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89 Burb Computer/Trany Problem

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by AZ_Overdrive, Aug 28, 2003.

  1. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    I have a good one for you all. I did a search and found a few issues with the 700R4 but nothing that talked about the problem I am experiencing on my 89'.

    I had the trans rebuilt last October. Shortly after getting the truck back it would "jerk" a little in OD for about the first 30min of driving (from cold). After taking it back to the trany shop 3 times (of course when I took it there the problem went away!) I drove the Burb while the tech had the AutoXRay hooked up. What was happening is that the computer was telling the trans to shift in and out of "lockup"(converter). So the trany is NOT the problem (according to the shop) but they could not diagnose the cause of the computer sending lock/unlock codes to the trans. It actually is sending the signal 2 to 3 times a second for the converter to lock/unlock. This is causing my trans to overheat and is what probably killed the old one last year.

    Please help! Does anyone know what scenario the computer in an 89' TBI use to send the lock/unlock signal to the trans? I hate to have to take it to the dealer and spend a couple of hundred bucks for them to tell me a wire (or something simple) was wrong.

    Your expert opionions and advice would be greatly appreciated.

    Thanks,
     
  2. azblazor

    azblazor 1/2 ton status Premium Member

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    What shop rebuilt it?

    I would look at the inputs to the computer that affect the lockup, make sure your brake switch is not sending a false signal, maybe MAP sensor (engine load).

    Do you have a factory service manual? They have pretty good troubleshooting trees.

    Another thought is to swap computers with somebody with the same setup and see if the problem continues...if it does you know its an input to the computer not the computer itself.
     
  3. HarryH3

    HarryH3 1 ton status Author

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    The first things I'd look for are a bad brake light switch and a flaky throttle position sensor (TPS). When the brake light swich closes, it tells the computer to unlock the converter. When the computer senses that the throttle position has increased by more than 2%, it will tell the converter to unlock. A flaky TPS could be sending out a signal that's changing just enough to trigger the unlock command from the computer.
     
  4. Blazer1970

    Blazer1970 1/2 ton status

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    I have heard that this problem can be caused by worn out bushings in the distributor. It causes an erratic output from the cam position sensor, which is part of the distributor, and that somehow causes the computer to lock/unlock the TCC. Pull the dist. cap and try to wobble the rotor side to side and front to back. If there is a lot of play, replace or rebuild the distributor.
     
  5. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    What gears are you running? What size tires? What RPM is the engine at when it's doing this?

    700 is notorious for in and out of lock up and OD with to tall of gear.
     
  6. sled_dog

    sled_dog 1 ton status

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    its also notorious for being the reason a 700R4 burns up. You shouldn't be driving in Overdrive all the time. On a 700R4 I never shifted in to Overdrive will like 70.
     
  7. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    Here is my setup:
    89' Suburban 1500, 14BFF Swap with 3.73 from a 1980 3/4T(the original 10B had 3.73s), stock TBI/engine(350), 2" lift(not that it matters with this problem), Tires: 305/70 R16 (equivilant to 33x12.5 15s)

    Ken's Transmission in Phoenix rebuilt and installed the tranny and I have had good luck with them (had 2 others tranys rebuilt by them). I actually watched the analizer, while driving and saw the computer telling the converter to lock/unlock, it was quite rapid.

    I came to the same conclusion, that an input to the computer is causing the problem.

    Based on the several suggested items to check, and the fact that I do not own an AutoXray analizer it looks like I will be forced to take it to the dealer. I will check the distributor play (I am mechanically inclined) but to test the TPS I am assuming I will need an analizer. I have read in this forum how the TPS works, voltage changes as throttle position changes. As far as the brake switch, if it was acting up wouldn't the brake lights be going on and off to indicate a faulty switch? I don't have a service manual (just a Hanes, which has very little info on TBI).

    As far as running in OD, I agree. Until I drop down to 4.10s I have been running around town in D. I only use OD above around 60-65.

    The problem seems to occur when the trans/engine is warming up. I would say, after about 20-30 minutes of driving. When I took the truck to the tranny shop to report the problem I was driving on the freeway for about 45 minutes and the problem went away. What has brought me to this forum looking for any assistance is the fact that I tried to head out of town last weekend and half way out of Phoenix the tranny blew the govenor cover off and spewed 3 quarts of fluid all over the interstate(heck of a smoke screen!). After I let the truck cool for about an hour I went to put the cover back on the side of the tranny and the case was as hot as a firecracker! I had been running it in OD but didn't notice the lockup problem. I wasn't towing anything but I did have the burb pretty packed with camping stuff.

