Hello all,

I understood points and condenser ignitions (at least well enough to know what to replace), but I need help on this one. I live in "bush" Alaska where I have to fix my own problems, so I could really use some help here on the web!

My truck is a 1992 multi-port fuel injected 318 V8 w/ 4-wheel drive and auto transmission...

I have had the truck for nearly 10 years and have never had an issue with starting or running...

A couple of days ago it started immediately and drove normally for several miles. I was going down the road at about 40 mph when it suddenly lost engine power as if the key had been turned off. I lost engine power only; electrical power to the instruments appeared to be fine...

I have performed several tests, and here is what I have found -

I have no spark at the plugs, and I am pretty sure I have no output at the coil (my test tools are limited)...

Resistance tests of the coil are outside of spec, but not by much...

Here is what I have found with regards to coil input-

At the feed wire, I connected a test light to battery ground. The light is initially off. I switch the key to run position. The light comes on for about 1 sec, then goes off. I crank the engine and the light comes back on for 2 or 3 seconds then goes back off (engine cranking entire time)...

At the signal wire, I connect test light to battery positive. The light is initially on. I switch key to run position. The light goes off for about 1 second, then comes back on. I crank the engine and the light goes back off briefly, then comes back on and appears to stay on solidly rather than alternate on off to discharge the coil's secondary circuit (engine cranking the entire time)...

From what I have read here and on other sites, my ASD relay must be working, or I would get no power at the coil input. My crank sensor must be working, or the ASD relay would never send said power to the coil. There must be no fault between the coil wires and the PCM, or I would not observe these behaviors at the feed and signal wires. Reason tells me same for the splice under the PDC mentioned in the FAQs..

One site I read seems to condemn my PCM at this point. Can anyone think of anything else that I should be testing?

 

The PCM is doing what it's supposed to at the ignition coil. You'd get the same result at the ASD relay, injectors and fuel pump relay.

Consider what happens when you crank the engine:

The PCM furnishes power to all the above by energizing the ASD relay (furnishing it ground) for about two seconds while you crank. Meanwhile, it looks for a signal from the cam shaft position sensor(CPS) in the distributor (aka; distributor pick-up).
If no signal is present indicating engine rotation, the PCM lifts ground from the ASD relay, effectively making your truck a paper weight with pretty lights, horn, a radio and a starter motor.
Your CPS could be bad, or the crankshaft sensor could be bad, dragging down the signal from the CPS. This is because they both use the same 8 Volt power from the PCM. The CPS is the disk shaped device with wires coming out of it under the rotor of the distributor. The Crankshaft sensor is at the top rear of the block on the passenger side. The signal from the CPS goes to the PCM on pin 44 of the 60 pin PCM connector, the Crankshaft signal is on pin 24 and the power for both of these emanates from pin 7 of the PCM (8 Volts) via splice K7 (probably in the wiring harness that's part of the mess behind the throttle body).

You state you saw power for two seconds, this tells me the PCM is doing its thing. From the FSM Ignition Systems Section, page 4: "Battery voltage is supplied to the ignition coil positive terminal from the ASD relay. The Powertrain Control Module opens and closes the ignition coil ground circuit for ignition coil operation." If, indeed, you have the proper conditions for coil operation and no spark during those initial two seconds, why not suspect the coil? People seem mighty quick to conclude the PCM is the cause of their problems. They replace it, then just as quickly conclude the expensive A$$ replacement is bad. I have an idea, maybe the problem lies elsewhere! How 'bout doing the ol' "three turns of the key" thing to get the PCM to return any fault codes via a blinking "check engine" lamp. Code 12 means the battery's been disconnected recently and "55" means its finished saying its piece. While its true that many of the things we're considering don't generate a code and bearing in mind that disconnecting the battery obliterates any info concerning existing faults until the PCM detects them again, the bottom line I present to you is; if the PCM talks to you, its probably just fine. The present mystery is why you appear to have lost one or more of the big three (fuel, spark, air) while under weigh. "The Splice" could do that. Also keep in mind that the relays in the PDC can be swapped. If the fault travels with the relay, then there you go! Anyway, once I eliminated the "Splice" from consideration, I'd be inclined to just suck it up and replace the CPS and/or the coil. If you had a fuel pressure gauge, the fuel relay was operational and any of this theorizing was correct, you'd see the fuel pressure rise for a couple seconds, then fall. Come to think of it, what about the fuel pump? If that quit, it would put you out of business. You can have a helper turn the key while you listen at the filler port, you should hear a whirr. A recently failed pump can often be coaxed to reveal itself by striking the tank with a soft hammer.

To recap:
The ASD relay will always energize, regardless of whether the Camshaft Sensor (not Crankshaft Sensor) is in working order for two seconds.
Does the coil spark at all, even for two seconds?
Does the fuel pump run at all?
Does the PCM respond correctly to a fault query?
If things come to a halt after two seconds of cranking and the PCM is responsive, suspect the CPS.

