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  #1  
Old 04-27-2004, 02:19 PM
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Default One Spark Key OFF -> ON

85 CJ7 258 CID TFI Upgrade

I get no spark from the coil when cranking, but a single spark when the key is turned OFF.

The IGN MOD, Coil, Solenoid are new. Cranks fine. I'm getting 12.30V to the coil in ON, and 10.16V under load. Any ideas on the one spark of death??

I'm getting ready to swap the IGN SWitch even though it cranks fine, and I've got hot on RED in On and hot on BLue in Run, going in to the IGN MOD, and the IGN MOD is properly grounded. Also IGN LPS fuse is good.
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  #2  
Old 04-27-2004, 03:18 PM
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Default Re: One Spark Key OFF -> ON

1. Please fill in your profile.
2. I assume that this is immediately post TFI upgrade.
3. Who's TFI?
Off the cuff.. sounds like the ignition module.
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Old 04-27-2004, 04:21 PM
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Default Re: One Spark Key OFF -> ON

No this was just prior to the Upgrade. I got a bit crazy and went and got the parts necessary, just in case... But no.

Both the IGN MOD I took off and the one I replaced it with bench test fine. It has ground through the BLK wire and the case.

TFI upgrade is installing Motorcraft (FORD) coil, Cap, Rotor, and necessary guts. Much hotter spark and better distribution. Again, this was prior to the upgrade.

I hate profiles. If I want you to email or IM me, I'll let you know. [img]images/graemlins/givemebeer.gif[/img]
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Old 04-27-2004, 07:11 PM
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Default Re: One Spark Key OFF -> ON

Obviously the coil's getting power during key on and cranking (I assume "under load" means cranking.)

Check the <nobr>pickup</nobr> coil - orange and purple wires - use a voltmeter across them while disconnected. Crank, should see at least 1 volt AC. On DC the meter will bounce all over too.
Digital won't work, must be analog - meter type.
Those orange and purple wires bend every time the vac advance moves - hundreds of times every mile - common for them to break inside.

And, the black wire is ground -- from the module it goes into the pick-up coil - through the rubber feed-through to a grounding lug. Check with an ohmmeter to make sure it's fine. Ford had tons of trouble with the black wire to lug connection. It's crimped, the solvents in the rubber corrode the connection. Easy fix, cut the wire and ground it on the vac advance diaphram.

You'll find using the TFI coil with the original Module the coil will put out the same as the old one. That coil's designed to be used WITHOUT an ignition resistor. Check coil output with a scope, you'll see.
If you eliminate the ignition resistor, the coil and module will get an early grave -- the module's not designed to take that kind of current, and the coil's not designed to take that long a duty cycle.

But -- Often times doing the "upgrade" makes it run better - because something else was wrong before, and it got replaced in the so-called "upgrade."

You can't arbitrarily mix and match parts without knowledge of how they work --- unless you "Get "Lucky."
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Old 04-27-2004, 10:48 PM
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Default Re: One Spark Key OFF -> ON

Well thanks for the feedback. The distributor pickup and, well, the distributor altogether, should be taken out of the loop when you spark test the coil wire to ground, but what the hell do I know? [img]images/graemlins/smile.gif[/img] anyway, I'll gladly check the pickup wiring as well.

Many many people report no problems with the implementation of the TFI ecoil setup without the inline resistor. In fact, I can't find a write up that recommends retaining the stock resistor or installing a new one. Who knew? Though your insight on the coil output with the stock IGN MOD is very interesting.

The village idiot approach to randomly upgrading (yes upgrading) the ignition components to support TFI was made under duress, and, fortunately is simple enough for even a simple minded village idiot like myself to install and troubleshoot, but the upgrade has nothing to do with the original problem, soooo.....

Thanks, I'll check up on the distributor advance.
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Old 04-28-2004, 12:15 AM
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Default Re: One Spark Key OFF -> ON

I forgot to explain the "why."
Normally when you turn the key on, the module goes into the "on" mode - just like the points were closed. The coil charges up like that. The "on" mode of the module is essentially at ground potential, so current flows through the coil, charging it. When you turn the key off, the module goes "dead" or "open" or "off" - current no longer flows through the coil - magnetic field collapses in the coil, creating the secondary spark. (Normally running the module's switching from "on" to "off," creating the sparks.)

That's why I suspect the pick-up coil - or the wires from it. The module is going to default, "on." Usually a burned module is "open"or "off." But, it could be shorted, so that's a 90% rule.

Checking the pick-up coil - making sure the pulses are getting to the module, and voltage is there, == bad module.


