Question about Ford F-100

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Transmission vac lines

I got a 73 F100 did some motor upgrades after i got all the lines back on the motor that were needed the two lines from the trany i hooked up, are they both suppose to be on constant vacuum? would that hinder it from shifting down. Or did i hook them up wrong, the truck runs fine other then that.

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  • spcaplette Dec 31, 2008

    thank you for your help, yes i did cam it too I don't think its more then 220 degrees mild edelbrock cam, and carb and intake and also put in a shift kit too!

  • Richard Scordino May 11, 2010

    Been a long time since 73... I think that one goes to manifold vacuum, the other to a fitting above the lower carb butterfly (no vac there 'till you hit the throttle)... even if I'm right, I dont remember which goes where on the modulator. If I could find one of my old chiltons, I'd be able to help better!! Actually the more I think of it, the less I remember seeing two hoses?? Are you sure the modulator is good?? I'm not even going to list this as a solution...Just some ideas...Also as a rule if you put a cam in there with more than 220degrees duration it can cause shift problems.



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Transimission will not go in revirse allother work fine

Posted on Dec 31, 2008


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The F100 ended in 1983, so there is some confusion on your 1987 Ford F100. The F150 started in 1983 and was available in 1987, but not the F100. You have this listed under the 1981 Ford F100 category.
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In order to best answer your question I have included both the 1981 F100 4.9L and 1987 F150 4.9L below. The only difference is with the Distributor Cap Configuration and Connections.

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Hope help with this (remember comment and rated this).

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The line w/ vac on it :
Blue - when in 4WD
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You can use a mighty-vac or other vac pump to test the lines.
Could be as simple as a cracked vac line.

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Start off with the vac test on the motor, and see what the next step should be ( repair vac lines or new vac motor / vac solenoids ).

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with trnsmission in neutral or park start engine measure vacuum on the two vac lines at front axle shift motor . one line should have approx 20" vacuum. other line should have none
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does shift motor plunger move at least 1/2 inch
no shift motor bad
yes check axle shift fork and vac motor for binging

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