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Gear Vendors overdrive?

Discussion in 'The Garage' started by mountainexplorer, Nov 14, 2003.

  1. mountainexplorer

    mountainexplorer 1/2 ton status

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    Has anyone ever run one of these before?

    I may have scored a deal on an '83 1-ton 4x4 with an overdrive on the back of the T-case. I know it can only be used in 2wd, but it would help for highway mileage I'd think.

    What might a used one be worth?
     
  2. VisionxOrb

    VisionxOrb 1/2 ton status

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    GV overdrives are the Chit!!!! those babys can handle around 1500hp stock and 2000 with a little work, plus u can split every gear, could be useful for towing.
     
  3. Desert Rat

    Desert Rat Fetch the comfy chair

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    I have one in the 76 Blazer I just bought. The pro is that it works like a 6 speed and the overdrive is great for keeping the rpm's down on the highway. The only con so far is the shorter driveshaft. But, I suspect it's similar to having a doubler in there as far as length goes.
     
  4. mountainexplorer

    mountainexplorer 1/2 ton status

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    Cool. /forums/images/graemlins/grin.gif

    Ive looked up new prices, and they seem spendy from what I've seen. What should a good used one be worth?

    Right now its all bolted up to a 205 T-case in an '83 1-ton 4x4. I think I can buy the whole truck (minus motor) for $1000. Im leaving 1st thing in the morning to go put money down on it. I figure the overdrive is nearly worth the price of the whole truck... plus the driveline and everything is all there... it's all set up.
     
  5. Desert Rat

    Desert Rat Fetch the comfy chair

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    I'd buy it too. I'm actually probably going to sell my GV unit along with the NP205 and the TH350 that it's hooked up to as a unit one of these days myself...
     
  6. mountainexplorer

    mountainexplorer 1/2 ton status

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    How old is your unit? The one under this truck is an "older" model I'm told, and it seems to slip some in reverse.
     
  7. Stephen

    Stephen 1/2 ton status Moderator Vendor

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    GV's system doesn't let you use it for compression braking, and you probably can't use it in reverse either for a similar reason.
    They require pump speed to keep the hydraulics locked so you can't use them under 20 mph they say. Also, they use a cone clutch and the thrust from the gear set locks the clutch harder as power is applied, in a forward gear. If you try to compression brake with them, the power input is the wrong direction and the clutch loses pressure and slips. That smokes the unit.

    Buy the 1-ton.

    You can call GV and get the right manuals and everything you need for the unit, there's some info you really need to have before you use it.
     
  8. mountainexplorer

    mountainexplorer 1/2 ton status

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    Thanks for the info. I went and picked the truck up today. He gave me the manual for the GV also.

    So, does that mean that (even in the low range) that the GV will slip in reverse? I have yet to read the manual. I guess I should start.
     
  9. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Why would you want to overdrive reverse? Are you racing backwards with someone or something?
     
  10. Desert Rat

    Desert Rat Fetch the comfy chair

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    Mine was installed new in 1991. The guy also rebuilt the TH350 and put in an NP205 at the same time. I've never had it slip and for towing, it's the shiznit.
     
  11. mountainexplorer

    mountainexplorer 1/2 ton status

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    No, I was told it doesnt work in overdrive in reverse. But in non-overdrive (low range/regular range...whatever) it slips. I would like to be able to back up hills or back up with a trailer. So Im concerned about why it slips like he said it does. He said he would lock it in 4 wheel when he would back up with a load.
     
  12. mountainexplorer

    mountainexplorer 1/2 ton status

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    Did yours act any different in reverse?
     
  13. azblazor

    azblazor 1/2 ton status Premium Member

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    I've got one. Works great. No problems in reverse or any other mode. It acts like a 1:1 direct drive when it's not in OD.
     
  14. BanksBurban

    BanksBurban Registered Member

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    16 years and 140,000 miles later, mine still works like new. I had slipping in 3rd and chatter in reverse. It ended up being the tranny.
     
  15. Desert Rat

    Desert Rat Fetch the comfy chair

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    I've never noticed any slipping in reverse, or any other gear.
     
  16. kennyw

    kennyw N9PHW Premium Member

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    Allan, I think the tranny is more than likely causing the problem in reverse. Pull the pan and check for metal in the tranny /forums/images/graemlins/thumb.gif
     
  17. mountainexplorer

    mountainexplorer 1/2 ton status

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    Well, it's an SM465. And I know the clutch wasn't slipping. It was solid going foreward. I got to drive the truck down the freeway and around before I bought it and he pulled the motor.
     
  18. dyeager535

    dyeager535 1 ton status Premium Member

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    To be honest, my dad has one in his truck with 454/456/4.10's, and I don't think it's effective.

    It does cut down on RPM's, but why it's not a ratio of something like .5:1 instead of the .7+:1 that it is, I don't know. (perhaps space considerations for cars) You aren't really splitting the gears, and with the 465, a "half gear" in each gear over 20MPH would make a huge difference, at least in his case. When towing, you STILL have the gear spacing problems of the 465, and the .7+ OD ratio isn't a big enough difference to help on big hills.

    I also don't like the way you can't use it under 20MPH, and that if you leave it in OD as you slow down, you can damage the unit and it doesn't sound/feel nice. /forums/images/graemlins/smile.gif

    Can't hurt if it was a good deal of course. No reliability issues, I just think it could have a more effective OD ratio.
     
