Electrical Tfi Sinking Too Much And Overheating, What Else Can I Check?

Discussion in 'Fox 5.0 Mustang Tech' started by boboli, Aug 28, 2012.

  1. I have an 89 lx 5.0 with an aod. Mostly stock with 166k. The car has only an explorer manifold with the act moved to the air box. The car runs great most of the time and just passed emissions with very good readings.
    This problem has plagued this car every time it gets warm for 3 yrs, and in Az its warm a lot. This summer it has stranded me 4 times and I'm sick of not knowing when I push the gas, if the rpms will go up or down. I have narrowed it down to the tfi is definitely overheating. I carry a couple and as soon as it dies, I switch the tfi out and the car starts right up.
    Things I've tried to fix problem
    New distributor, 2nd one since problem started ( parts store)
    New plugs and wires, tried 2 different ford (used coils) and also tested them and they are within spec.
    The tfi has been remotely mounted on the passenger side fender with a heat sink off a 94 thunderbird. It has been been this way for 2 yrs and I did have the overheating problem before I remotely mounted it. The problem doesn't seem any better but at least I can change the tfi in about 3 mins.
    I have a 3g alternator that seems to be charging OK. If I have the AC on, the volts do come down, ( I tested, just can't remember the actual voltage).
    I have 2, 4 gauge grounds added. 1 from front alternator bolt to passenger sway bar mount. 1 from a stud near the oil filter to the driver's side sway bar mount. From here a wire goes to the battery and the stock wire from the battey goes to the computer ground. When the car is hot, but running, I have checked resistance on all these grounds ( and also the one to the computer mounted next to the battery as explained in the checklists) to the battery and have no resistance issues. I know I have the orange ground wire on the back of the motor hooked up but I don't know and can't seem to find the other one mentioned.
    I checked resistance from the wires at the dummy tfi on the distributor to the remote one and there was no resistance on the wires.
    The only codes I get on KOEO are for neutral safety switch and egr not functioning. I can't check KOER because of the neutral safety code. After the tfi dies I get codes that seem to relate only to its failure.
    I also have an 88 turbo tbird with a similar remotely mounted tfi and on warm days I have driven both to get them warm and used my infrared thermometer to compare tfi temps. The mustang general has tfi temps 20 degrees hotter than the tbird, which never overheats with a similar standard brand ignition tfi.
    So any help on what to check to get the tfi to stop sinking too much energy till it overheats would be appreciated as I am certain on the cause of my problems, I just don't know what the remedy is.
  2. So no one has ever found a solution for this?
    How about some shots in the dark as to a solution
  3. The general opinion here seems to be that only the Ford Motorcraft TFI and PIP parts are any good. All the others seem to have short lifetimes and High failure rates.

    The only other clue is all that TFI's must use the thermal heat sink grease on the backside where it mates to the distributor or heat sink. No thermal heat sink grease and the thermal transfer rate decreases to the point that any brand TFI will overheat and fail.
  4. Thanks Jrichker for the help. The solution to my problem seems to be more than just using the ford parts with the thermal paste. Under severe conditions, hot day with AC on and some frequent idling in traffic, my 5.0 stang's tfi will continue to get hot until it finally shuts down. By comparison, my 88 turbocoupe has a parts store tfi that never fails in the same conditions with nearly the exact same set up. I have used the thermal paste from radio shack ( its in my tool kit so I can use it every time the tfi fails) and i have tried the ford tfi, which lasts a lot longer than the parts store ones but eventually has failed.
    My problem, from what I understand is that there is something wrong in the tfi system, either in the wiring from the coil to the tfi, from the tfi to the computer, the computer ground, or some other ground in general. This problem is causing the tfi to have to absorb a lot more energy while it is grounding the coil. Which then turns into heat that can't be dissipated fast enough and over heats the module. I was hoping someone had figured out the source of this problem since it seems to be endemic in many tfi based cars. I would appreciate any ideas from some one who might know electronics better than me on whether the coil side or the computer side of the tfi would cause the excessive energy and heating.
  5. Are you using the gray TFI? It is the only one that works properly with 86-93 Mustangs.

