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Thread: C4 woes

  1. #1

    Default C4 woes

    Alright, since doing my GT40P head swap, I can't get my Fairmont to make the 2-3 shift on the C4. Here are some things I've already done:

    1) Saw the kick down lever was lazy to return back to the normal spot. So I added a spring at the carburetor to make sure it goes back. Now it's worth noting that I have a holley carb on the car in place of the stock Motorcraft 2150. This puts the kick down lever in a slightly different spot over stock, but I feel pretty confident that it's in the full "forward" position regardless
    2) Checked for vacuum. Was pulling 11-13 whatevers of mercury by putting my vacuum gauge down on the vacuum control modulator end
    3) Replaced the vacuum control modulator
    4) Checkded for cracks in the rubber lines as well as pin holes in the metal line all along the route. I see nothing
    5) It does have a decent leak somewhere and had found over the time I had it down (we're talking 8-10 months) it was close to 2/3 qt low on fluid. I've since topped it off.

    So I'm out of ideas. It ran fine before. Could having a higher compression motor now (after the head swap compared to the stock heads) changed my vacuum levels? I'm making things up at this point. But I'm out of ideas.

  2. #2

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    So here are my three videos on the problem:

    Initially found - https://youtu.be/EAdf4g7oqAI
    First attempt at a fix - https://youtu.be/sglETXTZDdc
    Second attempt at a fix - https://youtu.be/yWRnDu-egr4

  3. #3

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    Make sure that the modulator is connected to manifold vacuum. 11-13 whatevers is awful low if you check it at the manifold.
    check your carb mixture adjustment by connecting the vacuum gauge to the manifold source and adjusting the mixture screws while watching the gauge to get the highest vacuum. Next with the gauge still connected adjust your timing to get the highest vacuum on the gauge then check with the timing light to see where you are and , make any changes to get it correct.
    There is an adjustment to the vacuum modulator on the c4 also but its best to get the low vacuum sorted first.
    http://www.superchevy.com/how-to/45638/
    https://itstillruns.com/adjust-modul...4-8049803.html
    1978 Mercury Zephyr boxtop 5.0 EFI T5Z 8.8
    1999 Ford Contour 2.5 V6 5 speed
    2016 Ford Focus ST 2.0 Ecoboost 6 speed

  4. #4
    FEP Senior Member BMW Rider's Avatar
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    Does it shift to 3rd at all? You could try removing the kickdown linkage and do a test drive to see if that sorts it. As for low vacuum, it should still shift up, the modulator only affects the shift timing and should not prevent it from upshifting entirely. Otherwise I'd say you have deeper issues.

  5. #5

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    I agree, if it worked before and doesn't work now, it is something you touched!

    Pull off the kickdown rod, it is unneeded to make the tranny work (it just won't kick down when you stomp it). Try it, if it works, then let the rod be lazy (the rod is always lazy, I can't think of one that is all the way forward, even stock). If I remember right, when you change to a 4V carb, you need to run a longer kickdown rod for it to work correctly. The 4V throttle shaft moves forward quite a bit from stock 2V location (though you may have already had a 4V intake on it, then this is all mute)

    Good luck
    Kenny

  6. #6
    FEP Super Member gr79's Avatar
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    From what little i know about auto trans, overall low engine vac could be an issue.
    Agreed 11-13 is real low if reading is taken at idle, not bad when driving. Try for 14" up.
    Is one reason why race type cars convert to manual valve bodys. Big cam, engine wont generate enough vac to auto shift the trans.
    When driving, vac will be a lot lower the more the throttle is pressed.
    Cruising on freeway is around 10". Street, light load, lower gearing, light throttle vac is higher to 20".
    Pre-PCM stock engines and tune have no trouble generating vac to operate auto trans, power brakes, a/c doors.
    Low vac is fooling trans thinking it is going uphill, accell or passing and needs to hold 2nd for the load.
    May be a simple vac leak or past tune causing bleed off of vac. Check for leaks and retune for high vac at idle.
    A tee in the trans vac line to connect gauge incar when driving will show whats going on.

  7. #7

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    Well I took another look at it today, and here's what I've done since that post -

    1. Took the kick down linkage off completely. That didn't solve the problem.
    2. Pulled the vacuum control modulator off and saw the new one didn't come with the pin like the stock one had. Luckily I haven't thrown out the stock one, where I swapped the pins (but again the car did the same thing with the stock one in it).

    So that leads me to believe what you all are suggesting. I probably could stand to do a tune up to help get vacuum levels where they need to be, then try it again.

    Oh and mudgepond...I'm running a holley 2bbl in place of my motorcraft 2bbl. So still a 2bbl.

  8. #8
    FEP Super Member gr79's Avatar
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    I was thinking if there was a good trans shop in town maybe they could look at it.
    Personally do not trust most of them.
    Had a C4 in my dd 67 Mustang 289 verti. Ran fine, eventually started slipping. Young driver- hard on trans.
    Rebuilder in town went thru it early 70's and added a shift kit to it. Not expensive, not cheap.
    "You won't blow it up or its on me". Never forget he aggressively test drove car with me before i paid. Said car was a dog.
    Oh well 17 sec in 1/4 mile cars are not that fast.
    Definite diff- firm shifts, could finally spin the F-60-15 tires no problem straight punchs off the line no brake torquing.

  9. #9
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    For a test, you could run the vacuum line from the transmission into the car and have a passenger apply vacuum with a hand pump at the proper time.

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