I've ruled out carb problems as the source of my 'missing' under load and have been looking at the ignition system. Using the shop manuals, I've went through all the tests (module unplugged) with everything within specifications except:

• resistance wire test measures 1.7 to 2.1 ohms - spec is 0.7 to 1.7 ohms
• with the key in RUN, my battery voltage is 12.36 V and I measure 12.10 V at the red wire into the module - spec says battery voltage +- 0.1 - I read in a thread that with less than 12 volts, the modules are unstable - anyone know about this?

With the module plugged in, I measure 6.14 V between the BAT terminal of the coil and ground with the key in RUN - I read in some posts this should be between 7 and 9 V? If true, then would contribute to weaker spark, yes?

A couple of other notes:

• upon close inspection of the ignition harness wiring, I see evidence of a fire or meltdown as the insulation is gone for about 1 ft around the brake booster and a PO liberally used electrical tape for each wire and to bundle them all together - this clearly needs fixed
• there is an interior kill switch spliced into the coil BAT wire - source of increased resistance?
• the module looks new, has made in china printed on it, and has at least 800 miles on it - everything points to using only Motorcraft?

So . . . does any of this makes sense with a missing engine under load? I'd rather not start swapping out parts (module, coil, cap, rotor, ?) unless there's some hope of improvement. Plus the $ will add up fast and I'm also wondering whether just to scrap the whole system for something aftermarket. Thanks!

 

The extra resistance you're picking up is likely either contact resistance, or your meter leads. You can easily check the resistance of your bare leads. It's difficult to accurately measure sub-ohm resistance with store-bought meters under $50.

It's fine.

No, it's fine.

Fix it regardless.

That depends on where it's placed, but it shouldn't be there regardless. This signal is too sensitive to add extra impedance in this fashion.

Although it's unlikely the problem in this case, Motorcraft is the only suitable source.

Nothing you've described points to the primary side of the ignition system as a culprit. The primary side of the ignition system rarely leads to run-time or driveability issues. It either works or it doesn't. The ignition system doesn't behave any differently or go into any sort of "mode" off of idle or under load. The carburetor and mechanisms behind ignition timing, however, do. If it idles fine, stop investigating the primary side of the ignition system.

That's not going to solve anything.

How have you ruled out the carburetor as the culprit? That's the cause 85% of the time. The other 10% are vacuum leaks, with the other 5% ignition timing. Notice how the primary side of the ignition system is not on the list. The secondary side can be (plugs, wires, cap, rotor) - if these haven't been replaced in some time, you should replace them anyway simply for tune-up purposes. But not because I think it's the problem. The fact that you're having the issue you're having (and knowing the causes and statistics behind it) means you probably actually haven't ruled out the carburetor. I'm not sure one could ever really say they have ruled it out anyway; you can't easily take black and white measurements like you can elsewhere. The only way you can really rule it out is to not have any problems.

 

I would start by replacing the burnt section of wiring. Once the copper has been exposed, moisture/corrosion sets in and may be affecting your readings. Remove the kill switch if not really needed. Do the resistance check on your distributor pickup. 400-700 ohms between orange and purple wires with it unplugged. The Chinese boxes are not known to last very long, but I would try the other things first.
Edit: Never mind. fmc400 has you covered.

 

Thanks for the fast responses. As for the carb side, it's more that I've exhausted any ideas on what to do there. That history is over in a thread called 'Holley 4160 on 351m ...' - if there's something I missed there, I'm all ears.

The plugs are new, but as you say it makes sense to replace some of the other parts. Out of curiosity, the wires measure around 6 kohm which is in spec, but I assume they could still be a problem in delivering the high voltage pulse? I'll definitely pull the kill switch - that will extract about 12 feet of wire and what looks like a 120 VAC switch. And I'll rewire the bad part of the harness. Also the pickup resistance is ~620 ohms which seems ok. I did see a post about spinning to check output - maybe I'll give that a try if all else fails to have an impact.

