Shotgun troubleshooting is generally good for the parts counter balance sheets but hard on the owner’s check book.
1993 Jeep Cherokee
Odo: 90,468 miles
Repair Order Analysis
I don’t know if this repair was attempted by the customer, a buddy of his, or a fly-by-night independent. Most shops see situations like this fairly regularly. The owner of the car turned the key on this Jeep one morning and the engine spun and spun. He opened the hood. He looked around under there, hoping to find something that might have come loose, but everything looked like it did the last time he looked. Maybe he called his wrench-wise neighbor. Maybe he dragged his Jeep to Jack’s friendly fix-it-while-U-wait Auto Shop. Quite a few dollars went whistling over the parts counter and the Jeep was still sitting there grinding away with no hint of any life of its own.
Whoever it was that did the work on this Jeep knew enough to be mildly dangerous. Spark plugs, wires, and a distributor cap don’t really hurt anything, especially on a high mileage engine. The Crank sensor is a fairly good guess, but no cigar. The “Pickup Coil” (cam sensor, actually) in the distributor can cause a no-start, but it didn’t do the job either. Finally, some wrecker driver’s phone rang the Jeep wound up at our shop.
Ignition System Basics Revisited
When I discovered the wonderful world under the hood working with my dad around 1970, we had contact points, a condenser and an ignition coil, and we were all very happy. Then Chrysler Engineers sent shockwaves through repair shops all over the country by making “Electronic Ignition” a household word in 1972.
There was “pickup coil” in the distributor which sent an analog signal to a mysterious box on the firewall called an “Ignition Module,” which, in turn, used a transistor to do what the points once did, repeatedly saturating the primary windings on the ignition coil with a magnetic field, collapsing the field, and producing a high voltage in the secondary coil windings.
This secondary high voltage was discharged through the coil wire to the center post on the distributor cap and then delivered to center of the ever-spinning rotor button. The spark traveled through the metal strip on the rotor and jumped out the tip, which happened to be in the vicinity of the spark plug terminal that needed the spark. In order for things to work right, the spark had (and has) to be delivered to each cylinder at the right time.
The spark plug wires have to be in the right order on the distributor cap or the engine will pop and snort, either kicking back or belching flame under the hood or with a great boom through the exhaust. Most of us have seen muffler shops reap a handsome profit from this frightening scenario.
Fine-tuning the timing was done by turning the distributor, which, in the old days, changed the time when the points opened and closed, and in the newer days, accomplished the same thing with the signal delivered by the pickup coil to the module.
Nowadays, even when a vehicle has a distributor, turning it this way and that sometimes only makes the engine run worse, because in many cases the “pickup coil” in the distributor has a different job than it once did.
On this Jeep, the signal the PCM uses to fire the coil comes from the Crank sensor.
There are three sets of four notches on the six-cylinder flywheel (V8’s have 8 notches spaced at 45 degrees, but that’s a different story). The Cam Sensor in the distributor (known as a “pickup coil” to Jack’s auto repair) tells the PCM which cylinder is #1, and the trailing edge of notch number four in each group of notches on the flywheel (read by the Crank Sensor) tells the PCM that the cylinder in question is at 4 degrees Before Top Dead Center (BTDC). Since it knows where #1 and #6 are based on Cam sensor input, it can use the Crank Sensor input figure out which cylinder is which to synchronize injector pulse.
To be honest, this info isn’t anything you’ll use very much in the field, because you can’t make adjustments to change it anyway.. Suffice it to say that, (generally speaking) when a crank sensor is present, the ignition timing can’t be adjusted by turning the distributor, even if it has one.
Thus turning the distributor on the 4.0L Jeep just fouls up the rotor alignment. Keep this in mind, because it’s an important part of the story. Earlier Jeeps (87-90) with that old bulletproof Bendix fuel injection system have a similar Crank/Cam sensor setup, but the flywheel sensor ring on those units has narrow notches all the way around the ring except three wide ones that are spaced 120 degrees apart. The principle was the same, but the older Jeep would start with the cam sensor unplugged. The 91/up Jeep 4.0L engines will run if the cam sensor is unplugged after the engine is started but they won’t start with it unplugged.
Under the Hood
This ‘93 Jeep was sitting out behind the shop where the wrecker driver had deposited it. Turning the key produced the sound of a strong battery and a healthy starter, but no fire in any of the holes. Popping the hood and looking around, I didn’t see anything out of the ordinary under there, but based on what “Jack’s” had replaced with his shotgun, I already had an idea what the problem was. The first thing I wanted to do was check for spark at the ignition coil, (it’s best not to check fuel pressure first, or you could start a fire) and just as I suspected, there was no spark at all. Some folks like to check for spark at the plugs, but I start at the coil. Obviously, if there isn’t any spark at the coil, there won’t be any at the plugs. It looked like somebody had checked for spark at the plugs, found none, and replaced the plugs and wires. When they still had no spark, they replaced the Crank Sensor, and when that didn’t produce any results, they replaced the “Pickup Coil.” This is a natural progression for a shotgunner. In some cases, they meet with success using this method, but it generally costs more in parts than it needs to.
