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Is it my tranny or my engine?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by txbartman, Jun 18, 2001.

  1. txbartman

    txbartman 1/2 ton status

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    I've got a 85 Jimmy with a 305 connected to the 700 tranny. At highway speeds with the slightest incline or wind or load, anything, my truck downshifts from overdrive (4th) to 3rd for about 2 seconds then back to overdrive. It runs there for about 10-20 seconds then does the "hiccup" downshift again. Is it my engine not providing enough power and "confusing" the tranny? Or does my tranny have a problem? Someone said something to me about maybe vaccuum? Can anyone help, please?!?!?!?

    85 GMC Jimmy, 305 w/TH700-NP208, 32X11.5 BFG ATs, 15x10 Eagles, Smittybuilt tube nerf bars and bumbers
     
  2. Emmettology 101

    Emmettology 101 3/4 ton status

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    Its the tranny. I believe Summit Racing sells something that will eliminate this.... Give them a call.... Number is at: <A target="_blank" HREF=http://www.summitracing.com>http://www.summitracing.com</A>

    <font color=green>MIKE</font color=green> - "Hukd on fawnics,werkd fer me!"
    <A target="_blank" HREF=http://www.geocities.com/emmettology101>http://www.geocities.com/emmettology101</A>
     
  3. muddin4fun

    muddin4fun 3/4 ton status

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    That type of tranny doesn't really like to stay in OD. They do make something that will let you push the peddle down 3/4's of the way and it will stay in OD, but also when in OD the tranny slips more and makes more heat and heat is what that tranny does not like! If you do get that servo (or what ever it is from summit) make damn sure you get a tranny cooler. You should get one anyway, but definatly if you put that in.

    <font color=purple>Have you hugged your K5 today?</font color=purple>
     
  4. txbartman

    txbartman 1/2 ton status

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    Thanks. I called and they said the 700 was well known for doing the, as he called it, the 3-4 shuffle. He said I should install a shift kit. He recommended the TransGo shift kit. Said it was something I could do myself. I have worked a lot on engines, but never much on transmissions. Anyone ever installed one?

    85 GMC Jimmy, 305 w/TH700-NP208, 32X11.5 BFG ATs, 15x10 Eagles, Smittybuilt tube nerf bars and bumbers
     
  5. txbartman

    txbartman 1/2 ton status

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    My truck came from the factory with a heavy duty trailering package. It already has a tranny cooler installed. When I found that out I thought it was pretty cool. (no pun intended)

    85 GMC Jimmy, 305 w/TH700-NP208, 32X11.5 BFG ATs, 15x10 Eagles, Smittybuilt tube nerf bars and bumbers
     
  6. Blue85

    Blue85 Troll Premium Member

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    This is the famous Chevy clueless transmission. It has nothing to do with vacuum, only vehicle speed and throttle position. You have a couple of options. One is to order the B&M overdrive kickdown valve mentioned above and tear into the transmission valve body to "fix it". However, you don't want to work the transmission with much of a load in 4th gear. This is because it doesn't cool as well in OD and th lockup clutch is not that strong, and you always have more torque in a higher gear for the same wheel load. So I don't know whether all the shifting or the higher load in OD is worse for the tranny. The best way is to change the load by putting lower gears in your axles or going back to stock sized tires. This would make it a lot better. You could also change the TV cable a little bit, but that wouldn't help much and could be harmful to the tranny. You could change the governor weights and lower all of the shift points, but your acceleration would suffer. The easiest thing is just to shift to "3" when you're going up a hill and put it back in "D" when you crest. This saves from a lot of shifting, but is also more work. If you are in a really hilly terrain or are pulling something, you should just be running in "3" all the time anyway. But all the 3-4 and 4-3 shifting can wear out the forward sprag in the 700. Oh yeah, the final other thing that would help is to make sure that the engine is running it's best, so that it's putting out the most power with the least gas pedal applied. That reduces the TV pressure that triggers the downshifts.