 

In looking at prices for replacing some of the ignition components with decent quality, the $ easily approaches $100 - cap, rotor, ignition module, coil, etc. if I end up going down this road. Rather than spending the time experimenting with this and fixing all the wiring, what about just replacing it all with something like this? It's $169 and also eliminates the open question of whether I need to replace my current vacuum advance which is additional money.

http://broncograveyard.com/bronco/i-...black-cap.html

 

While you seem to have done a pretty thorough job in your other thread, there really is zero reason to spend money on the ignition system - other than standard tune-up parts (plugs, wires, cap, rotor). All of your measurements with regard to the ignition system are perfectly normal. The ignition system seems fine, pointing to a fuel delivery problem; but the fuel system is fine, pointing to an ignition problem. One of your observations is misleading you. The ignition system is pretty cut and dry; there's not much finesse or mystery, so it has to be something hidden or missed with ignition timing, vacuum, or carburetion. While I understand you were lead here and started this thread because you sought clarification on some minor deviations between your observations and specified guidelines, the difference you're seeing is in the noise and explainable. I wouldn't spend any more time on ignition modules and coils than I would take Pepto Bismol for a headache.

There's certainly no harm in the HEI conversion you've highlighted, other than wasted money. HEI and Duraspark II use the exact same technology, so there's little to be gained. It's nice having everything in one place, but now you're out of luck if you're stranded and have to walk to a parts store that only carries common OEM parts. It's also something extra you have to wire up.

 

On the basic system. I carry a spare Motorcraft brand ignition box from Rock Auto behind the seat in case my original installed in 1978 ever fails. It has not yet. I think its a robust system. Its also nice to know I can find most anything I might need at a parts store anywhere in a pinch.

As a general guideline, with the problem you are experiencing, first step would be a basic "tuneup" if not done recently. Plugs, wires, cap, rotor. It matters a lot and is relatively inexpensive. Also fix the wiring and eliminate the kill switch. It would be a shame to buy a new ignition system only to find the problem persists. If you use the better grade parts store stuff it should work fine. Stay away from the "value" brand. On wires I like to use the good stuff... they last longer.

On mine, I had clear carb problems. A rebuilt carb fixed it 80% but still not quite right. I did the basic tuneup mentioned above and it was 100%. The last tuneup was not many miles past... but many years as the truck was not used much. I think the wires had started to break down with age.

 

Missing under load could be caused by weak spark - not hot enough to ignite heavier fuel mixture. Chased this problem for a long time until I found a loose plug wire (when it sparked me instead of the plug).

Sometimes it's something simple.

 

fmc400 - your points are well-taken. Right now it's appealing to think about swapping out the entire system versus experimenting with piece parts trying to find the problem (under the assumption it's in the ignition).

The tuneup suggestion is good - I'll swap out those parts as it's not much cost and makes sense to do anyhow.

I keep thinking there's something simple I'm missing. That most frustrating part is it seems to run just fine in neutral (or clutch in), but misses under load while moving with not much throttle open. But acceleration from a stop is fine.

I took out the kill switch and replaced both the damaged wires to the coil. No difference at all. Rats.

A couple of questions/observations:

The distributor shaft moves up and down about 1/8" - the spec is much lower than this - any impact here?

The truck runs better cold than hot

Overall, the problem seems to be getting worse. I'll make some minor change and it seems to help initially, but the behavior seems not different when it appeared a month ago - even though I've fixed many things.

To the touch, the coil is pretty hot. Should I expect this? Might it's secondary output be degrading? Consistent with worse at operating temperature than cold?

Right now, the only thing that makes sense any more is weak ignition and/or I'm still running lean. I'll pull the plugs tomorrow and take a look for the later.

Thanks again for all the comments and help - this site/forum has been such a great resource since I got the truck and gives me hope that I'll get this fixed!

 

The coil dissipates power on its own, and is bolted to the engine. It will get hot.

The electronic components of the ignition system have no sense of load on the engine. Ignition timing, however, does (vacuum advance). I'll defer to someone else with regard to your question about distributor shaft play. But you can rule out everything upstream of the coil.

 

Thanks. I'll say it's ok then - it's hot, but not so much that I can't touch it.

Regarding the timing, I was thinking about this on the way home. I'll put the vacuum gauge back on and see how it varies between my no-load/load states when I see the problem. I think I'll also plug the vacuum advance and see what effect that has.