In some cases, spark will be present at the coil tower, but it’ll be too weak to coax any fire into the cylinders, where the atmosphere is compressed and rather unfriendly to being punctured by a spark. The trick here is understanding how weak is too weak. I’ve seen more than one seasoned mechanic who was bumfuzzled to the point of being body slammed by a no-start and a weak spark. If the spark won’t jump but about a quarter of an inch, the engine probably won’t start, even though some shop manuals use that distance for a good measure. A half-inch jump is pretty good, but if it won’t jump any farther than that, I’ve seen hard cold starting as a result. While it can be tough on the ignition components to stretch the spark to the point where it looks for somewhere else to go, I like to see at least three quarters of an inch of bright blue lightning, and a comfortably loud pop.
(Important author’s note: Most of the old round oil-filled ignition coils produced from 40,000 to 60,000 volts. When the newer coils started appearing in the mid-seventies (like GM’s HEI unit), that voltage jumped to around 80,000 volts. With the advent of distributorless ignition, coil voltages have increased to over 100,000 volts. It’s not good to horse around with that sort of voltage. If it runs through your chest cavity the right way it can stop your heart, so don’t play tricks on your buddies with these systems and don’t take unnecessary chances yourself to prove how macho you are. A heart attack could ruin your whole day. It’s best to use the plastic pliers you bought from the tool guy for this exercise or perhaps one of those handy adjustable spark testers.)
Finding the Problem
Scan tool trouble codes are usually helpful, but they can be misleading, particularly on one of these Jeeps. In this case, Chrysler’s DRBII threw me a curve, and if I hadn’t been familiar with an old Jeep recall program I might have been sidetracked. The DRBII readout said “FUEL PUMP RESISTOR BYPASS RELAY CIRCUIT FAULT.” That trouble code message rang a bell in my hard drive.
As I scanned my mental files, I remembered that we had been required to disable that relay on Cherokees in the mid-‘90s, but as I write this I can’t remember why we had to do it. At any rate, if you happen to run across this code on an early ‘90’s Cherokee, don’t spend a lot of time chasing it.Bringing up the Inputs and Outputs on the DRBII screen, I found that the Jeep’s ASD (Automatic Shutdown) relay was operating normally. The ASD relay on a Chrysler product feeds battery power to the injectors, the ignition coil, and the generator (alternator) field.
If for some reason the ASD relay fails to power all that stuff up, the unit obviously won’t start. The orange wire feeding the Hall Effect Crank and Cam sensors receives an eight-volt feed from the PCM (the other sensors get 5 volts from a different place in the PCM), and if that orange feed wire is chafing against ground somewhere no signals will be sent from the Crank and Cam sensors to the PCM.
In this case, the 8 volt feed was there, because my DRB showed the Cam and Crank sensors to be operating as expected. At this point, since I had my crosshairs on the lack of ignition spark, I could have checked the primary lead on the coil with a test light while spinning the engine to see if a signal was present, but instead I plugged in a substitute coil I keep on hand and received the healthy blue pop you see in the photo. Everything seemed fine until I popped the coil wire on the new coil and tried to start the engine… I had fixed the spark (which was the original reason for the no-start), but now I had to fix the problems that had been inserted by the other hands that had been laid on this machine.
Pop, Snort, Start-and Die
With a strong coil installed, the engine immediately gave its best attempt at firing up, but bumped and snorted, then yanked itself to a stop rather abruptly, as if a pair of spark plug wires had been crossed. When Jack’s (or whoever it was that worked on it) had replaced the “pickup coil”, they had put the distributor back in place a tooth out of phase.
This distributor has a set of ears (see photo) that keep would-be do-it-yourselfers from trying to turn the distributor and set the timing. In this case, the pop-snort condition was present because the rotor alignment was out of sync and the spark was jumping to the wrong posts and firing the wrong cylinders. In order to untangle this hopscotch and get things back in line, you simply bring the timing notch up on TDC (#1 compression stroke!), then use a distributor cap that has been cut out to reveal the number one firing post.
The distributor turns clockwise, so set the distributor so that the rotor has just passed the number one post. Per Jeep instructions, I use an old distributor cap for a tool to properly index the distributor this way.
In the late ‘80’s, Jeep actually told us to hacksaw the ears off the base of the distributor and turn it until the rotor was in this position if we had an annoying skip or surge at cruise. If you set the rotor pointing straight at the post, you’ll get hard starting and an annoying surge at highway cruise because when the PCM advances the timing beyond a certain point, some or all of the spark will jump back and hit the previous post in the cap instead of the right one. Some old timers might remember the days of Ford’s EEC III system where rotor alignment had to be set with a special tool for the same reason.It only took a few minutes to set the rotor alignment and we had main engine burn on the Jeep once again. R.W.M.