The TFI system has a higher available voltage, about 40KV - a higher amperage output than conventional systems (thicker spark,) and at the lower voltages required by a plug (8-18KV) it has a much longer spark duration. A typical conventional ignition, (points or most electronic ignitions) have a spark duration of about .75 milliseconds. The TFI, Duraspark II, and the GM HEI typically have a duration of about 2.5 milliseconds. The longer burn time does a better job of getting a fire started where it counts.


The TFI module -- and the Duraspark II and HEI systems - have a variable dwell and a current limiter in it. At lower RPM's the coil initially charges up for a few milliseconds, then the current limiter in the module lowers the current through the coil - to just enough to maintain the magnetic field without it collapsing. Current for the first few milliconds is about 12 Amps, then it drops to about 4 mps. And, since the Dwell at lower speeds is far less -- the duration that all this is happening, the duty cycle on the coil, is low. Dwell at idle is about 8 degrees, increasing to about 30 at 2500 RPM.

The reason for all this is to charge the coil as much as possible without overcharging, and without overheating the coil. Remember - "Variable dwell and current limiter."


The plain old original Ford Module (Jeep), the DuraSpark, had neither.
When the key is on, current flows just like the points were closed. The signal from the pick-up coil tells the module to "open", to stop current flow for an instant. Default is always "on."


The TFI coil primary has a lower resistance than the original Ford/Jeep coil, so it will draw a little more current than the original. That alone puts a strain on the module. But that's if the original ignition resistor is retained. The coil output - max voltage, is about the same as the original coil, since it's turns ratio is very similar. The max voltage is about 28-32 KV, same as original. I know some folks say otherwise, but it's very easy to measure the max available voltage with a scope.

Now - if you want the TFI coil to put out more voltage, and more current, and a longer spark duration (more power,) it can, but the ignition resistor must be removed so the coil primary can "see" a full 12 volts. (Energy in = energy out.)
But -- then you are straining the module -- you are asking it to carry almost twice as much primary current as originally designed for.
Remember the variable dwell? At lower speeds the dwell is still 30 degrees on the module, where the TFI module would only be 10 degrees. That means this excessive current is there 3 times longer!
And -- the current limiter - the TFI module had one, the Duraspark does not. The TFI lowered the current after the coil was charged, but the Duraspark continues hitting it full blast! That's really straining both the module and the coil - the coil wasn't designed to carry that much current for so long, the module wasn't designed to carry that much current ever!
Try it - key on, engine off. Wait a few minutes, note how hot everything gets. Now try it with a stock TFI system on a Ford, cool. Look at them with a scope.

Yup, lots of folks get away with it. Some win in Vegas too. Someone proved you can hijack an airplane and jump out with extorted money and a parachute over a mountain too.

If I had on, I'd sure carry a spare module and coil, and good walking shoes.

Check for pulses to the module from the pick-up coil.



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  #7  
Old 04-29-2004, 01:24 PM
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Default Re: One Spark Key OFF -> ON

RRich~

After it occurred to me that the PO had just replaced the P/U Coil on another spark issue (another village idiot [img]images/graemlins/smile.gif[/img]!) I jumped the EECU on the ORN/VIO circuit and got spark. The P/U coil is grounded and Distributor BLK is grounded, so I can only assume that the EECU is F'd.

Thanks for the assistance, this is quite an informative thread.
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Old 04-29-2004, 02:23 PM
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Default Re: One Spark Key OFF -> ON

EECU -- Electronic Engine Control Unit? Is that what I've been calling Module?

I didn't know you could jumper -- I assume short between them - Orange &amp; Purple to get a spark. I'd think that means the module's OK, since it's switching.

When the P/U coil connector is disconnected, measuring the P/U coil's Orange and Purple should NOT be grounded. The black should be - that's the ground for the module.
I have no idea what you'd see looking toward the module down those wires. You are probably looking at a transistor's input, so you'd get two different readings when you reverse ohmmeter polarity.

If you have a spare P/U unit - without installing it, plug it into the harness, key on, touch a screwdriver tip to the P/U coil - watch the ign coil make sparks.

You can swap in a different module easy too - they don't have bolted or ground (black wire does that.)

Or - use an old seat belt buzzer wired to 12 v so it buzzes constantly, key on, place it on top of the P/U coil - watch the ign coil spark like crazy.

I forgot about those old tricks -- we used to diagnose lots of 70's Fords with the buzzer trick. The complaint would be car would die, after cooling a bit it'd restart - could be module, PU, coil, fuel etc. Often the module.
We'd line the Fords up and use the buzzers -- the module runs hot like that -- then every once in a while go look - once it quit it's relatively easy to find why, but still running it's just a guess.
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