 

I found that the rotor contact in the cap was almost gone so I replaced both the cap and rotor - no impact.

I also tried another coil - no impact.

At this point, I'm thinking the ignition system is in good shape unless I'm missing something with the plug wires. However they're within spec when I measure the DC resistance.

Working down a list of suggestions from another thread, I switched from ported to manifold vacuum for the advance. This didn't seem to make any real difference with my fundamental 'missing/vibrating' problem off-idle during cruise. There were subtle differences, but these are probably in the tuning realm if I ever get the bigger issue resolved.

Guess I'll go ponder the carb again and see what I can experiment with . . . probably should go back to the #65 jets (I have #63 in now) and get back to the 'out of the box' set up. Arrggh.

 

It could well be a lean condition causing your problem... that said it could still be wires. Resistance across the wires matters... also resistance to ground or another wire. This would likely be in Mega ohm and I don't know of a good way to measure. I know mine was sensitive to wires... low mileage but 5-6 years old wires didn't work so well. A fresh set of wires (Bosch spiral core) plus low cost Autolight conventional plugs got mine to 100%. Still worth finishing the "tune up" with plugs and wires I think.

 

The wires are on my list (also just replaced the plugs). How did you end up choosing the Bosch spiral core? There are a surprising number of wire choices out there.

 

Unfortunately I haven't had much time to work on the truck (and hence no posts), but I finally got to the bottom of all this. After checking and rechecking everything with the carb and with the ignition including rebuilding/replacing most of the components, I searched more broadly for vibration problems. Long story short, I investigated the u-joints and found one of the bearing caps with no needle bearings and a just bit of orange dust left. I replaced the u-joints and the vibration is gone!!!!!!

So, what have I learned after this experience and a tremendous amount of internet searching? The problem I was experiencing - off-idle vibration under cruise a low RPMs - is a potential symptom, in general, of lean carburation causing missing, weak ignition causing missing, or driveline problems such as bad u-joints or driveshaft imbalance. For me, I found issues in all areas that combined to exacerbate me solving the problem:

Carburation:

• From my old carb experience, I assumed my new carb was running rich, which was consistent with the Holley manual stating I should drop jet sizes due to my altitude. So I went from #65 to #61 (all the shop had). This created the lean condition (and started me down the ignition path) as I know have #67 jets and the engine runs smooth and the key item is the plugs finally have a light brown porcelain compared to snow white previously.
• In parallel, my old stock fuel pump was putting out too much pressure which forced me to lower the floats too much which I think compounded the lean issue. A pressure regulator fixed this which provided some improvement.
• My clear fuel filter by the carb would go completely empty from time to time. I replaced the fuel pump, but that didn't solve it. I tightened all the hoses and replaced a few - that didn't solve it. Finally, I went to see where the tank vent went - I turned the gas cap and heard a slight swoosh of air. Inside I could see the vent tube. Evidently it was pulling a vacuum (probably worse since I tightened all the hoses) on the input side which the pump couldn't over come explaining disappearance of the fuel. Removing the cap had fuel filling the filter again.

Ignition:

• Heading down the weak ignition path, I made many measurements and found some specific issues in the wiring and the cap and rotor. These changes made some improvement, but I had I not found these, I imagine I would be stranded on the side of the road in the future.
• I also found my vacuum advance wasn't working and I had blindly hooked it up to the ported vacuum as on the previous carb. Switching to manifold vacuum and setting the timing with more advance helped as well.

Driveline:

• All the above helped, but didn't fully solve the issue. Running out of ideas, I started searching the symptoms rather than what I thought might be the cause. In a few website forums, I found others that were going down this road and had success.
• Wiggling the driveshaft by hand, I could feel a bit of motion, but I had no idea one bearing cap was completely devoid of needle bearings and another had considerable play. Fortunately, I was able to replace these in a few hours.
• Working on the u-joints, I noticed a fare bit of oil coming out of the differential front seal. Thinking I needed to replace this as well, I did some searching and found the problem could be a plugged vent - sure enough, mine was. We'll see if this helps.

Overall, this was a somewhat frustrating experience these past few months but I learned SO MUCH from advice here and other FTE posts. Thank you all! Hopefully others out there will find this helpful.


Joel