With the advent of the latest OBD II systems, most vehicles have a cam and a crankshaft sensor. Most are distributorless ignition, but some still use a distributor.

The engines that use a cam and crank sensor with a conventional distributor housing and distributor cap, I call "traditional" engines.

Examples of traditional engines are the Jeep 4.0L, the Chevrolet V6 and V8s and the Dodge V6 and V8s. There may be more, but these are the most common.

These engines still use a secondary distributor cap and rotor to distribute the spark to the plugs. This I call issue No. 1, with the position of the distributor mounted camshaft sensor as issue No. 2.

Issue No. 2 seems to cause us the most problems as it must be exact, plus or minus very little as the cam sensor often signals the firing of not only cylinder one on the engine, but in many cases also the firing of the fuel injector sequence.

A lot of horsepower has been lost on the older Buick V6 Turbo Grand Nationals because the engine builder didn't set the sequence of the distributor shaft mounted cam sensor, and injectors were fired too early or too late!

The first vehicle case study was picked up from an ASA meeting one evening as a fellow shop owner discussed the Jeep that no one could start after an engine replacement.

It seems as if shop "A" had replaced the engine on a 1991 Jeep Cherokee with the 4.0L in-line six-cylinder engine. Upon replacement, the engine would not start and would spark twice and then go to a no-spark condition. After some time, the vehicle was towed to an area Chrysler dealer to have them start the vehicle. A day later, shop "A" was called to pick up the vehicle and was told it still would not start. They believed the flywheel was damaged when the engine was replaced. The owner could either let them replace the flywheel or pick up the vehicle. The vehicle was towed back to shop "A" where the flywheel was replaced with a new unit and with the same results.

Shop "A" then sent the vehicle to another area Chrysler dealer for another opinion with almost the same results. The vehicle was looked at and a repair estimate was put together to include ECM/PCM replacement.

Being somewhat skeptical, the vehicle was picked up and towed to the local "electrical wizard" (appropriately called because the shop is way above average!) for another opinion. After some parts replacement and testing, the opinion was the same. Everyone seemed to know the problem, but no one knew how to fix it.

Listening to this discussion at the area ASA meeting, I committed the cardinal sin of automotive repair. I agreed to have the vehicle dropped off at our shop just for the bragging rights and emotional thrill of fixing this vehicle, as there couldn't have been any money left in this repair! We all do this from time to time - don't we?

Our testing showed that the vehicle would spark two times and then shut off all spark, just as the shops involved told us.

Knowing everyone involved was of above average caliber we reluctantly rechecked everything involved. The distributor seemed to be timed correctly, injectors were spraying fuel, and it appeared that the vehicle anti-theft system (VATS) wasn't causing the problem. As the vehicle came to us with three extra ECMs in the front seat, we ruled out the ECM as being the problem.

Using the Chrysler Jeep training manual we viewed the overall operation of the system to make this engine run. I feel that this is the missing ingredient in many repairs. Understand the system you are working on!

It appeared as if the engine needed the crankshaft sensor signal (confirmed to be good with lab scope from rear of engine) and also the cam sensor (distributor mounted and also confirmed to be good).

Upon viewing the Chrysler manual timing schematic, it also appeared that the two signals must be in correct order.

At this point we hooked up the Simu-tech to view all these units at the same time and compare to the book (see Figure 2). Ah - ha! Now we knew what was wrong!

The cam-to-crank sensor relationship looked nothing like the book had shown in Figure 1. Either the chain was put in wrong or the distributor was installed incorrectly. Having previously checked valve timing and compression (or at least someone said they did) we looked to the distributor as the culprit. Upon cranking and turning distributor housing a dozen different ways and rechecking our positioning many times, we were no better off than before. Still two sparks and then none!

Just to check our story out, we ran another ignition system from a simulator to the Jeep's distributor cap center coil terminal, and the engine did start. (Again confirming the engine condition was OK.)

Remembering a similar vehicle, our ALLDATA and Mitchell systems were again consulted (we had checked early in the game), as well as the iATN archives, for TSBs or anything that we could use. No exact situations were found!

The real breakthrough came when another customer called in for some help with a vehicle and I asked if he had ever had a Jeep problem like this? He said not exactly but remembered something in the Snap-on Troubleshooter manual concerning this exact problem.

Off through the shop to find this manual and there on page J3 was the distributor index procedure that we were looking for!

The problem was that this bulletin was for a 1988 vehicle, bulletin #18-53-88! That explains why both our information systems and lookups had missed the culprit! It didn't appear that either of us looked for an older vehicle! Lesson learned.

Now would this indexing actually fix our Jeep? Yes, it would!

We quickly hacksawed the hold-down bracket off and cut a window in a distributor cap (see Figure 3) and installed the distributor in the engine. Ah-ha! The engine starts and runs!

But why would you ever need to perform such a fix on a vehicle and why is it that critical?

Further testing showed that someone in the group had replaced the cam sensor in the many repairs this vehicle had over time. Ever notice how distributor gears have an odd number of teeth? With an odd number of teeth, it is impossible to install a distributor 180 degrees off. Think about it; you could be off 175 or maybe 185, but NOT 180 degrees - 180 out is a myth unless you're an old Chrysler guy! (Sorry guys, but I remember those days!) That seemed to be the problem with the index.

Why is it that critical to be plus or minus .020 on rotor index?

The answer is shown in Figure 4 with the Simu-tech waveforms. Everything must happen exactly in order for this engine to run. With the key on :

1. Pin # 8 sees voltage change from ASD relay.
2. Dst Sync (cam sensor) MUST be seen prior to crank signal.
3. Crank signal is x amount of degrees after cam signal.
4. Primary ignition signal is seen.
5. Injector signal is turned on.
6. Once in order, the PCM sees everyone is OK and allows the engine to run. Any other sequence and the PCM shuts down after two sparks!

Distributor Indexing Tip
Caution:
A few of the "traditional" engines still using a distributor are also using a crank-cam shaft position sensor. Some of these engines use a positive distributor position notch or clamp that enables the distributor to be placed in only one position, while others use a method of indexing to verify the distributor position.

A misindexed distributor may exhibit surging, light bucking or intermittent engine misfiring. This will most likely occur when the vehicle is at operating temperature and under a light load at approximately 2,000 rpm. This condition may be caused by a misindexed distributor.

The factory procedure must be used as outlined in the service manual. Some of these may be indexed with a scan tool, while others require the use of a volt meter to verify position.

Jeep, late-model Chrysler V-6 and V-8s, and some GM trucks require indexing.

Jim Linder is owner of Linder Technical Services, an automotive technician support facility in Indianapolis. For more information on these topics, call (317) 487-9460, or visit his Web site at http://lindertech.com.

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