    See paragraphs II & IV -7 below...

    How the TFI ignition works in 86-93 model Mustangs:

    Tools needed: DVM, noid light, safety pin.

    Theory of operation:
    The TFI ignition in 86-93 Mustangs has 4 main components: the ignition switch, the coil, the TFI module and the PIP sensor inside the distributor.

    The ignition switch gets power from the two yellow wires that are supplied power by a fuse link located in the wiring harness that connects to the starter solenoid.

    Diagram courtesy of Tmoss & Stang&2birds

    I.) The coil is mounted on the driver’s side strut tower on most EFI Mustangs. It gets power from a red/green wire and a brown/pink wire from the ignition switch. That wire from the ignition switch feeds a 20 gauge blue fuse link that connects to the red/green wire. The fuse link protects the wiring and the ignition switch, since the fuse link for the two yellow power supply wires has a much higher current rating. Without the smaller fuse link protecting the smaller wiring used in the ignition circuit, a short there would cause the red/green wire to overheat and burn up.

    II.) The TFI module is mounted on the side of the distributor and supplies the ground for the coil. Every automotive power supply circuit uses the ground as the return path to carry power back to the negative side of the battery. The TFI switches the tan/yellow wire coming from the coil to ground. It gets power from the red/green wire when the ignition switch is in the Run position. The red/lt blue wire supplies a signal to turn on more power (dwell time) when the engine is cranking. The increased dwell can cause excessive current draw if the red/blue wire remains energized when the ignition switch is in the Run position. The trigger signal comes from the PIP sensor when cranking and the computer when the engine is running. The SPOUT jumper plug enables computer controlled spark advance. When the SPOUT is removed, spark advance is locked at the setting determined by the mechanical position of the distributor.

    III.) The PIP sensor is in the bottom of the distributor under the shutter wheel. It is a Hall effect magnetic sensor that senses a change in the magnetic field when one of the slots in the shutter wheel uncovers the sensor. Then it supplies a pulse that triggers the TFI module to provide a ground to the ignition coil. A bad PIP will often set code 14 in the computer and cause hot start problems. Replacing the PIP sensor requires removal of the distributor and pressing the gear off the distributor shaft to expose the sensor. For most people, a remanufactured distributor ($55-$75) is the solution, since they may not have access to a press.

    IV.) Troubleshooting the ignition system – no spark or weak spark. All the tests are done with the ignition switch in the Run position unless specified otherwise. A safety pin may be used to probe the wiring connectors from the back side.
    1.) Check for 12 volts at the yellow wires on the ignition switch. No 12 volts and the fuse link near the starter solenoid has open circuited.
    2.) Check for 12 volts on the red/green and brown/pink wires coming out of the ignition switch. No 12 volts, replace the ignition switch.
    3.) Check for 12 volts at the ignition coil. No 12 volts and the blue 20 gauge fuse link has open circuited.
    4.) Check for 12 volts at the red/green wire on the TFI module. No 12 volts and you have wiring problems.
    5.) Remove the small red/blue wire from the starter solenoid (looks like it is stuck on a screw). This is a safety measure to keep the engine from turning while you are making measurements. Have a helper turn the ignition switch to Start and look for 12 volts on the red/lt blue wire on the TFI module. No 12 volts and you will have starting problems, but push starting the car will work OK. No 12 volts, replace the ignition switch. Be sure to reconnect the red/blue wire to the starter when you finish.
    6.) Check the red/blue wire to make sure that it has less than 8 volts when the ignition switch is in the Run position.
    7.) A noid light available from any auto parts store, is one way to test the PIP pulse. The computer uses the PIP signal to trigger the fuel injectors. The noid light plugs into the fuel injector harness in place of any easily accessible injector. Plug it in and it will flash if the PIP is working. No flash from the noid light and the PIP is suspect. To confirm the PIP is being the source of the non flashing noid light, look for 12 volts on the red injector wiring. Good 12 volts and no flashing noid light means the PIP has failed.
    8.) Remove the SPOUT plug from the harness and try to start the engine. If it starts, replace the PIP. This is a common no start condition when the engine is hot.
    9.) The TFI module is a go/no go item when you have a no spark/weak spark condition on a cold engine. It either works or it doesn’t.
    The TFI failure mode on a running car is usually a high speed miss on a warm engine. Many auto parts stores will test your TFI module for free. Bring along a hair dryer to get it hot while testing it and run several test cycles, since it often gets weak when it heats up.

