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Diagnosing a 700r4 Lockup convertor

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by BorregoK5, Feb 9, 2002.

  1. BorregoK5

    BorregoK5 1/2 ton status

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    I have a friend with a 91 GMC Jimmy which thumps and surges on the freeway. I've seen 700r4 lockup diagnostics on previous posts and searched out a few. Its my understanding that you can unplug the 3 wire plug going to the tranny which disables the elctric lockup switch. It's also my understanding that this switch still uses fluid circuit mechanisms to perform the actual lockup. Other than making sure not to drive around in overdrive while this plug is disconnected are there any other gotchas I should be aware of? I'd like to eliminate or identify the problem as being the lockup before it goes out for service! Thanks!

    I started with nothing and I still have most of that left! - <a target="_blank" href=http://community.webshots.com/user/borregok5> Pictures</a>
     
  2. jimmyjack

    jimmyjack 1/2 ton status

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    Mine did that when I first had my blazer. I had a tranny guy tell me to feather the brake with my left foot when it did that. Just enuff to lock up the convertor but not slow the truck down. It worked til my tranny blew up (totally unrelated)

    That jeep thing? Yah I understand it....... Like my Blazer? <a target="_blank" href=http://community.webshots.com/user/blazerk5>http://community.webshots.com/user/blazerk5</a>
    Jim
     
  3. woody9

    woody9 1/2 ton status

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    Any other symptoms? Mine is acting similar, but I'm going down a whole different trail to figure it out.
    First it's an intermittant thing (which I hope indicates that it's not anything terminal in the tranny).
    I've replace the Throttle Position &amp; O2 Sensors and it really improved the drivability. Guess you're supposed to do those @ about 75K (mine is about 90K over that, but no problem before this. around $50 for the pair; some assembly required!
    ALso running 3 bottles of the Chevron Techron EFI cleaner thru the tank (38 gallon tank, was way below 1/4th and only put about 20 gallons in with it). (Oh, and make sure fluid is A-OK too!)
    Drivability sure has improved a ton. (tune up done recently as well).
    That converter is controlled by the ECM which gathers up input from all those pesky sensors, and tells it when to do what.
    Also ordered a GM Drivability troubleshooting guide (used/$30.00) and hope that will unravel the mystery! It's due to arrive mid week.
     
  4. BorregoK5

    BorregoK5 1/2 ton status

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    If its any help, this problem is intermittent as well. I imagine its the same problem. I could swear I've hear this problem pass through here several times before and that it was something to do with a faulty switch.

    I started with nothing and I still have most of that left! - <a target="_blank" href=http://community.webshots.com/user/borregok5> Pictures</a>
     
  5. woody9

    woody9 1/2 ton status

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    Oh, forgot to post the best part.. The guy i'm getting the book from had a 91 as well (it went down the road and he's got a sweet 99 now, but that's another story).
    Well, he chased down the same problem in his, replaced it methodically (sp), and after some 'quality time' with his local GM dealer &amp; electrical shoppe, deceided to tackle the rascal himself. He had a multi-meter, did his homework on all those sensors/wiring to see input/return voltages/ohms and all those fancy electrical terms, and still problem wasn't obvious.
    well, by this time, he's about to tear the thing down to the frame. Even replaced the ECM (can you say Cha-Ching boys &amp; girls?).
    Well, he goes to the next level, and starts pulling the wiring harnes from the back of the ECM that goes thru the firewall, and unraveling it looking for frayed spots.
    Lo &amp; behold, he finds one! repairs where's it's chaffed, tapes it, puts it in conduit, tapes that. Quite the assembly once he's done!
    Seems that harness rides right near where those luxury items (heat, A/C, defrost/etc) have their controls and such. I personnaly haven't been in this terratory, but fear it's in my near term future!
    THe items I changed really was money well spent, and it acts like a pup now except for this problem. and as mentioned, it's only interrmittant (and today, only acted like it wanted to chug in/out of 'lockup'; didn't really buck like it had been).

