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What is the vaccum pump for?

Discussion in '1982-Present GM Diesel' started by afroman006, Aug 16, 2005.

  1. afroman006

    afroman006 1/2 ton status

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    I did a barebones 6.2 swap in my trail truck and dont have anything hooked up to the vacuum pump. What is normally hooked up to it? Also, where is the temperature gauge supposed to be hooked up? There are several of the same appearance plug on different areas of the engine and they all have two male blade terminals on top, are they temp senders?
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Vac pump is usually for the heater controls.

    If you don't have AC you don't need the vac pump. My 6.2 has no vac pump at all...

    Temp sender is on the drivers side head towards the front of the motor.

    Rene
     
  3. afroman006

    afroman006 1/2 ton status

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    So do you have a plate or something over the hole? Can you buy them or will it need to be fab'd? Thanks, thats what I thought.
     
  4. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    There is just a plug where the pump would normally go. Should be a dealer part, mine is factory...

    Rene
     
  5. rd durham

    rd durham Registered Member

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    The vacuum pump at the back of the engine also runs the oil pump. There is a gear drive quasi plug that can go in place of the vacuum pump to run just the oil pump.
     
  6. diesel4me

    diesel4me 1 ton status Premium Member

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    It runs a few things..

    The vacuum pump also supplies vacuum for the transmission modulator on the TH400 tranny,and the EGR valve and solenoids(disconnected on my 6.2!)..it also supplies vacuum to the A/C controls,like Rene said..and the "EFE" valve on the drivers side exhaust pipe--(same thing as a "heat riser" on gas engine)--IF your truck came equipped with one factory(trucks without EGR valves,the "C" code engines didn't have one)..

    I saw one diesel that was swapped into a gas powered truck--the guy tried using the vacuum assist power brakes by hooking it to the stock vacuum pump on the 6.2!--it worked,but had very little power assist!--he nearly totaled it a few times before he found a hydroboost brake system at the boneyard to swap in it!..

    The 6.2's I saw with a manual tranny had no vacuum pump--looked like a freeze plug in the block corking off the hole it normally goes in!--I guess its important to make sure that "dummy" shaft for the oil pump drive is installed if you decide to ditch the vacuum pump! :eek1: :crazy:
     
  7. afroman006

    afroman006 1/2 ton status

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    Well I am running a th400 so how do I connect the vacuum pump to the tranny modulator?
     
  8. rd durham

    rd durham Registered Member

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    On my CUCV engine that I dropped into a '79 Jimmy the vacuum first runs to a kickdown switch on the throttle cable bracket and then to the vacuum modulator on the transmission. The truck isn't at my house so I don't know when I could take pictures and post them but I'll try sometime this weekend if possible.
     
  9. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    That's exactly how mine is...

    Rene
     
  10. afroman006

    afroman006 1/2 ton status

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    I have a 6.2 parts truck with a manual right now and it does have a vac pump, but it has A/c and heater so thats probably why. It wont be any help figuring out how to hook up the tranny though, if anyone has pics of the setup that would be great. Thanks
     
  11. diesel4me

    diesel4me 1 ton status Premium Member

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    Its a bit complicated...

    The vacuum pump supplies vacuum to a switch on the injector pump,called a VRV valve...this switch interupts the vacuum to the tranny modulator...what it does is regulate the vacuum so it acts just like a gas engine does--high vacuum at idle,and progressivly less vacuum as the throttle is opened wider..since a pump put out constant vacuum,this valve is needed for the tranny modulator to work properly( I.E. "fool" it into thinking its hooked to a gas engine!)..

    The VRV valve on my truck was pooched--the tranny would hang in second gear until 50+mph,then SLAM into 3rd when I first got the truck--which supposedly had a fresh rebuilt TH400 installed when a boneyard motor was put in--

    The shop that did the swap must have never seen or worked on a diesel GM truck before--they succeded in installing the engine and tranny,but couldn't even get it started!--nor did they have a clue HOW too! :haha: --when I asked on CK5,tRustyK5 told me how to bleed the fuel system,and what too do about fuel lines and glow plugs,and I got it started in no time,and its been one of the best trucks I've owned so far(and my first GM diesel!)..

    Back to the VRV valve!--mine wasnt working right,I adjusted it (its similar to a throttle posistioning sensor on EFI)..but it acted the same,or sometimes it would work perfect,and shift normal,then act up again!..a search at the dealer showed the valve was no longer a "service part"--and every one I saw at the junkyards were pooched as well,or they would not "break up" parts off a good running 6.2..SO---

    I asked at a few tranny shops locally and 2 guys said the same thing--run the hose directly to the modulator valve from the vacuum pump!--and screw the adjustment screw on the modulator (usually located INSIDE the nipple the hose goes on!) with a tiny screwdriver IN until it bottoms out gently! (DONT TIGHTEN IT TOO TIGHT!) and the tranny should shift normally!..... :laugh:

    I've had mine set up that way since I got the truck roadworthy 1-1/2 years ago--it shifts a bit early and too smooth for my tastes,but at least I dont have to hit 50 before it shfts into 3rd anymore,and it hasn't blown the tranny after plowing 25 BAD snowstorms last winter,so I guess the "soft" shifts havent hurt it any..

    One other tranny shop said they use a "mechanical" modulator that is CABLE operated on diesel trucks like mine with the vacuum switch dilema--they said places like Jegs or Summit sells them for cars using a 6-71 Blower or other supercharger,because the blower will make the modulator act goofy!!...I looked on TCI's website--they sell one for about 120 bucks! :eek1: :crazy:
     

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