If it were I up there in the bush, counting on a twenty year old truck getting me around, a fuel pressure gauge and OBD1 reader would be on my shopping list. I'd also probably end up going on ebay and investing in a FSM and the Powertrain Diagnostic Procedures (just sayin'). Finally, I'll point out that in a truck of this vintage, more than one fault can exist simultaneously and complicate the bejeebers out of your troubleshooting task.

 

Yes, I have come to the same conclusion that the PCM is doing what it is supposed to. However, no, there is never any spark generated during cranking, nor does the signal wire to the coil behave in such a way the coil would produce a spark even if it wanted to...

I had tried swapping the relays in the PDC to no effect...

I found another older post by a guy with the exact symptoms as mine and his was resolved by replacement of the hall effect sensor...

I believe this is my issue as well, as it explains all the behaviors I am seeing with regards to the coil feed and signal wires. I ordered one yesterday after determining that I have the signal generating switch type rather than the hall effect pick-up...

Thank you for your writeup. It includes some information I had not yet discovered. I have a manual, but it is vague, incomplete, and near worthless with regards to this type of an issue. A friend loaned me his reader, but it is an OBD2 reader (where is the connection for the readers anyway?). I will go out and try the 3 turns of the key thing now and see what that does...

 

The OBD1 connector is a proprietary Dodge style with six slots arraigned in a sort of "L" shaped black or grey plastic receptacle dangling from the wire harness inside the engine compartment on the passenger side. It's usually right below the blower resistor located on the firewall. If you get hold of an OBD1 reader, it should come with several connectors, one of which'll be a Dodge. Just select the one that matches the receptacle.
The key technique is Key On, Engine Off (KOEO) three times. The third time, if done correctly, you'll see the "check engine" lamp flash. All OBD1 codes are two digits long, "55" means the PCM is done.

Yeah, the CPS is a hall effect device on my '93. Hall effect refers to the fact that old school bipolar transistors turn on when a magnetic field sweeps through the base of the transistor, inducing a current. Base current equals a turned on transistor. I am not familiar with the "signal generating type".

Concerning the coil, if you hook up battery power via a jumper and momentarily ground the other post of the primary, a spark should be generated by a good coil. You can also bypass the ASD points with a jumper. As I recall, this'll energize the coil and fuel pump. The coil still needs ground provided by you or the PCM to spark. The fuel pump will run for as long as the relay is bypassed, however, since ground is hard wired to it.

What do you have to say about the fuel pump?

 

You have probably long forgotten me, but thank you for your suggestions. You can probably tell by the passing of time, but I do not rely on my truck for general transportation. It is nice to have when I need to haul something, and it is my only 4-wheeled vehicle, but I go for weeks without even starting it even when it is running. My primary mode of transport here is bicycle; it is good for me and more importantly, it is free in a plce where gasoline is about $7 a gallon right now...

Anyway, ss of this afternoon, my truck is back to running. In case it helps anyone else with the same issue, I will answer your questions and make some notes below...

Yep, it is there. I found the purchase price of a reader to not be worth it to me, though. I was still pretty sure I could work this out with a test light and my DVM...

I was unable to get anywhere with this method. There are two possibilities of why not - 1) my ignition switch is so worn out I am not sure I could activate the switch fast enough to cause the code to flash, and 2) none of the lights in that lower part of my dash show up very well; all I can see illuminate of the "check engine" light is the word "check;" maybe check is flashing, but I cannot see it...

The signal generator is cheap and easy to replace, but it did not fix my issue. Given the architecture of the generator and the components that it interfaces with inside the distributor, I could not see how it could possibly control firing of an eight cylinder engine; I later learned that it in fact does not (I still don't really know what it does do).

I never did really test the coil as 1) I had put in a new one when I found resistance through it to be out of spec and 2) I had later determined that I was not getting a fire signal to it anyway...

I dumped the fuel pressure by depressing the valve at the pressure gauge port with a small implement. After cranking briefly, I again purged fuel at that port. I thus determined I had an operable fuel pump (of course this did not tell me whether I had proper fuel pressure).

So, following your and other threads' advice, I tore my wiring apart down to the splices. I found them to be fully operational. While I had the PDB apart, I probed it from underneath with a test light and verified that my relays were all operating and that power was passing back out the output side of the fuses...

Through more forum reading I found evidence that the CPS is what signals actuating of the circuit that tells the computer to fire the coil. Given that that fire signal seems to be missing, I ordered a CPS. When I took the old one out today, here is what I found: The mount of the old one had broken long ago (years maybe) and the sensor end was laying loose in its receptacle, held in place only by the flywheel itself. The flywheel finally wore through its bitter end, exposing and disabling its working innards. Upon replacement with a new one, my truck fired right up and runs as well as ever...

Thanks again for trying to help me. Take care...