouled oxygen sensor, pinched fuel lines, frayed wires and damaged electrical connectors. Engine cooling issues usually follow a similar pattern, with worn belts, a leaking water pump, ruptured hoses, a split radiator seam, a clogged radiator, fan clutch failure or simply a stuck thermostat. In each of these cases, a pattern of neglected service and overused equipment is apparent.

     To reduce the need for field fixes, begin with preventive care at home. Do your routine service properly, and prep your Jeep 4WD before each outing. This will lower the risk of more common field failures.

     Many field fixes can be performed with basic tools. If you plan an extended trip into rugged back country or across Mongolia, you need more tools, more spare parts and access to this website! (We could discuss your Jeep 4WD troubleshooting problems using the E-mail 'Q & A' approach!) Here are some simpler diagnostic tests that you can perform readily in the field.

     Note: Elsewhere at the 4WD Mechanix Magazine website, you will find tune-up information, service data and additional, in-depth Jeep 4WD troubleshooting tips.

Illus. 1: The most common battery problem is leaving the lights on with the engine not running. Next would be dirty posts and clamps. If you want to know the true state of charge on the battery, use a quality volt-ohmmeter. This digital type meter is very accurate.

Cautions: For safety sake, I test for battery voltage away from the battery at the heavy charge wire on the back of the alternator. For this TJ Wrangler, I run the ground lead to the alternator's housing.

Battery related facts: 12.6 volts is a full state of charge. Sometimes, voltage will read this high on an unloaded, defective battery. If the voltage range reads normal but the engine still won’t crank, try cleaning the battery posts and cables. Should that not fix the problem, check voltage once more at the alternator source while someone else attempts to crank start the engine. (Keep your hands and fingers away from the fan and belts!) If voltage drops to 9.6 volts or less while attempting to crank the starter, there is either starter motor drag or a dead short (like a “dead cell”) within the battery.

     For isolating a starter problem alongside the trail, remove the starter motor and attempt to run the motor “on the bench” (or a rock shelf in this case). Use a pair of jumper cables and a good battery for this test. Keep fingers free of the starter drive! If the starter spins fast and strongly, suspect the Jeep's battery. If the motor will not spin, immediately quit trying—or you will damage the jumper battery!

     Note: I had an AMC/Jeep era (Ford-type) starter that stopped working suddenly. The symptoms of this locked-up starter motor were identical to a battery with a dead cell or short. When I bench tested the starter motor—off the engine—and load tested the battery, the facts surfaced! The starter motor was defective, and the battery was fine.

    Caution: When attempting to start a Jeep equipped with electronic fuel and spark management, you need at least 12.4 volts. Do not jump start a dead battery. Allow the jumper cables to deliver a charge for a while (to reach a 12.4 volt minimum on the low battery); then complete the startup. Full-charge voltage will read 12.6 or more volts.

Illus. 2: This inexpensive “induction” meter, purchased in the early 1970s, has been a great companion on the trail. By merely laying the meter over the flow/charge wire from the alternator, you can tell whether current is moving and how much. Gauge reads in amperes. I keep this meter in my backcountry tool kit!

Illus. 3: A companion to the charge meter is the starter draw meter (at right). This is a quick way to note the amperage draw at the starter motor. You simply lay the meter over the heavy cable to the starter. Choose a safe, easy access point along the cable and keep your hands away from any moving parts. As someone else cranks the engine over, read the amount of current draw. This is not pinpoint accuracy but rather the fastest way to narrow down troubles. A defective starter will draw very high amperage, sometimes enough to melt battery cables!

Illus. 4: This is the secondary wiring circuit for the spark plugs on a distributor-type ignition: the distributor cap, wires to each spark plug, coil lead to the high tension tower on the distributor cap and the coil unit (at firewall). Plug wires have separators. Firing order for Jeep fours is 1-3-4-2; sixes fire 1-5-3-6-2-4. Rotation of the distributor rotor is clockwise. Late Jeep 4WD engines use the newer distributorless ignitions with coil-on-plug technology.

Illus. 5: Checking spark plug firing can be done readily on distributor-type systems. You can use an old spark plug if handy and not have to remove a spark plug from the engine. Otherwise, remove the most accessible spark plug and wire (#1 in this case). Attach a spark plug lead as shown here. Ground the spark plug metal shell solidly. Keep hands out of the way of spark and moving parts as you crank over the engine! Spark should fire crisply across the plug gap. If necessary, remove all spark plug(s) to confirm that the spark plugs are not gasoline fouled.

Illus. 6: Later engines have distributor-less ignitions. This means that the computer controls all spark and timing functions; there is no cap or rotor. Coil packs mounted above the spark plugs on this 2002 4.0L Jeep engine fire the spark plugs directly, a highly efficient spark that burns hot and clean. Timing is precise, and wear parts have been minimized. The computer gets a signal for the position of the crankshaft and continually changes spark timing based upon engine speed, load conditions, coolant and manifold temperature, manifold pressure and other relevant input.

Illus. 7: Oil pressure is critical to engine performance and lifespan. 45 PSI registers on this YJ's 2.5L four at 1,000 rpm, a very good sign. Monitor this gauge frequently, especially when you subject the vehicle to steep four-wheel drive ascents and descents. The oil pump and pickup in the oil pan must be able to draw oil; the oil level is critical for this reason. Watch your gauges! The oil pump, built to last between engine rebuilds, is mounted within the oil pan. Clean oil assures that the pickup screen will not become clogged.

