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Thread: Intermittent Tach drop

  1. #1
    Join Date
    Jan 2021
    Posts
    3

    Intermittent Tach drop

    Good evening, I'm a new SC owner and am looking for advice on an issue with the 92 I just purchased. It drove fine throughout the day but then the check engine light came on and tach dropped to zero. It didn't register any rpms for about a mile and then check engine light went off and tach started to work again. Any help would be appreciated.

    Jake
    Last edited by pjfirla; 01-05-2021 at 06:14 AM.

  2. #2
    Join Date
    May 2018
    Location
    Irmo SC
    Posts
    199
    I belive the most common problem is the camshaft position sensor

  3. #3
    Join Date
    Apr 2002
    Location
    St. Louis, MO
    Posts
    6,548
    While the cam sensor can apparently fix the problem, I have yet to determine the cause of mine. I've tried 3 different cam sensors (two new, and one known good used sensor) and still he the intermittent issue. The only constant seems to be related to under-hood heat.
    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!

  4. #4
    Join Date
    Jul 2005
    Posts
    493
    Good evening Mr. Kurt K

    Maybe this might help.

    I have also run up against the same problem. Intermittent dropping RPM, EEC light, bucking. Sometimes a hard fail. Troubleshooting only lead me to the CAM sensor and or DIS harness. I even replace the DIS module with silver heat sink compound. Would work but within 3 months and especially when outside temperatures exceeded 115 + degrees it was a failure. So would begin troubleshooting/parts cycle.

    So in Aug 2020 I removed the DIS harness. I decided to use the EVTM and ohms check the entire DIS harness staring from CAM/CRANK sensor connectors to both the GRAY and BLACK DIS connectors. Checked my spares. All bad. The surprising clue that I ignored over the years was when removing the harness it would do a strange "twist memory movement" at the wiring connectors. Slight twisting while ohms testing was the key to finding the issue. I believe over the years constant twisting/ un twisting/pulling/heat cycling causes the insulation to break and expose wiring to the elements, corrosion and or fraying. For me this was especially true at the CAM sensor connector.


    Ordered/acquired all necessary connectors, wiring/ looms, 3M electrical copper shielding and TESA electrical tape. Overhauled/Repaired the DIS harness Sept 2020. At the time of this post, no intermittent EEC light codes for CAM sensor, no bucking, no tack drop. Car runs and sound better. Can't wait for the summer.

  5. #5
    Join Date
    Apr 2002
    Location
    St. Louis, MO
    Posts
    6,548
    Quote Originally Posted by sam jones View Post
    Good evening Mr. Kurt K

    Maybe this might help.

    I have also run up against the same problem. Intermittent dropping RPM, EEC light, bucking. Sometimes a hard fail. Troubleshooting only lead me to the CAM sensor and or DIS harness. I even replace the DIS module with silver heat sink compound. Would work but within 3 months and especially when outside temperatures exceeded 115 + degrees it was a failure. So would begin troubleshooting/parts cycle.

    So in Aug 2020 I removed the DIS harness. I decided to use the EVTM and ohms check the entire DIS harness staring from CAM/CRANK sensor connectors to both the GRAY and BLACK DIS connectors. Checked my spares. All bad. The surprising clue that I ignored over the years was when removing the harness it would do a strange "twist memory movement" at the wiring connectors. Slight twisting while ohms testing was the key to finding the issue. I believe over the years constant twisting/ un twisting/pulling/heat cycling causes the insulation to break and expose wiring to the elements, corrosion and or fraying. For me this was especially true at the CAM sensor connector.


    Ordered/acquired all necessary connectors, wiring/ looms, 3M electrical copper shielding and TESA electrical tape. Overhauled/Repaired the DIS harness Sept 2020. At the time of this post, no intermittent EEC light codes for CAM sensor, no bucking, no tack drop. Car runs and sound better. Can't wait for the summer.
    All of my symptoms started after removing heat-wrapped shorty headers and replacing them with are stainless steel mid-le gth headers. The under hood Temps significantly increased. I also would have been twisting and connecting the sensors. I've suspected I've needed to attempt what you have done for a while, but have lacked motivation to tackle the job. It is encouraging that you fixed your issue that was definitely more severe than mine

    Thanks for the detailed post.
    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!

  6. #6
    Join Date
    Jan 2021
    Posts
    3

    Should I order all new sensors

    Quote Originally Posted by sam jones View Post
    Good evening Mr. Kurt K

    Maybe this might help.

