Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

Swapping 100mph for 85mph speedo

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 77fixer, May 5, 2002.

  1. 77fixer

    77fixer 1/2 ton status

    Joined:
    Nov 22, 2001
    Posts:
    464
    Likes Received:
    0
    Location:
    Meridian, ID 83646
    Is an 85mph speedo a direct swap for a 100mph or will it give me inaccurate readings? I've got the 100 in my 77 4wd and an 85 in my 80 2wd. The 2wd has the tach and I would like to swap the entire cluster because of different fonts and colors. Will I have to run a separate wire for the tach hookup at the distributor? Does the fuel guage work without rewiring? What about the speedometer gears in the T/C? Can they be swapped to make everything work OK?
     
  2. 77fixer

    77fixer 1/2 ton status

    Joined:
    Nov 22, 2001
    Posts:
    464
    Likes Received:
    0
    Location:
    Meridian, ID 83646
    Hasn't anyone done this? Either swapping in a 100mph speedo or vise-versa.
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    26,979
    Likes Received:
    189
    Location:
    Roy WA
    Really only gotta know if the '85 is auto trans or not. If it is, its not going to be a direct swap. Converter lockup and Cruise is controlled by VSS (partly) and that is bolted right to the speedo. Not present on '81 or '82 up speedo's.

    Prolly the easiest thing to do if auto trans is find a sticker that goes over the 85MPH speedo, and have the speedo recalibrated. $40-50 locally.

    Other problems are wiring differences and spedo cable hookup, both/one of which can or will be different. Make sense? : )
     
  4. riz

    riz 3/4 ton status

    Joined:
    Mar 7, 2000
    Posts:
    6,026
    Likes Received:
    0
    Location:
    Wisconsin
    <font color=blue> Do you plan on going over 100 m.p.h. in it ?
     
  5. MudbogAD

    MudbogAD 1/2 ton status

    Joined:
    Jun 29, 2001
    Posts:
    672
    Likes Received:
    0
    Location:
    Las Vegas Nevada, US
    I have a 100 mph speedo in my 79 Jimmy.
     
  6. 77fixer

    77fixer 1/2 ton status

    Joined:
    Nov 22, 2001
    Posts:
    464
    Likes Received:
    0
    Location:
    Meridian, ID 83646
    The 100mph is in my 77 suburban 4WD and the 85mph is in an 80 suburban 2wd with the tach. Neither has any computer controls, both have the vacuum operated cruise control. No matter what I will probably swap and deal with the results just to keep the guages similar. The 77 has blue kph and the 80 has orange and the fonts are different. I won't mind an 85 mph speedo since even as a daily driver it probably won't see 75 very often. Although maybe it would be a good time to try those white-faced stickers.
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    26,979
    Likes Received:
    189
    Location:
    Roy WA
    Sorry mis-read post. 77-80 shouldn't be as difficult, since I was thinking '77-'85 /forums/images/icons/smile.gif
     
  8. Medman74K5

    Medman74K5 Registered Member

    Joined:
    Jun 7, 2001
    Posts:
    48
    Likes Received:
    0
    Location:
    Arizona, Prescott Valley
    I did that swap on my 74 seems to be accurate.
     
  9. 77fixer

    77fixer 1/2 ton status

    Joined:
    Nov 22, 2001
    Posts:
    464
    Likes Received:
    0
    Location:
    Meridian, ID 83646
    Thanks for the info. Now I just have to get an electrical sending unit for the oil pres.
     
  10. Blue85

    Blue85 Troll Premium Member

    Joined:
    Jul 26, 2000
    Posts:
    9,075
    Likes Received:
    486
    Location:
    Grand Rapids area
    I think the speedo will be fine. The big three generally a standard for speedo's/odo's. So many revolutions of the cable per mile or something like that. So different tire sizes/gear ratios were all compensated by different drive gears. You should be able to take any cable driven speedo from the 70's/80's and it should read accurately. I don't know about imports, though.
     
  11. Blue85

    Blue85 Troll Premium Member

    Joined:
    Jul 26, 2000
    Posts:
    9,075
    Likes Received:
    486
    Location:
    Grand Rapids area
    The cruise control reads revolutions inside the speedometer. The torque converter lockup uses vehicle speed from the governor inside the transmission; it is independant of the speedometer.
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    26,979
    Likes Received:
    189
    Location:
    Roy WA
    I don't think in any case the governor controls lockup. I think ultimately this is a "test it and find out theory" but this http://community.webshots.com/rz/r1/6/55/41/27265541GNaQnxOSAg_fs.jpg and http://community.webshots.com/rz/r1/6/31/18/27263118tjFhUzfaMU_fs.jpg show converter clutch output from the ECM, and show a VSS pin on the ECM connector, but no wire at that connector, which is odd, since all the other unused connectors are blank. However, it shows the pressure switches in the circuit (as I read it) as well as a temp switch in one year as well.

    In any case a service manual would probably tell, but I'd expect it to be slightly different every year or so. http://community.webshots.com/rz/r1/6/21/4/27262104XheYIgCdeJ_fs.jpg looks more like pressure switches and 4wd are the controls for lockup, although IMO, it's an unknown if VSS is used by TCC in the ECM.

    I know my '83 didn't have an ECM though, just the box above the gas pedal, but I don't have an '83 manual to read up on TCC. It seemed to be so consistent (lockup) that I always believed it was governed at least partially by VSS, but from what I can see from these diagrams, if there is no ECM, VSS probably is/was not an input for lockup. But again, an '83 or 84 service manual or wiring diagram would probably tell for sure.
     
  13. Blue85

    Blue85 Troll Premium Member

    Joined:
    Jul 26, 2000
    Posts:
    9,075
    Likes Received:
    486
    Location:
    Grand Rapids area
    No, you are right. I thought that his truck was an '82 for some reason and described how it worked for those. In an '87, the ECM controls the lockup completely. The governor does not come into play.
     

Share This Page