Question about 1991 Ford F250

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I have a 1991 ford f 250 diesel with a4od transmission the problem is my tachometer stopped working and the light on the od button came on this interfers with transmission shifting it will not take off in 1st gear and will not shift into overdrive i changed tach sensor and tach still will not work and light on overdrive button will not go out

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  • Contributor
  • 3 Answers

Check to see if the computer is getting proper voltage on the yellow wire if that is not it ckeck the large red wiresmy dads truck had this problem last year the yellow wire was defective in the harnes the tack and the transmission work off the computer trust me it is a simple fix run a new wire from the solinoid on the pasenger side to the computer relay on the driverside and replace the relay too

Posted on Nov 10, 2010

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  • Ford Master
  • 76,132 Answers

These are two separate issues, the 1st the tach is most likely the tach is defective, this is fairly common, the 2nd problem requires that u have the transmission control module tested for fault codes, anytime the OD light flashes it means the transmission control module has detected a problem in the transmission operation, this can be a shift error or More commonly excessive slipping has been detected, u may not even fell this but never the less the difference between the input and out shaft speed sensors of the transmission showed a speed difference that triggered the OD light and a fault code, not saying this is the problem but based on my experience of 35 years at a Ford dealer it is the most common fault, if that is the problem then the transmission will require a rebuild. THERE ARE MANY PROBLEMS WITH THIS EARLY E4OD, LATER VERSIONS HAS BBEN GREATLY IMPROVED.

Posted on May 30, 2010

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  • samlejeune May 31, 2010

    od light does not flash it stays on this transmission was just rebuilt and has less than 30,000 miles on it when tach worked at times the transmission worked fine then tach stopped working and transmission started messing up right after transmission rebuild i brought back to shop and mechanic said every thing in the transmission was working fine

  • yadayada
    yadayada May 31, 2010

    how can everything in the transmission be working fine, we both know that isn't true, of course he is going to say that so he doesn't have a problem, u mean he road tested it and said it shifted fine? a free 2nd rebuild on his hands, personally I would have bought a factory rebuilt from a Ford dealer, we seldom if ever have that problem. , something was done wrong in the rebuild, the transmission and the tach are not related

  • samlejeune May 31, 2010

    no he put it on the computer and checked the transmission this problem did not occur untill the tachometer stopped working

  • yadayada
    yadayada May 31, 2010

    find out is the Tach is receiving a signal at the Tach connector, but the shift commands come from the ECM and shift commands mostly come from the vehicle speed sensor, it is very likely the ECM uses the Tach signal, but don't remember that to be the case. Y may have a wire connector issue at the engine bulkhead, but finding that takes a wiring diagram and hands on careful inspection, at this point if the transmission shop can't figure out the shift issue they should send the truck to a ford dealer and pay for the inspection, saw this many times while working at Ford, the shop that did your transmission is over there head here.

  • yadayada
    yadayada May 31, 2010

    just because he put it on a scanner and said it was ok means nothing , this is really a AOD transmission with an overdrive and locking torque converter added, would u like to call me, will be here for the next 30 minutes USA 925-459=0571

  • yadayada
    yadayada May 31, 2010

    did u check to see if the Vacuum pump on the engine is working and the vaccum line to the transmission shift control vacuum modulator has vacuum and the line is ok and not split or leaking? this can cause very harsh shifts

  • samlejeune Jun 03, 2010

    sorry i said a4od but actually it is a e4od there are no vaccum lines to transmission it is all electonic shifting

  • yadayada
    yadayada Jun 03, 2010

    THIS TRANSMISSION DOES HAVE A VACUUM LINE AND A VACUUM MODULATOR, THE VACUUM IS SUPPLIED FROM A VACUUM PUMP ON THE PASSENGER SIDE OF THE ENGINE, I HAVE REPLACED THE VACUUM MODULATOR IN THIS TYPE OF TRANMISSION MANY TIMES. THE ONLY THING THAT IS ELECTRONIC IN THIS TRANS IS THE OVERDRIVE AND THE TORQUE CONVERTER LOCKUP. I WORKED FOR FORD 30 YEARS AM SURE ON THIS.



    HERE IS THE PART NUMBER AND THE PICTURE OF THE VACUUM MODULATOR





    MOTORCRAFT Part # TM43 (VAC MODULATOR E4OD 7.3 DIESEL)




  • yadayada
    yadayada Jun 03, 2010

    I STAND CORRECTED, THE E4D DOESN'T USE THE MODULATOR, WAS THINKING OF THE EARLIER C6 TRANSMISSION WHICH DOES, U NEED TO CHECK THE BULKHEAD CONNECTION TO THE ENGINE FOR THE TACH SIGNAL, IF IT IS PRESENT THERE BUT NOT AT THE ECM (COMPUTER CONTROL OF THE TRANSMISSION). THEN THE WIRING IS AT FAULT, WHICH I BELIEVE IS THE PROBLEM HERE, THIS ENGINE IS HARD ON WIRING HARNESSES DUE TO THE EXTREME MOVEMENT OF THE DIESEL

  • yadayada
    yadayada Jun 03, 2010

    already got credit for this fix, thanks

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  • If you have any comments please feel free to leave them here.
  • Hope this helps and thank you for using FixYa.


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