1991 Ford F150 - 5.0L No Spark

[autoxperts]

the customer said it died while driving. so i check module it was bad , so i put test dist.in that had new module and pickup and still no spark.so i put test coil still no spark. I have power to everything ,i have traced all wires, i have power into the coil and power out , just no pulse. the injectors are pulsing ,also customer had changed ignition switch. the ground system is good.

[Tim Martin]

Boy, we used to work on alot of them over the years. If I recall correctly, the PCM controls the minus side of the coil to create the pulse. Assuming that the ignition coil pickup and the module are functioning properly, I would have to say that the PCM is faulty and in need of replacement.

[steven kiser]

With the information provided i would lean towards a pcm also. I had one kicker that drove me nuts and i found the wires at the data link (one or more ports have dual wires) had corroded internally. I believe that the link actually is the ground feed for the pcm. Make sure the module is powering up and the eek relay is ok. It's been a while but i remember having huge issues with the two relays (main or eek and the fuel pump relay) on the drivers inner fender skirt. I stocked them along with the plugs. A lot of corrosion from salt on the roads. Another issue i would run into is the inner skirt (drivers side) chafing through the harness.

No stupid ignition kill in this one is there? Had a few of those. Look closely at the harness for the ignition switch and see if it's been taped over.

[Jeff @ Able Auto]

What is fuel pressure? Possible that the inertia switch has failed. Was the customer driving and quit or were they stopped somewhere and came out to find it a no start? A slight bump in a parking lot can trip these.
I personally drive a 1992 F150 (290000K) and for whatever reason seems every 5-6 years the ecm craps out so it is a possibility.

[Tim Martin]

I thought of fuel pressure also and I would advise checking that. I suppose I was thinking that if there is a fuel injector pulse, then an obvious problem would likely be the PCM.

[Jeff @ Able Auto]

SORRY..I didnt read the post throughly...no spark.

[autoxperts]

the module does have power,the eek relay is good. i had a good computer the numbers match tried it and still same. the data link connector has no corrosion. .

[steven kiser]

There has to be an interruption somewhere. A broken wire, corroded plug, etc. This calls for a Ford diagnostic tool called a wiggle test. I was shocked when i saw this for the first time as a ford approved diagnostic procedure.
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I have an old remote crank set up that is a push button with 8' leads that clip onto the solenoid and a push button to activate it. With the key on and button pushed cranking the engine over "wiggle" the harnesses and see if it kicks. This is a basic TFI or hall effect system that fires on negative impulse. You can remove the trigger wire from the coil and using a test lead connected to the post and dragging the other end along a grounded surface trigger the coil. Power on both sides of the coil means there is no impulse reaching it and i've had issues with the wire rotting internally on the pcm side of the coil. This could be an issue of some hammer head probing the harness and not sealing the wire causing it to rot internally. Look for swollen areas in the wires. Look for non oem taping and investigate. I've seen areas that were attacked with scotch locks and taped over and when exposed it looked like a meat cleaver was used.

The ignition switch should be tested real quick making sure it has power on the run side. Sometimes when testing the pcm for power i've mistook the kam feed for the power one. Take a good look at the plug for the ignition harness that feeds the dist.

[Tim Martin]

Ok. So we have the ability of the engine to crank over. We have injector pulse which indicates the PCM recognizes engine rotation. Already we have ruled out alot of the possibilities. Assuming there is fuel pressure and good compression, we still could have a bad ignition module. But if that all is good, I am inclined to be agreeing with steve. In looking at a wiring diagram for this application, I see what they call an 'Ignition Suppression Resistor' and am wondering if there is a problem with it. I have not seen one of these go bad or open that I recall but I suppose anything is possible. One thing I would do is to ohms check the wiring from the PCM to the minus side of the coil.

Hey! Is this one of those that has a tachometer in the dash? What is the possibility that is giving some grief and causing this? It is on the same circuit as the minus side of the ignition coil. Have seen this in the past.

[autoxperts]

already ohm wire from pcm to - on coil its good.i"ve checked power at run on ignition its good and no it does not have a tach on the dash. does have fuel pressure,i"ve never had this much problem before, but this one has throwed me for loop

[brother bubba]

...

