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Thread: Intermittent Tach issue (stumped)

  1. #1
    Join Date
    Apr 2002
    Location
    St. Louis, MO
    Posts
    6,446

    Intermittent Tach issue (stumped)

    I've had an intermittent tach issue for a few years now. I will be cruising around and then the tach will go to zero rpms--sometimes just briefly, sometimes long enough to throw a check engine light. The issue seems to occur more recently the hotter, or longer I've been driving the car. I finally decided to change my cam sensor with a brand new Motorcraft sensor. Unfortunately, this did not fix the issue and about 20 minutes into my test drive, the tach went to zero and through a check engine light. I have not tried pulling codes. The light goes away as soon as the tach starts working again.

    Ideas?
    Kurt K (e-mail)
    SCCoA Member #: 443
    '92 SC AOD -- 11.521s @ 116.748mph, 2.0 AR power
    . . . . . . . . . -- 13.547s @ 101.01 mph, only w/ bolt-ons
    '95 SC 5spd -- All Stock, except 17" Simmons wheels.
    '90 SC 35th Anny 5spd -- 3rd owner, 16k miles
    2 '89 XR7 5spd's -- on their way out, really!

  2. #2
    Join Date
    Jan 2003
    Location
    Woodbridge, VA
    Posts
    5,168
    I can tell you this. I installed a WOT box on my 5 speed car. I wired the rpm signal wire into the crank sensor. And the coil power wire goes thru the WOT box. When I set it on the 2 step for a few seconds the tach drops in and out as the WOT box controls the rpm and the check engine light will illuminate then go out a few seonds later after I stop the 2 step.

    So maybe something to do with the coil or coil power wire?
    ~Chris
    SCCoA Member #1826
    Bettie Riding jack stands
    SCARlit on drag duty now

  3. #3
    Join Date
    Jul 2005
    Posts
    419
    Good evening


    i would inspect the connectors at the DIS harness. Might have two separate problems. The shield connector for the intermittent EEC light and exposed/frayed wiring connectors on the CAM sensor and DIS for the intermittent RPM drop.

  4. #4
    Join Date
    Sep 2012
    Location
    Raleigh, North Carolina
    Posts
    685
    I had the same problem in my car for a long time. I swapped dis modules and thought that fixed it for a few months. Swapped the crank sensor and thought that fixed it. Later swapped the cam sensor and thought that fixed it. My next step was to swap the section of harness by those three items. I suspect that it worked for a while because I was moving the harness everytime I replaced one of those items. I ended up doing the 351 swap before I ever solved the problem. I also have a gauge that reads rpm for the shift light that picks up it's signal from the dash harness. I doubt that's it, but that is an idea that I had not thought about.
    Any idiot can make a Mustang fast...
    It takes a special idiot to make a Thunderbird fast!!!
    89 SC 4.3 stroker 384rwhp 473ftlbs
    Best ET 13.085 Best MPH 106.6... got some work to do!
    2016 V6 M90 Dyno Champ 2016 Mod 1 Runner Up

  5. #5
    Join Date
    Jul 2002
    Location
    Roanoke, Virginia
    Posts
    3,291
    For what it's worth, I had a similar situation where it was a bad tach and I went through all the same steps and head scratching as you are doing.
    -Tim
    Current Collection
    1994 Thunderbird SC - Auto converted to 5spd

    1995 Thunderbird SC - Auto

    1994 Thunderbird SC - 5spd
    1989 Thunderbird SC - 5spd - Daily Driver
    1990 Thunder/Cougar XR7 - 5spd
    1994 Thunderbird SC - 3v Project

  6. #6
    Join Date
    Mar 2014
    Posts
    9
    Going through exact same problem. I have replaced cam sensor, dis module, coil pack in that order. Wierd part is I never got the upshift light while cranking which means bad crank sensor. After replacing all those parts,now I get the upshift light and will not start what so ever,not even intermittently. I have a new crank sensor that I will be puting in this weekend. Fingers crossed, if that doesn't work I'm stumped after that.

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