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Factory tach/instrument cluster

Discussion in 'The Garage' started by AZ79K5Project, Mar 17, 2007.

  1. AZ79K5Project

    AZ79K5Project 1/2 ton status

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    Can an 84 factory instrument cluster w/tach bolt into a 79 w/out a tach (and work)? What mods need to be done?
     
  2. Goose

    Goose 1/2 ton status

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    The wiring harness is different. That is, where the wires are routed to the big plug are in different places due to the tach if I remember correctly.
    If you get some good wiring diagrams, it shouldn't be that hard to fix it.
     
  3. AZ79K5Project

    AZ79K5Project 1/2 ton status

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    Thanks.

    Anyone have a link to a factory cluster diagram for this? 84 Chev P/U.
     
  4. mofugly13

    mofugly13 1 ton bucket of rust Premium Member

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    Haynes manuals have a good set of wiring diagrams, and where individual wires land in the connector. I plan on doing the same, putting an 81 cluster w/ tach in my '73. It's just gonna be tedious tracing out different wires on the diagrams to figure out what needs to go where.
     
  5. AZ79K5Project

    AZ79K5Project 1/2 ton status

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    Thanks - I just found the harness diagram in my factory service manual. I've got a nose bleed trying to read that thing.
     
  6. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Actually, in 79 the wiring changed, and stayed that way until 1990 when they switched to an electric speedo. Assuming both trucks have oil, temp, and volt gauges, not idiot lights on one or the other.

    You will need to switch the sending units from the newer engine over to the older one to keep the gauges reasonably accurate.

    The tach wiring is totally indepenant from the rest of the cluster's wiring, so you'll need to move the tach harness over as well.

    The clusters wiring was the same from 73 - 76, then 77 & 78, then 79+. They did change color in 1980, and began being equipped with speed sensors in 83 when the electric cruise system was introduced.
     
  7. AZ79K5Project

    AZ79K5Project 1/2 ton status

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    Thanks for the information. It gives me some direction.
     
  8. wazzabie

    wazzabie 1/2 ton status Premium Member

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    I've heard that if you mix years it may cause a fire. Something about the circuit boards not being compatable. Don't know if it true. Anybody have any experience with this?
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    Had a friend throw a tach cluster into an early truck, IIRC it didn't burn, but he got a lot of smoke and toasted the cluster somehow.

    As mentioned, it has to do with the wiring. It's actually pretty simple, even without a manual. Pull your cluster, and trace the wires on the PCB. If they all go to the same place, no problem. It's only a 16 pin connector or so, and not all the terminals are used every year.

    It's not an incompatibility issue, all the cluster connectors are the same, it's just the way the wiring and PCB are arranged.
     
  10. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    The major interchange issue rises with mixing up volt and amp gauges. From 73-76, they ran an amp gauge, which measures current, and is hooked up inline with a positive power wire, then from 78-87 they ran a volt gauge, which measures potential difference between a hot wire and a ground wire. If you mix them up, it will cause an electrical fire by overloading the coils in the gauges, and overheating them till they start burning.

    Technically speaking, the basic wiring pinout change was made in 77, when they switched to the volt gauge, but in 77 & 78, the trucks still had mechanical oil pressure gauges. In 79 they were changed to electric gauges, and the wiring for the gauges was adding, making the final wiring change until the electric speedos were introduced in the burbs / K5s etc.
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Have you looked at the '87+ stuff?

    I know the PCB is greatly simplified on the '90-91 stuff, but of course the electric speedometer was a major change anyways.

    Just curious if the PCB was improved before the electric speedo. It only uses about half the wires on the connector because all the power feeds and grounds are on the same pin.
     
  12. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    I'm assuming he is working with a 73-87 truck, so I didn't get into the 88+ stuff, where yeah, things definitely changed again :D
     
  13. dyeager535

    dyeager535 1 ton status Premium Member

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    I'm talking '87-89 R/V, our body style. But going off topic. :)
     
  14. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Ah yeah, good point. Haven't really delt with the R/V stuff much. I can definitely imagine that the 90 - 92 stuff is quite different with the electric speedo, but didn't know that the pinouts changed in 87 with the K5s, Burbs etc...
     
  15. wazzabie

    wazzabie 1/2 ton status Premium Member

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    Are you sure 77 - 78 still had the mechanical oil gauge cuz my 78 just has the wire hooked up.
     
  16. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Suppost to, but there are always exceptions :)
     

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