Here's some oddballs I've run into ....

1. Power to the EEC power relay and a bunch of other stuff goes through a seperate contact in the ignition switch in the run and crank positions (seperate from the actual crank signal). Check for power at the coil in the crank postion.

2. The injectors pulse so this tells me the hall effect sensor is working and the signal is being passed to the computer. This does not mean the section of the TFI module that controls the coil is working properly. Possible bad ignition module. New dont mean good. I have a known good distributor (painted red) that I can plug in to the harness, ground to the engine, spin and observe spark.

3. The ground for the distributor is where the distributor bolts to the block. See if you measure a voltage drop between the distributor and ground.

4. I have seen the contacts at the coil corrode and/or work loose in the connector at the coil. Backprobing showed good power at the coil.

Hope this helps

...

[steven kiser]

With the scanner attached are you seeing rpm and spark advance? If not seeing spark advance input or pcm is bad. Another possibility is that the ground to the pcm is faulty so even if the pcm is calling for a trigger it's not there because of the ground.

THINGS I'VE SEEN THAT SET ME ON THE WRONG ROAD

Missing fuses so even if all test ok the one that's missing is causing the problem. Forgotten kill switches. Embarrassed customers withholding information. Distributors that aren't spinning. Assumed known good parts failing. Assuming the "new" part is good (i see a lot of "new" parts for older vehicles that have made the rounds) and are junk. Common mistakes such as not grounding the distributor when you plug the "good" one in. Confusing ohm readings on a wire when there is a common ground throwing you off. Not using a bar graph on my multi meter when trying to read an impulse. Not seeing the forest through the trees. Overcomplicating the heck out of something.

I learned a long time ago that new doesn't mean good. I installed a battery in a vehicle because it had an internal short and a week later the car came back in with the same issue. I spent days looking for an intermittent short or draw. Thought the automatic locks were acting up, etc. I had everything unplugged and still it would die overnight two or three times a week. I never thought of testing the "new" battery and on a whim i did and it was junk.

[jbadenoch]

90 percent of the time I find its the ignition module, wouldn't hurt to try one more

[Tim Martin]

90 percent of the time I find its the ignition module, wouldn't hurt to try one more

I have an ignition module tester and we test even new ones before installation. Brand new out of the box some do not work, even national brands. Amazing!

Bottom Line: New does not guarantee that it works.

[steven kiser]

Just took a quick look and there is a fuseable link that feeds the pcm. Hot at all times. I'm not 100% but there may be a link grouping near the battery and if the battery is loose could wear through and short. I've run into this before.

[Tim Martin]

Just took a quick look and there is a fuseable link that feeds the pcm. Hot at all times. I'm not 100% but there may be a link grouping near the battery and if the battery is loose could wear through and short. I've run into this before.

Well steve, I didn't look at a wiring diagram but I woulda thought that if the fusible link to the PCM was open then there wouldn't be any fuel injector pulse. But you do bring up a very valid point about the fusible links on these vehicles. All of us have seen melted or corroded fusible links that have really given us a run for our money.

[steven kiser]

I'm not sure of the internal workings of the pcm but i think the constant power would be the k.a.m. feed. The power fed by the key through the eek relay would be the main feed. Again i'm by no means an expert in the internal workings of the pcm but if the k.a.m. is trashed it just may kill the spark. In the older vehicles I think (again i'm no expert on the internal workings) the fuel impulse wasn't an adjustable issue so it just fired. However the ignition was so the memory would be an issue. I know on the older G.M' s there were replaceable calibration chips that could kill one or both of the systems.

I would love to be able to delve into the internal workings of the electronics of vehicles but don't have the patience. I have hands like feet and am limited in the finer hand motor workings. Back probing at my level is like trying to jamb a tomato stake into a hook eye. It's not going to happen and the frustration overtakes my better judgment. I know of techs that could and still open a module and spend time figuring out the workings and repair it. That's beyond my scope. I trace the issue and if it's at a module it gets replaced.