    I can keep ya posted on it if you like...
     
  6. ftn96

    ftn96 1/2 ton status

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    So you think it is an electrical chaffing problem to check for before droping the mother out?! I dig. But Im having a very simular problem right now but mine doesn't seem to be TC related. More of a solid metalic thunking , I just skipped a tooth sorta thunk. But I would not rule out electrical ball means. Mine seems to do it going up hills on the interstate, down hills, flat land, dont matter. Does this sound like the same thing????

    **WARN**ing, Something winching this way comes! [​IMG]
    <a target="_blank" href=http://www.mccords.s5.com/myk5/index.htm>NashvilleK5</a>
     
  7. woody9

    woody9 1/2 ton status

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    Ouch! that sounds different than what mine is/was/is doing. Mine feels like the TC is trying to Lock/Unlock/Unlock rapidly! It also seemed to do it only in a select 'band' of movement on the 'go' pedal/speeds. Stomping the 'go' pedal seems to bring it out of it, but just having WFO &amp; Off don't seem quite right to me! (especially if mamma drives it!)
    The chaffing (as explained) was on the back (firewall?) side of ECM and easier to access by pushing the cable bundle OUT under the hood for further examination. (don't like the sound of that 'labor' wise, and figured I'd check all sensors/clean injectors, etc first. Maybe i'm just allergic to electrical troubleshooting? ROFL).
    If you've got a metal clunk, hills/corners, that sounds more like a spider/ring-pinion affair and i'd think it'd jolt ya a bit when it happened. Any recent hard-wheeling? drag race starts? etc?
    When that book arrives, I can look for anything that sounds similar to yours.....?
     
  8. 95 Silverado

    95 Silverado 1/2 ton status

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    unplugging the transmission connector will disable the lock-up convertor, but you can still drive it in overdrive. the overdrive and the lock-up are two separate functions. the solenoid inside the transmission that controls the hydraulic operation of the lock-up convertor is prone to problems if the problem goes away when it is unplugged it could very well be the problem.

    '95 Chevy Silverado 1500
    5.7 V8-NV4500- 3.73 rears
    See My Toys: <a target="_blank" href=http://community.webshots.com/user/wdb172454>http://community.webshots.com/user/wdb172454</a>
     
  9. woody9

    woody9 1/2 ton status

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    I'd agree that the solenoid COULD be the problem. But i'm trying to insure all the 'input' to the control module is on target and problem actually lies within instead of a $15 sensor or loose/aged connection.
     
  10. BorregoK5

    BorregoK5 1/2 ton status

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    Oh, I was under the impresion that you could burn up the tranny driving in overdrive without it locked up. It would have excessive slippage driving near the stall on the torque convertor in high gear should it hit overdrive and be cruising at the right speed.

    I started with nothing and I still have most of that left! - <a target="_blank" href=http://community.webshots.com/user/borregok5> Pictures</a>
     
  11. 89GMCSuburban

    89GMCSuburban 1/2 ton status

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    Does he have a heavy foot? That's how I've blown three torque converters in 700's I've owned. If it's a jerking, like the motor is cutting in and out, but the engine RPM isn't changing, it's the lockup. Unplug the plug and put it out of the way. I've been driving for about a month with it like that, but expect the tranny to go out soon. Pieces of the TC get into the clutches and destroy it. My baby is going on for a tranny rebuild today.

    Red 1989 GMC Suburban SLE V1500 - TBI 350, 700R4 w/ shift kit, Quad-shock front, 31/10.50/15 Pro Comp MT, 4.10 gears, Duals off cat
     
  12. BILLY RAY

    BILLY RAY 1/2 ton status

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    HAVE THEM PUT IN A 4L60E CONV. AND WELD THE FINS AND YOU WILL NEVER HAVE ANOTHER PROB. I PROMISE!!!
     

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