    So in summary, I should be able to check the distributor out but as for the other items, TPS, Brake Switch, and MAP sensor I don't know of another way to test them except with an analizer.

    Oh, one more point for Grim-Reaper, the lock/unlock condition happens under very light or even throttle while in OD (while on a flat stretch of road) so I am not sure of the RPM but I would say around 2100-2300.

    Thanks everyone for the help./forums/images/graemlins/burb.gif
     
  8. Blazer1970

    Blazer1970 1/2 ton status

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    I had an '88 pickup that started doing exactly the same thing. It was basically stock, running 3.73s and 265X75R16s, and I had run it about 100,000 miles before that with no problems, so I know it was not anything to do with the gear ratio/tire size. Unfortunately, I got sick of screwing with the truck (due to a variety of other problems), so I traded it in on the new Dmax before I ever figured out the TCC problem. I hear about this same problem on one board or another about twice a month, so I am curious as to the solution. Please check the distributor just so I will know one way or the other.
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    Could be at least partially related...is your speedometer and what the ECM says you are doing MPH-wise the same?

    I'm not sure how much the VSS is plugged into the converter lockup on the trucks, but it stands to reason that the throttle position, vacuum signal, and vehicle speed are all tied together in the ECM, and if the speedometer/VSS is off, that may be throwing the TCC signal out of whack.

    Obvious way to rule this out is to make sure the speedometer gears are correct for your combo. If they are, you can pretty much disregard the above. /forums/images/graemlins/smile.gif
     
  10. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    Since the stock truck came with 3.73s and 235/75 R15 tires and I put 305s (33s) on the speedo is about 5mph off at 49MPH and gradually gets farther off the faster you go. Is there a VSS on an 89? I thought Chevy didn's start using them on Burbs until 90 or 91. Reguardless, it is looking more and more like a trip to the dealer. If I had the $200 bucks I would pick-up an AutoXray. I tried to use the free ALDL reader software by creating a custom cable to interface with the truck and a serial port but I could never get it to work.
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    VSS was used for the 700 since it's inception on the trucks. (meaning '82 IIRC)

    5MPH difference shouldn't be enough to cause problems like you are experiencing.

    An OBD1 autoxray shouldn't be but $150 or less if you can find it. I got mine off ebay for I think $125, with all 3 US automaker cables included.

    Check the cable diagrams, I know there are smoe differences based on year. Power to the cable may need to be external as well.
     
  12. K5 NUTT

    K5 NUTT 1/2 ton status

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    Snap On has a book that comes with there scanner...its a gm prom listing...gm just burned a new chip when they had problems...assuming you have the 7747 ecm...i would need your prom id number which will show on the scan tool to cross reference the prom and follow the chain till it runs out of supercedes...

    DW
     
  13. thebigdaddyof2

    thebigdaddyof2 1/2 ton status

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    The problem you described is identical to the way my rig is acting...now. Let me back up and say that I had a TOTAL ecm failure about 6 months ago. No check engine light- nothing. Luckily, I was able to drive it home in limp mode.

    NEway, since I swapped the ecm, my rig goes in and out of lockup at slower speeds....very cyclic like. I swapped in a used ecm and have thought since the day I put it in that it may be the fault. This rig is just a toy anymore so it is rarely driven. I have another (used) ecm that I will swap in just to make a comparison.
     
  14. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    I checked out the TPS this morning. I noticed that it doesn't look like some of the aftermarket ones. It doesn't look like it has any adjustment to it. There are NO slots where the mount bolts are for it to pivot. Does the "factory" TPS have any adjustment? I checked on prices and AutoZone, Napa, and Cheker all run around $30 but the dealer part is $80.

    Also, how do you check the voltage? What wire (of the three) do you check for juice? Looking at the harness, I am assuming that I will need to Pierce the wires with the multimeter probe. I have read on what voltage should be at idle and at WOT.

    /forums/images/graemlins/burb.gif
     
  15. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    Nevermind on how to wireup the TPS to check it. I found the link, need to use some jumper wires in between the harness and the TPS and then stick the probes in those.
     
  16. dyeager535

    dyeager535 1 ton status Premium Member

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    Don't know what's up with the TPS'.

    Got an original one on my '88 TPI setup, and it's not adjustable, but the brand new Delco replacement I bought *is* adjustable.

    Probably just a way for GM to use the same part # on multiple vehicles, letting the mechanic figure it out, and the customer pay for the additional time. /forums/images/graemlins/smile.gif
     

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