Illus. 8: If you suspect that a YJ Wrangler's TBI 2.5L four is not getting fuel, you can check for fuel spray and signs of gasoline flow within the TBI throat. This assembly looks and functions, in some ways, like a traditional carburetor. You can see signs of fuel flow on the throttle plate. If a timing light is available, I hook the light to #1 spark plug wire, shine the light at the TBI throat as shown here, and watch the fuel spray pattern with the engine cranking and running!

Illus. 9: On MPI engines with one injector per cylinder, you can determine whether pressurized fuel is at the rail. Remove the cap from the check point. (The black tube with cap is visible here.) The valve is similar in appearance to a tire stem valve and serves as the test point for pressure testing the system. See precautions below:

     With the engine shut off and key in the locked position, disconnect the battery negative cable and relieve pressure at the fuel tank (remove the filler cap); pack rags around the check valve and fuel rail area to absorb gasoline; protect your eyes from possible fuel spray! In the Jeep professional shop, a special hose and gauge take the fuel away from the engine. You likely do not have these kinds of tools in the backwoods, so take necessary precautions to avoid spilling fuel on hot manifolds! Be very careful; the rail pressure range is typically 19-39 PSI and could cause fuel to spew out with force. You can open the valve slightly to see if pressurized gasoline is at this point.

     If no gasoline is present, attach the negative battery cable, and turn the key to the “On” position while listening for fuel pump noise at the tank. The pump should be running. Disconnect the negative cable once more, and repeat the fuel pressure test at the check valve. If there is no pressurized fuel and the pump has run, you likely have a plugged fuel filter. Depending upon your Jeep model, there may be two filters: one at the base of the fuel pickup in the tank (sock filter), the other mounted along the frame rail. Late Jeep models use a pressure regulator/filter just above the fuel pump at the tank, an item that serves for a very long time without the need for replacement. There is no routine service interval for the later filter type; these filters simply last a long time.

Warning: Do not allow pressurized gasoline to spray over a hot engine! This could ignite and cause a fire, far more serious a problem than your engine not running! Protect hands and eyes from the gasoline, and use rags to catch the spray. Be very careful here, or you could risk serious injury or burns!

Illus. 10: Fuse and relay box has clearly marked locations for the fuses and relays. If the fuel pump is not working, check here first. Also, check the PCM (computer) plug for looseness, and tighten the bolt on the 60-way connector if so equipped. This bolt is only tightened to 35 in-lbs—yes, inch-lbs not ft-lbs! (If simple remedies do not suffice, look for additional information elsewhere at this 4WD Mechanix Magazine website.)

Illus. 11: If you do not have induction meters in your tool box, you can use the on-board voltmeter for rough diagnostic procedures. Battery voltage at rest and during cranking can be noted. The state of battery charge without the engine running can be determined by simply turning the key to the “On” position without starting the engine. Used creatively, a voltmeter is much more useful than an ammeter. This YJ Wrangler gauge indicates a charging state with the engine idling and battery in good condition.

Illus. 12: An oil pressure reading of 45 PSI at warm-up idle (1,000 rpm or so) is more than adequate for the 2.5L four. AMC-design fours and sixes do, however, have higher oil pressure readings than many other engine designs. Expect 45-60 PSI at cruise or highway speed and when spinning the engine up in low range. Lower pressure than 40 PSI at highway speeds is questionable. On the trail, you want normal idle pressure. Monitor the oil gauge.

Illus. 13: The Jeep oil pressure sender can often be the culprit when oil pressure reads low. This device has a small orifice. With age and oil crusting, the orifice can become plugged. Before condemning your engine’s oil pump or pickup screen, check the oil pressure sender. If necessary, at your home shop, attach a mechanical oil pressure test gauge in place of the sender. Check the pressure.

     Note: If pressure reads normal at the mechanical test gauge, try grounding the oil pressure sender lead. If the oil gauge swings to full pressure with the key “On,” the sender is defective and needs replacing. No engine knocks or rattles? Crankcase has plenty of oil? Maybe the gauge sender is not working!

Illus. 14: If the clutch does not feel right or requires “pumping the pedal” to work, check the fluid level at the clutch master cylinder. The clutch slave cylinder on YJ models is built into the clutch release bearing, a fine idea as long as the bearing seals properly! If you find the fluid is low, look for leaks or drips from beneath the bellhousing. If the leak is brake fluid, the release bearing could be at fault. Top off fluid as often as necessary, and try to get home this way. You will need to remove the transfer case and transmission to access the Jeep YJ Wrangler's release bearing. (See transmission and clutch service elsewhere at this magazine website.)

Illus. 15: If you need to bleed the clutch linkage system on a Jeep YJ Wrangler, this is the release bearing bleed tube at the clutch bellhousing. Bleeding procedure is similar to brake bleeding: pump up the pedal, hold it down. Crack the valve open to let out aerated fluid, close the valve, and release the clutch pedal. Repeat, making sure you remove all air without running the clutch master cylinder out of brake fluid. Add brake fluid as necessary during the bleeding procedure. Make proper repairs as soon as possible!

    Note: Leaking hydraulic brake fluid will ruin the clutch disc. Repair or replace these parts as soon as reasonable, or you may become stranded with a slipping clutch!