    The coil is somewhat more difficult to pinpoint as a problem. A good coil will make a nice fat blue spark 3/8”-1/2” long. The problem is that one person’s perception of a fat blue spark looks like may not be accurate enough to spot a weak coil. The coil is cheap enough ($13-$16) that having a known good working spare might be a good idea.

    diagram courtesy of Tmoss & Stang&2Birds



    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

    Ignition switch wiring

    Fuel, alternator, A/C and ignition wiring

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

    Vacuum diagram 89-93 Mustangs

    HVAC vacuum diagram

    TFI module differences & pinout
  6. Revisiting this for more help.
    I checked everything in the no start tfi checklist above and all the voltages at the specific times listed are correct. I tested resistance to the computer on all the wires from the tfi as well as the computer ground to the battery. They all were 0 ohms resistance.I tested the coil when it was cold and then after the car ran for 20 minutes and the reading are all within the specs I found, although the warm readings were getting close to the limits.
    Just to be specific I am using a ford tfi and I do have the radio shack white thermal paste on the back. I have used an entire tube this summer because I've had to switch the tfi so many times.
    I took the car through emissions about 3 weeks ago and it passed the 1st time with right down the middle #'s or less on all test parameters. I compared them to my previous 2 tests and they were all very similar.
    I didn't drive the car till tonite. The weather has cooled off, so I figured it would be fine to drive it. I drove it to 3 different places and when I got it home, I 1st used my hand to test how warm it was, it was noticeably hot. I then used my infrared thermometer which showed 140ish degrees. That seemed hot for a < 90 degree night with no AC on. I decided to take my 88 turbo tbird out and put gas in it so that I could also compare the temperatures between the 2 tfi's. I have the exact same tfi relocation on both these cars ( relocated to the carb snorkel hole on the passenger side inner fender with a carb snorkel feed tube installed inside the fender). When I got home, I touched the tfi and it was barely warm to the touch. My thermometer showed 115 degrees. So its running 30 degrees cooler than the mustang. Also, the tbird is running a parts store tfi that has never failed me.
    So what can I do. Is the mustang somehow supplying too much dwell and overheating the tfi? I followed the tests listed above and they were all within specs.
    I thought my 3g alternator may not providing enough power but tonite I drove with only the lights on and the tfi still heated up.
    If I am missing something, I wouldn't be surprised, so if someone can point my obvious misunderstanding on how the tfi works or what I'm not doing right, I would appreciate it. I have just about had it with this problem. I wish there was a way to remove the tfi from the equation without going to megasquirt or some other system. Can an aftermarket ignition box be used in a way that would use the tfi as a trigger only so the tfi doesn't have to ground the coil and obsorb all the energy mine seems to be?
    thanks again for any help!
  7. I am bumping this again to at least add some more info to my problem. I am hoping that by posting new info, this will eventually help solve this for me and others.
    To start, all my tfi's are remote mounted
    I am starting to wonder if this is really a TFI overheating problem. To recap, I own an 89 5.0 and also an 88 tbird turbocoupe. The mustang has a no start/ shutting off problem that I concluded was a TFI problem. I can't drive it when the summer heat here is above 100*, or it will strand me. The TFI was definitely getting extremely hot (190ish). Although I found that when I swapped another in, it would start right up. This 2nd one would overheat very quickly with the same results.
    My tbird is my DD. All new ford ignition parts, except coil. I can drive it any time, or so I thought. No problems in town. I went on a road trip and about 90 miles in, I hit some steep hills. I stupidly left on the AC. I was having to use continuous boost and down shifting with large boost spikes, to keep going up the hill. After about 5 minutes of this my temp gauge spiked nearly instantly, oil also was way lower than normal. I figured blown head gasket. The car stayed running until I shut it off. Then it acted like the TFI had overheated. Sputtering when it would start and pushing the pedal did almost nothing( rpms remain same/engine misfiring cylinders). I had my IFR temp gun and the TFI was only 130*. This is average from what I have seen. It took about 30 minutes, but the tbird started and ran again like nothing was wrong. The gauges still seem a little wrong. My temp was still nearly pegged, while the oil was still low. I still DD this car and have changed nothing from that day.