  19. BanksBurban

    BanksBurban Registered Member

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    I didn't know you could damage it slowing down while engaged. I leave mine on, set at 40 MPH all the time and drive it like a 4 spd auto. Like I said, 16 years and 140,000 miles later, it still works like new. It doesn't make any funny noises and feels fine when I slow down.
     
  20. azblazor

    azblazor 1/2 ton status Premium Member

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    From Owners Manual
    General Information - all models

    A. The GV Under/Overdrive is a hydraulically operated auxiliary transmission. It is activated electronically by opening and closing a solenoid. Opening the solenoid allows the unit's shaft actuated pump to create nearly 600psi and perform the shift. Due to the hydraulic requirement the unit cannot be activated until approximately 20mph, and should not be operated below this speed.

    B. The GV has an electronic function that automatically turns the unit on and off at predetermined speeds. This function prevents premature shifting of the unit before proper hydraulic pressure is achieved, and, prevents the unit from activating in reverse, which could cause internal damage. A manual override (on-off switch) deactivates the automatic overdrive function if so desired, This switch used in conjunction with the transmissions manual gear selector allows gear splitting (see appropriate instructions for manual or automatic transmissions).

    C. Always deactivate the GV auxiliary before backing up; on manuals push down on the hand switch and on automatic models depress foot switch so red light does not illuminate. Automatic models can be left on if reverse speed does not exceed 10mph.

    D. Mechanical spring pressure (540lbs) allows for engine braking (deceleration) and reverse, however caution should be used. The proper sequence for slowing on hills is to use your brakes to slow down and then downshift to keep your speed in check. Neither the vehicles transmission or the GV auxiliary are designed as brakes.

    E. Gear Splitting delivers a final drive ratio between your present factory transmissions gear ratios. Choosing the proper gear ration keeps you from overrevving or lugging your engine. ......


    Automatic Transmissions:

    A. Vehicles equipped with automatic transmissions are equipped with electronic controls that will activate the overdrive automatically. For overall economy and performance it is best to operate your GV Over/Underdrive with the dash switch depressed to the right (40mph activation). A small red light on the dash switch will illuminate when the key is turned on letting you know the unit is set for automatic overdrive. If not red light appears, depress the foot switch to activate the unit. Place your vehicle's gear selector in drive and take off. You will shift through your normal gears until the vehicle reaches approx. 40 mph, the small red light will extinguish and the green light will illuminate and activate the gear change. As you decelerate the overdrive will automatically kick off at approximately 18 mph.

    B. Gear splitting is used to increase acceleration when moving a heavy load from a standing start or adding performance when climbing hills. Gear splitting can be accomplished under full power if necessary. To use all available gear ratios begin by depressing the dash switch to the left. This enables the unit to activate at approximately 20 mph ( disengagement occurs around 18 mph). Place the gear selector in low and ensure the red light is on. As you gain speed the vehicle will automatically be shifted into 1st over. When you are ready to shift, move the gear selector into second---just as you feel the shift occur depress the GV foot switch. This deactivates the GV and you are in 2nd direct. As engine RPM increases depress the foot switch for 2nd over. .........;and so on

    C. If crusing down the highway in overdrive and you encounter a hill............

    D. Deceleration in 1st overdrive is not recommended

    Manual Transmissions:

    A. Manual transmissions are controlled by the on/off pull switch on the vehicles gear selector. The unit is activated when the switch is pulled up, and down is off. At least a half-clutch depression is recommended for shifting.

    B. Manual transmissions have only one setting on their dash switch. With the unit depressed to the left the grenn light will be on at approximately 20 mph indicating ;overdrive available;.
    For normal operation leave the dash switch in the left position. Due to the GV's hydralic requirement it does not allow splitting between 1st (compound or granny) and 2nd. This same hydralic requirement causes a slight delay from when the switch is pulled and the actual shift. Therefore the proper sequence when shifting between 2nd and 3rd is to pull the hand switch , wait a second, and then to depress the clutch. As speed increases the delay diminishes so the wait between pulling the switch and depressing the clutch is reduced between thrid and forth , or after.
    When towing a heavy load up a hill.................;
    When shifting from a lower overdrive ratio to a higher gear, prevent gear grind by waiting until the selector has been moved into neutral, or the higher gear, before switching off the overdrive.

    C. The GV electronic control box automatically disengages the unit as you come to a stop, hoever, it is necessary to always turn the hand switch off, depress button when your stopped or vehicle speed drops below automatic deactivation speed.

    D. Due to the hydralic requirement it is not recommended to use 2nd over for engine braking or deceleration; second or third gear is recommended for this.

    From the troubleshooting section:

    5. Check oil level and road test
    Level should be even with the bottom of 1" fill hole located opposite speedo assembly.

    A road test can determine if there has been clutch failure in the overdrive. In high gear overdrive, back off on the throttle, downshift into the next lower overdrive gear and check for engine compression braking. Turn off the overdrive and check for direct drive compression braking. If there is no compression braking in overdrive, the forward clutch has failed. A chatter in reverse, no reverse, and no compression in direct drive indicate the reverse clutch has failed. If in either case you have no compression braking, call Gear Vendors ......
     

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