    <font color=green>If a K-5 is "Built not Bought", why does this stuff cost me so much money? [​IMG] </font color=green>
     
  7. Emmettology 101

    Emmettology 101 3/4 ton status

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    A few of my friends have done them and they say they are not too hard. Just watch because when you drop the valve body there are some check balls(small metal balls) that may pop out. Just keep a very watchful eye outand be very steady.
    BTW Trans-go is a good name! Rossler Transmission in Girad, OH uses them and they do a lot of high HP tranny work. Example, they build trannies or John Lingenfelter, and the fastest street car in america(as seen in Hot Rod mags)

    <font color=green>MIKE</font color=green> - "Hukd on fawnics,werkd fer me!"
    <A target="_blank" HREF=http://www.geocities.com/emmettology101>http://www.geocities.com/emmettology101</A>
     
  8. txbartman

    txbartman 1/2 ton status

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    I always keep it in drive when pulling my boat or much of a load (and at 4,000 lbs, my boat is a load!). As well as when I am in rolling terrain. But, when it bounces between 3 and 4 at 50MPH on a flat Texas highway with only me and my girl in it, I believe it has issues! She doesn't weigh THAT much! I will probably do the shift kit and see what that does for me. Besides, I love to tinker. Thanks for the thoughts!

    85 GMC Jimmy, 305 w/TH700-NP208, 32X11.5 BFG ATs, 15x10 Eagles, Smittybuilt tube nerf bars and bumbers
     
  9. CaptCrunch

    CaptCrunch 1/2 ton status

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    The shift kits are easy enough to install but make sure you follow the instructions to the letter. You may want to call up a tranny shop and tell them you deal... a good tranny shop can do the mods you need to correct your problem and improve the shift quality. Chances are it will cost you the same as the shift kit and you don't even have to get dirty or worry about messing it up... all the parts the shop uses are a just a couple of bucks and it only takes an hour or so...

    BTW... 305's IMO don't have enopugh power to tow a loaf of bread... a weak emmision engine for late 70's and 80's.... the dark ages in vehicle performance. A 350 would be much more caple of doing any towing and of hustling at freeway speeds better

    -Mikey
    1987 Chevy K5 Blazer- 350 TBI
    <A target="_blank" HREF=http://www.geocities.com/milnerlives_2000/blazer.htm>http://www.geocities.com/milnerlives_2000/blazer.htm</A>
     
  10. Blue85

    Blue85 Troll Premium Member

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    Yeah, that does sound like it's too sensitive. It would probably hold OD better at a higher speed, but when you go too fast, then the throttle is open so much that it starts with all the shifting again.

    Just make double sure that the kit you getting addresses this issue specifically. Most shift kits just firm up your shifts and some even adjust the shift points, but generally they do nothing for the 3-4 downshift. Also, make sure that you are hip to pulling out the valve body and drilling some holes and stuff without losing any of the checkballs.

    <font color=green>If a K-5 is "Built not Bought", why does this stuff cost me so much money? [​IMG] </font color=green>
     
  11. txbartman

    txbartman 1/2 ton status

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    I have full intention of replacing the engine. But, due to financial constraints, that won't happen until next Spring. So, I am getting all the other mechanical issues out of the way right now. And, the actual power rating on a 305 is, "Won't pull the bottom out of a wet paper bag".

    85 GMC Jimmy, 305 w/TH700-NP208, 32X11.5 BFG ATs, 15x10 Eagles, Smittybuilt tube nerf bars and bumbers
     
  12. CaptCrunch

    CaptCrunch 1/2 ton status

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    Ahhh... paper bag... I must of confused it w/ an 86 305 LOL [​IMG] A little work to modify the trannt definetly won't hurt... but I bet you and your truck will be a lot happier w/ a new motor [​IMG].

    -Mikey
    1987 Chevy K5 Blazer- 350 TBI
    <A target="_blank" HREF=http://www.geocities.com/milnerlives_2000/blazer.htm>http://www.geocities.com/milnerlives_2000/blazer.htm</A>
     
  13. Hathaway

    Hathaway 1/2 ton status

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    My 86 305 does just fine thanks [​IMG] &lt;going off to sulk now because I can't afford a new engine what with all the other repairs AND school coming up&gt;

    Dan, '86 K5, 5.0L v8, NP208, TH700R4, open 10bolts front and rear, 3.43gearing, K&N filter, 31x10.50R15 Trailhandlers
     

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