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308 GTSI Tachometer

Discussion in '308/328' started by Weaselwee, Jun 27, 2009.

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  1. Weaselwee

    Weaselwee Formula Junior

    Dec 20, 2005
    320
    Long Island
    Full Name:
    Jim
    #1 Weaselwee, Jun 27, 2009
    So I have had the car for about 7 weeks, 1981 308GTSi. When I bought the car, the mechanic said the tach was low. I asked him how he knew, he said it sounds low. Well respected guy, I trust him. It was on my list of things to take care of until today....
    So I am driving on one of the rare nice days on Long Island, I am doing about 40 coming off the parkway when all of a sudden the tach jumps from 3K to 4K. This freaked me out! My first thought was of course, snapped belt. After calming down, I recalled the issue of the low tach. The 5 min drive home was fine, only now the tach is about 1K above where it seemed to be. I am still getting familiar with the car and was trying to hear if it sounded any different than previously.
    So here are the questions:
    Does anyone know how to measure the tach for accuracy? I remember a thread where someone posted a formula about MPH vs RPM in certain gears.
    I know the tach is electrical, could I have had a faulty connection which righted itself from a bump, etc?
    I don't think I have the time to drive tomorrow, but I will let you all know what I find on the next drive.

    Thanks for the help.
    Should I have posted this in Tech Q&A?

    Jim
     
  2. irondogmike

    irondogmike F1 Rookie

    Sep 8, 2006
    2,532
    San Diego area
    Full Name:
    Michael Tucker
    #2 irondogmike, Jun 27, 2009
    for my 85 308qv,if I'm in 5th gear and doing 60 mph the tack reads 3k rpm,80 mph-4k rpm,100 mph-5k rpm,as for the other gears I never paid attention
     
  3. Weaselwee

    Weaselwee Formula Junior

    Dec 20, 2005
    320
    Long Island
    Full Name:
    Jim
    #3 Weaselwee, Jun 28, 2009
    Thanks Mike. I will compare next time out.
     
  4. mike996

    mike996 F1 Veteran

    Jun 14, 2008
    6,911
    Full Name:
    Mike 996
    #4 mike996, Jun 28, 2009
    You can check the tach based on RPM/speed in gear tables but this assumes that the tires are the same size as OEM and is an indirect/imprecise way to measure tach error. A better way is to use a photo tach and compare the actual engine RPM displayed on the photo tach with what is shown on the dashboard tach. It will probably take two people to do this - one to read the dash tach and one to operate/read the photo tach unless it has a remote sensor but will give you the precise variation in RPM displayed/actual RPM at any/all ranges you check. IMHO a photo tach is a very helpful tool for engine work on any vehicle engine (car/boat/plane)
     
  5. irondogmike

    irondogmike F1 Rookie

    Sep 8, 2006
    2,532
    San Diego area
    Full Name:
    Michael Tucker
    #5 irondogmike, Jun 29, 2009
    I did notice also when in 4th if the tach read 4krpm I was doing 60mph,so that might help?
     
  6. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,367
    NY
    #6 Steve King, Jun 29, 2009
    I would just hook up a known good tach to the coil and check the dash tach.
     
  7. Weaselwee

    Weaselwee Formula Junior

    Dec 20, 2005
    320
    Long Island
    Full Name:
    Jim
    #7 Weaselwee, Jul 5, 2009
    So I finally got a chance to drive for a while today. The tach was low this morning when I started her up. After about 10 minutes it jumped up again and stayed that way for about 20 minutes. On the next drive, it was normal for a while, but as I was driving I saw the tach creep down slowly as I was maintaining speed. Oh well, this will remain on the list of fixes.
    For the record it looks to be about 1K low compared to normal conditions and the numbers Mike gave, thanks Mike.

    Jim
     

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