    I have also run up against the same problem. Intermittent dropping RPM, EEC light, bucking. Sometimes a hard fail. Troubleshooting only lead me to the CAM sensor and or DIS harness. I even replace the DIS module with silver heat sink compound. Would work but within 3 months and especially when outside temperatures exceeded 115 + degrees it was a failure. So would begin troubleshooting/parts cycle.

    So in Aug 2020 I removed the DIS harness. I decided to use the EVTM and ohms check the entire DIS harness staring from CAM/CRANK sensor connectors to both the GRAY and BLACK DIS connectors. Checked my spares. All bad. The surprising clue that I ignored over the years was when removing the harness it would do a strange "twist memory movement" at the wiring connectors. Slight twisting while ohms testing was the key to finding the issue. I believe over the years constant twisting/ un twisting/pulling/heat cycling causes the insulation to break and expose wiring to the elements, corrosion and or fraying. For me this was especially true at the CAM sensor connector.


    Ordered/acquired all necessary connectors, wiring/ looms, 3M electrical copper shielding and TESA electrical tape. Overhauled/Repaired the DIS harness Sept 2020. At the time of this post, no intermittent EEC light codes for CAM sensor, no bucking, no tack drop. Car runs and sound better. Can't wait for the summer.
    Would it be a good idea then for me to order all new sensors (cam, crank, and DIS module) and replacing all?

  7. #7
    Join Date
    May 2018
    Location
    Irmo SC
    Posts
    199
    Personally I would replace the cam sensor and work from there see where if anywhere that leads you just be really careful with the pig tail make sure it doesn't end up contacting the pulley that it's right behind

  8. #8
    Join Date
    Jul 2005
    Posts
    493
    Good morning


    Do not start ordering parts until you identified/verified the cause of the problem.


    Start with the simple. Visual inspection of the DIS harness. For example:


    Is the CAM sensor section of the harness rubbing against the supercharger belt?

    Are the DIS connectors secured to the sensors and or modules?

    Do you have both DIS harness shielding connectors (crank sensor and the main engine harness) installed? Are they serviceable?

    Are any DIS connectors insulated wires exposed and or fraying?

    Is the DIS module broken at either end of receptacles?

    Is the heat sink underneath the DIS module still intact or does it need to be replaced?

    Any corrosion on the 4 DIS module bolts which serve as ground?

    Any corrosion found in the DIS harness connectors? Sensors and or modules? If needed use quick dry and or @Deoxit 5 electrical contact cleaner spray.


    Next do a Key On Engine Off and Key On Engine On scans for codes. Identify and correct. Hopes this helps.

  9. #9
    Join Date
    Jan 2021
    Posts
    3

    thank you

    Quote Originally Posted by sam jones View Post
    Good morning


    Do not start ordering parts until you identified/verified the cause of the problem.


    Start with the simple. Visual inspection of the DIS harness. For example:


    Is the CAM sensor section of the harness rubbing against the supercharger belt?

    Are the DIS connectors secured to the sensors and or modules?

    Do you have both DIS harness shielding connectors (crank sensor and the main engine harness) installed? Are they serviceable?

    Are any DIS connectors insulated wires exposed and or fraying?

    Is the DIS module broken at either end of receptacles?

    Is the heat sink underneath the DIS module still intact or does it need to be replaced?

    Any corrosion on the 4 DIS module bolts which serve as ground?

    Any corrosion found in the DIS harness connectors? Sensors and or modules? If needed use quick dry and or @Deoxit 5 electrical contact cleaner spray.


    Next do a Key On Engine Off and Key On Engine On scans for codes. Identify and correct. Hopes this helps.
    Awesome, I appreciate it. I'll have to learn as I go but I'll definitely check all of this out. I'm still learning how to work on vehicles but considering it will take a while to get it titled and registered I'll have plenty of time. Thanks again.

  10. #10
    Join Date
    Apr 2002
    Location
    St. Charles, MO
    Posts
    21,184
    Quote Originally Posted by pjfirla View Post
    Would it be a good idea then for me to order all new sensors (cam, crank, and DIS module) and replacing all?
    Not sure about the Cam Position sensor, but the DIS module and Crank Sensors haven't been available for many years. Bill Evanoff at SCP just started offering an aftermarket Crank Sensor last year.

    David
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

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