Overheating tfi/msd question

Discussion in 'Fox 5.0 Mustang Tech' started by Brandon110100, Mar 20, 2012.


  1. Brandon110100

    Brandon110100 New Member

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    Been having a overheating tfi module problem for years now. Tried remote mounting on a heat sink of a 94-95. Worked for a while and now the problem has returned. All the wiring and pinpoint tests all prove out as stated in my older post as Brandon11010. been thru about 5 modules in the past years, upgraded distributor, got ahold of a like new engine harness, replaced that. and its always the same symptom after a while. Tfi gets hot, wont start/barely idle. let it cool runs perfect. So iv reached my tollerance limit with this problem. I want to completely eliminate the tfi from the ignition system. If i install a MSD 6al does that remove the factory tfi from the ignition system?
  2. Blown88GT

    Blown88GT Founding Member

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    In a word, "No".
    Something else is going on. Where are you getting these TFI modules? Are you applying thermal paste? Remote wiring does require shielding & grounding of the harness. Most don't even know what this means, let alone know how to do it, so no reflection on your skills. Shielding & grounding of analog signals (even digital) requires knowledge & experience that almost all engineers lack. I happen to be very experienced in this area & have designed & fab'd many cabling systems for aerospace. I have studied the Ford remote harness which contains a foil overwrap & drain wire. You have to ground 1 side of the cable & float the other.
  3. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Get out your voltmeter or DVM and prepare to look for voltage drops between the TFI and the computer and the TFI body and engine block. The TFI is overheating because it is sinking too much current. Check the red/blue wire on the TFI. It should have 12 volts or cranking voltage when starting the engine and no volts with the key in the run position. The red/blue wire causes the addition of dwell time when cranking and makes for a hotter spark.
    Swap the coil if you have not already done so: a primary winding with out of spec resistance could increase TFI current to the point where it overheats and shuts down.
    The PIP sensor in the distributor will cause similar problems to an overheated TFI.

    The following ground test path will help show how to test for voltage drops...

    Grounds

    Revised 26-Feb-2012 to add testing of voltage drops with maximum load on the circuit or connection under test.

    Grounds are important to any electrical system, and especially to computer controlled engines. In an automobile, the ground is the return path for power to get back to the alternator and battery.

    1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.


    2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges.

    Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just as much current, so it needs to be 4 gauge too.

    The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

    The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
    Picture courtesy timewarped1972
    [​IMG]

    Correct negative battery ground cable.
    [​IMG]

    3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
    In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
    In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
    You'll find it up next to the starter solenoid where the wire goes into the wiring harness.


    4.) All the sensors have a common separate signal ground. This includes the TPS, ACT, EGR, BAP, & VSS sensors.

    5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.

    6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself. The TFI module ground terminates inside the computer.

    7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.

    8.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally.


    See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. Be sure to have the maximum load on a circuit when testing voltage drops across connections. As current across a defective or weak connection, increases so does the voltage drop. A circuit or connection may check out good with no load or minimal load, but show up bad under maximum load conditions. .

    [​IMG]

    Extra grounds are like the reserve parachute for a sky diver. If the main one fails, there is always your reserve.

    The best plan is to have all the grounds meet at one central spot and connect together there. That eliminates any voltage drops from grounds connected at different places. A voltage drop between the computer ground and the alternator power ground will effectively reduce the voltage available to the computer by the amount of the drop.
  4. Brandon110100

    Brandon110100 New Member

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    Well I have used multiple tfi's. From the MSD one that came on the distributor I purchased, to used ones I got off old distributors, and a couple of motorcraft ones, i bought a brand new motorcraft one last year when i did the remote mount. I used a heat sink off of a 94-95, broke the fins ect and mounted on the fender in the air box area. As for the harness all i did was re-route it over to the fender, no cutting or splicing, so all the shielding is there and intact. Is there an extra ground wire im going to need? I have used motorcraft di-electric grease on the back of the module, i was going out today to get some heat sink grease but i have a feeling im looking at a bigger issue. Even with the heat sink it seems to get excessively hot. I did replace the coil when I was having this problem a few years ago and did not seem to make a difference. I also replaced the pip sensor on the factory distributor which had no effect. I since have replaced it with a msd distributor. This has been a tricky issue, while i was attending school I had multiple Ford FACT instructors investing alot of time trying to diagnose this problem to no avail. I also have attempted to pick the brains of my fellow Ford techs who have been working with ford almost as long as iv been alive and no solution seems to surface. We even went as far as to try different pcms. Ill be doing more testing from the info jrichker posted and update when I am all out of ideas.
  5. Blown88GT