    Today, I was driving the mustang. It's a 101* out but I have installed a small computer fan in the fender snorkel hole, that blows air directly on the TFI. I drove across town and then back to walmart. About 20 minutes in walmart and when I come out, its only sputtering on a few cylinders. I figure its the TFI. I also notice the temp gauge is reading higher than normal and the oil is only 2 pegs from the bottom. I wait 30 minutes and try it again. Fires right up and I drive home. The gauges were now back to normal. The oil was reading the highest it had ever. I tested the TFI temp at home. With the fan blowing on it, it was only 135*. Should be fine.
    So here is what I am seeing.
    The TFI's aren't overheating, at least by my judgement.
    The gauges seem to act strange as this problem approaches. I now the temp pegs when grounded ( testing it). Does the oil pressure bottom out when grounded? I wonder if voltage is being lost and the gauges are just acting like they are grounding. Instead they are losing their voltage.
    Just mentioning this now, but when the sputtering/ misfire occurs, the fuel pump always primes with the key. No telling what the pressure is. On the tbird, I thought I heard the pump shut off after it would restart and run for a few seconds. It seems to me something else is overheating and then the TFI over heats as a result. Or maybe a main efi relay overheat?
    I also tried swapping in a known good aftermarket ignition system in the mustang. It worked fine, but it didn't seem to change anything the day I installed it. The next day, I went to check out the car and discovered the ignition control box was extremely hot. The key wasn't even in the car. I removed the system and returned it to the car it was on. Hooked it up the same and its been fine for 6 months ( as well as the 3 years before I put it on the mustang). Any ideas about this one?
    I am open to any theories about this. If no one has any ideas, I hope that this info will help others. Maybe they can look for similar problems with their gauges
    thanks for reading this long post!
  8. I would like to tell you that we had similar problems with a MSD box on our car. This happened a couple of years ago. We took the box off and have had no problems since. We are not sure if the problem was from the high discharge from the ignition box or what. We know that a bad coil will also kill the TFI too. Check out our website for more products. www.fatfoxx.com. I would suggest ditching the box and replacing your TFI Module once more. Glad to see you are still on board for the research portion of this problem. We are constantly monitoring others experiences with this problem. Remote mounting the heat sink is the only way to go. Keeping the heat off the module is a must . I am hoping that when you did your kit that your PIP harness was shielded too. Keep us up to date brother.
  9. Start by looking for bad grounds. See step 2 below and see if you have any of the symptoms.


    Revised 28-Oct-2012 to add signal ground description & possible problems if it is bad

    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.

    Make sure that all the ground places are clean and shiny bare metal: no paint, no corrosion.

    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. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C.

    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

    Correct negative battery ground cable.

    3.) 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: it 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.

    All the grounds listed in items 1,2 & 3 need to bolt to clean, shiny bare metal. A wire brush or some fine sandpaper is the best thing to use to clean the ground connections.

    4.) All the sensors have a common separate signal ground. This includes the TPS, ACT, EGR, BAP, & VSS sensors. This ground is inside the computer and connects pin 46 to pins 40 & 60, which are the main computer grounds. If this internal computer ground gets damaged, you won't be able to dump codes and the car will have idle/stall/ performance problems

    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.

    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. .

    Voltage drops should not exceed the following:
    200 mV Wire or cable
    300 mV Switch
    100 mV Ground
    0 mV to <50 mV Sensor Connections
    0.0V bolt together connections


    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.