Electrical In theory what ELECTRICALLY kills TFI modules?

IN2DEEP

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Jan 1, 2004
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1978 Jeep CJ5, fiberglass body, 1990 Ford 5.0 EFI engine.

I've been chasing a TFI killer for 10 years. It hasn't been a constant. Frustration leads to the vehicle (Jeep) being pushed into the garage and ignored.

In the spring I get excited, find a new solution or a mechanic to "fix" it and I'm ready to give it another try. But once again it has kicked my butt.

The modules aren't lasting long enough for it to be a heat issue. This latest one probably had 10 hours of run time at BEST. I've remote mounted them in the past with no new benefit. New module works for a short period, then quits.

I've replaced the wiring in the engine bay with a Ron Francis plug-n-play "kit" (last years fix). All the wiring is new. I've swapped distributors (new and used). I've swapped ECMs hoping that would provide some answers. Nothing changes. The body harness was new in 2000 and it hasn't been butchered. The Jeep is garage kept, so there's not rodents or weather to deteriorate the wires. For over ten years this Jeep was my "nationwide" cruiser, up and down the East coast. Now I can barely drive it home from the mechanics before it fails.

So in a very general sense....... what causes TFI failures ELECTRICALLY? Too much power? Too little power?

If this was your vehicle and you suspected an electrical gremlin is in there reeking havoc, how would you diagnosis it?

On a side note, how much gasoline does it take to burn down a Jeep CJ5?......UGH!
 
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7991LXnSHO

wanna catch the space herp
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The last question first- if you have an electric fuel pump, and a leak in the high pressure line that sprays near the exhaust manifold, four Oz. should be enough to get the process started. Any more will speed it up.
 
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Mustang5L5

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If you are burning them up in 10hours, I don’t think heat is the issue.

I’d be checking the wiring to make sure the correct wires are routed to the correct pins and one isn’t receiving more voltage than it should.

I can provide a wiring diagram for the factory setup, but you’ll need to trace it out to your aftermarket setup to see if it all matches out.
 
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dgollem

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A couple of things can consistently kill electronics. Over voltage or voltage spike or surges, I don't know what the TFI likes to see for an input voltage but if you are hitting it with more than called for it, it will cause a problem. Under voltage usually won't harm the electronics but can cause all sorts of weird and intermittent problems. Shorts will usually cause an over voltage or surge. Heat can kill electronics and it does not always take a lot of time. Can also very be inconsistent, electronics are speced at certain operating temperatures, exceed those specs and who knows what will happen, it may last minutes or months. Your problems seem to be in summer when temps are higher so I would not rule that out. If you can find out what the max input voltage is for the TFI you could add a voltage meter and or voltage regulator in-line to monitor or protect. You could also add a fuse in line if the input is shorting out, the fuse should blow before the TFI takes the hit. As for the heat they make a relocation kit for the 94-95 5.0 that moved the TFI from the distributor to the fender because of heat issues. Has something to do with the different airflow getting to the distributor in the SN 95 cars vice the foxes. Your jeep may have a similar problem because of the differences in the engine compartment, etc. How to diagnose, if it’s not heat or over voltage then it’s got to be a short or intermittent connection causing a voltage spike, painstaking wire by wire, connector by connector, with some good schematics and a good multimeter. Anyway a few thoughts hopefully they help.
 
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HemiRick

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Does it have a good battery, that says connected, a charging system needs the battery to act as a buffer to prevent voltage spikes.
 

90sickfox

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Part of the tfi wiring is supposed to be shielded. Not sure which ones because I'm away from the car. Over time the thin aluminum foil type shielding breaks down on factory harnesses and could cause electrical noise that may cause one to fail. Some aftermarket harnesses aren't shielded.

Have you ever taken a tfi module to advance auto to be checked. They have the tool and it's free. ( I worked there for years a long time ago ) Last time I had one checked was about 5 years ago. It's an ignition module tester. You want to make sure your grounds are good and clean. The paste that comes with a bunch of new modules is the wrong stuff. It'll cause the module to fail. The last few I bought came with a clear type dielectric grease....it should be a white heat paste.

There are two types of tfi modules. One is used on the older fox and trucks. The other one was used on 94 - 95 mustangs. Ones gray and ones black. The wiring is different. They aren't interchangeable.

Your problem could be electrical interference. I'd start by making sure the voltage is correct. Make sure wires are going to correct places. Check the ground continuity. Use an oscilloscope and read the signal from the PIP wire. Make sure the pick up assembly is clean and tight in the distributor below the rotor button. Put shielding on the wires to isolate the circuit. If it keeps blowing it would have to be a computer issue and not the harness or tfi.

New TFI parts are junk. I'd rather go to the JY and buy a few and get them tested. Keep a spare and ride on if it makes you feel better.

Hope you find the solution man.

JRichker no start list has a test using a noid light for the PIP signal
 
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IN2DEEP

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If you are burning them up in 10hours, I don’t think heat is the issue.

I’d be checking the wiring to make sure the correct wires are routed to the correct pins and one isn’t receiving more voltage than it should.

I can provide a wiring diagram for the factory setup, but you’ll need to trace it out to your aftermarket setup to see if it all matches out.
Hey Mike, Let's see that wiring diagram. thanks!
 

Mustang5L5

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Here you go. This is the 1990 wiring diagram

TFI in the bottom right quadrant
F0C91A13-8FD0-4EB8-A384-8DCFE9D27662.jpeg



wire 16 off the TFI is also referenced here
0B4D77CA-78C2-4575-830B-4CE620658E36.jpeg
BE404DF8-E5C2-4FEC-81DB-A23F867D027F.jpeg
 
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