    Blown88GT Founding Member

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    1. You never stated what year your Fox is.
    2. You stated: "...As for the harness all i did was re-route it over to the fender...". How did you get from the TFI pins to the distributor? You have to fabricate this harness from something.
  6. Brandon110100

    Brandon110100 New Member

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    Ah okay. Its a 89 5.o as for the 3 wires i just used 3 wires with filed down male connectors and ran those to the 3 pins on the module with female connectors. Didn't think i would need anything special for a plug.
  7. Brandon110100

    Brandon110100 New Member

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    you give me the feeling thats where i need something special lol
  8. Blown88GT

    Blown88GT Founding Member

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    No, but the 3 wires need to be shielded & the shield grounded at the heatsink.
    I used a modified Method #2. I figured this out before I even read the link.
    http://www.therangerstation.com/tech_library/remote_tfi.htm
    I didn't cut the TFI to run wires out from it, I used the existing receptacle on the end of the TFI. Soldered jumper wires inside to pick up 3 of the existing 6 pins. I made a harness using the same plug. I can switch to either mounting location in minutes.
    1. Remove existing plug from dizzy TFI.
    2. Plug into remote TFI.
    3. Remove dizzy TFI.
    4. Install modified gutless TFI.
    5. Plug in new harness.

    You can get the makings of the harness from the same junker you got the heat sink from. Carefully unwrap the drain wire & foil from the harness & re-wrap to make your new harness. Without the shielding there will be too much noise on the circuit because it is no longer close coupled to the PIP module which is inside the dizzy.

    Right now, I'm using by new ProBillet. If & when the TFI craps out, I can switch over.
  9. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    The symptoms of excessive RFI would show up as a miss at all speeds or a certain range of engine speeds. Since the computer uses the PIP signal passed by the TFI, there would also be problems with the fuel injection due to RFI riding on the PIP signal.There is also the potential for noise in the radio and stereo that would be very noticeable..

    I would be very interested in hearing an explanation of how RFI could cause overheating. I learn new things here all the time. The two main things that I can think of that would cause overheating are poor performance of the heat sinking system and excessive current being switched by the TFI. There may be more useful information that is not explained in the original post.
  10. Brandon110100

    Brandon110100 New Member

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    Is there anything in particular you would like to know? I have not noticed any RFI interference from the radio.
    Ill attempt to describe how this problem seems to duplicate:
    I will be driving enough to get to opperating temp. I will shut engine off for approx 5-10 min, Then when I start it up engine barely idles/wont idle/stumbles, will not rev past 1k. No codes.
    When i went to open the hood and inspect i found that heat sink and TFI are hot to the point where it hurts to hold hand my hand on it. I let vehicle sit and cool off and vehicle starts right up and runs fine. No overheating issues, 190 thermostat. Never seems to happen while driving, altho sitting at stop lights makes me nervous. All harness and wiring were tested with a breakout box with no excessive resistance, Also nothing from wiggle test. I was experiencing this problem before i attempted a remote mount, just after i did it ran fine for the rest of the year. Before i remote mounted pip sensor was replaced, coil was replaced and distributor was replaced. Also dash harness and engine harness were off a stang with 12k miles. (dad got back in the 90's for a project that didnt get used, sat in a climate controlled garage since a few years ago when i installed it, if that means anything. Could some excess resistance in the wiring to the coil cause an overheating issue? Would the base timing cause an issue with the tfi as well?
  11. Blown88GT

    Blown88GT Founding Member

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    I've seen EMI (RFI) do all kinds of crazy things. Heating would not be out of the realm of possibility. Eddy current heating comes to mind & I suspect that is what is occurring, since there is no shielding/grounding. I have seen this in a large facility switchgear room where the green-wire ground measured 25A with a clamp meter. Remove the ground, no spark. Grab it with you hand, no shock. Where was the 25A? Why was the meter lying? Never found the answer to my satisfaction, but did find excessive resistance to ground. Created a new ground with low resistance, no current in that ground. Probably doesn't explain this TFI situation but demonstrates the effect they can have.

    Grounding & shielding in aircraft is a big deal, but mostly for the interference issues, eddy currents are a secondary issue, largely overlooked when standard practices are successful. When not, you start looking elsewhere.

    I will have to study the PIP/TFI schematic drawings (not wiring diagrams). If you have them, can you link or show them here?
    http://en.wikipedia.org/wiki/Eddy_current

    Does this car have a large stereo system or any other electronics? Can you provide pics of the engine compartment showing remote location? Something is creating an eddy current which is passing through the TFI.

    I thought about this some more. If you have lost 1 or more of the diodes in the alternator, that would put AC voltage onto the vehicle which could cause the issues you are having. If you have a good multimeter, see if there is any significant AC voltage across the battery when the engine is running. An alternator is an AC polyphase generator.

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