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89 Burb- What Tells Computer it's in 4th Gear?

Discussion in 'The Garage' started by AZ_Overdrive, Feb 3, 2004.

  1. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    89 Burb- What Tells Computer it\'s in 4th Gear?

    Well, it's been a year and a half and I still am having OD shifting problems with my 700R4. I broke down and purchased the AutoXray Scanner and here is what I have done:

    1. Replaced and recalibrated the TPS.
    2. Replaced the MAP sensor.
    3. Replaced all spark plugs and wires.
    4. Replaced cap and rotor. ( also verified that the distributor drive gear does not have any play in it).

    During a highway trip I had the scanner hooked-up while the problem was occuring. The TCC is locking and unlocking at a rapid rate (I posted about this condition about 4 months ago) while the engine is at a steady cruise in OD. I scanned through the codes while the condition was occuring and noticed the 4th gear code ( I think it's code 24 or 25) was also turning on and off but the transmission was definately staying in OD. So, it looks like whatever tells the computer the trans is in 4th gear is faulty (possible short?). So as the computer thinks the trans is going in and out of OD it is locking and unlocking the TCC.

    I called the trany guys up to tell them the results of my investigation and they said that there are NO electronics inside the trans that tells the computer that it is in OD. So... What tells the computer on an 89' Burb that the tranny is in OD?

    I can't tell you how many times I have banged my head against the wall trying to figure this out! Any and all help would be appreicated.

    Other than the in & out of TCC the truck runs great with the new gears and detroit!

    HELP!

    /forums/images/graemlins/burb.gif
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    I'm sure there will be more competent 700R4 posts following mine, but there IS a 4th gear pressure switch in (all?) 700r4's. Since it's showing a code for 4th gear, I guess it means the ECM "sees" this data, right?

    I would *assume* the 4th gear is a simple open/closed switch. Just don't know how that is tied into TCC.

    Have you forced lockup via the ALDL connector, and if so, what happened? If it's a mechanical failure in the TCC, you'd expect it to keep happening, if it were the switch, the "override" should keep TCC locked up. At least that makes sense to me... /forums/images/graemlins/smile.gif
     
  3. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    I am assume the TCC is locking and unlocking due to the computer "thinking" the tranny is going in and out of 4th. If there is a "switch" on the tranny that tells the computer it's in OD why would the tranny shop not know about it (Ken's Transmission in PHX, they have bulit two other trannies in the past for me)?

    Any idea where I should look for this switch on the trany body?

    Thanks for the info.

    Oh, almost forgot, I am not sure how to trigger the TCC using the ALDL port...
     
  4. dave_90_blazer

    dave_90_blazer 1/2 ton status

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    the 4th gear pressure switch is inside the trans pan, screwed into the valve body
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    I *think* you jumper the top two terminals of the ALDL connector to force lockup. Paper clip works for this. yeah, it's pin A and F that need to be jumpered to force TCC. Here's an actual pic of an ALDL connector with the pins marked Be careful though, the truck ALDL connector is mounted differently, not sure which is "top" and "bottom" the way the truck ALDL is setup.

    Kinda cool for the trouble codes. Not a troubleshooting flow chart, but trouble codes with some data is more in depth than just "code 12=normal" like most trouble code sites. Sorry, not what you need, but I just figured I'd share for anyone else CCC or TBI/TPi that is following along.
     
  6. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    How/where does the pressure switch wiring exit the trans? Is this somehting that I can replace myself? I have dropped a few trans pans in my time so if is just screwed into the valve body I should be able to replace it.

    Man! If this IS the problem I am going to be really pissed! Why would the tranny shop be jerking me around? Does anyone know of a reputable tranny shop in Phoenix? If I have to have a shop fix it I am going to take Ken's Transmission to court!

    Thanks for the info.
     
  7. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    Ok I am trying to remember where I picked this up. There are several things that come into play to cause the converter to lock and unlock.

    wherever I read this it mentioned engine vacuum as being one of them. Something about when the ECM see vacuum drop it perceives this as an indication that the engine is under load. I think it monitors the MAP sensor to make this deterimnation.

    Now I chased a bunch of drivability problems on my 88 Burb. No lock up since it has a TH400 but I had plenty of serges, skips and other crazy things.

    It came down to ALL of them being vacuum related in the form of leaks once I got the ignition problems all sorted out.
    Several leakes were the plastic lines they use for EVAP,EGR,PVC developing hair line cracks from age. The biggest single issue I found was the base plate gasket for the TBI was toast. That base plate gasket is a common fail item on these trucks.

    Start from scratch with the dumb stuff and get out the carb cleaner. Fire it up and start looking for leaks. Put a Vacuum gage on it as well and see what it's pulling. You can clip it to the whindsheild wiper and drive around and watch it and see what it's doing while you watch the auto X-ray.
     
  8. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    I think I will try the vac leak tests next. However, based on what I am seeing with the scanner (4th gear in/out at the same time as the TCC lock/unlock) it appears that the problem lies with that pressure switch inside the trany. When I drive in D only, the TCC locks and unlocks normally. No surge, or jupming in and out of lock.

    The truck is due to have it's vacume lines inspected though as it appears that all the lines are original. Is there a specific type of vacume line replacement for those hard plastic originals? Will you basic AutoZone/Checker vacume tubing work?

    One last note about the TCC/4th gear issue... After a while of driving the problem appears to go away. An example would be on a long trip, traveling between 65 and 75 it will exibit the lock/unlock behavior for about 15 minutes then subside. Then, after stopping for gas and starting back on the highway the problem will occur again for another 15 minutes, then quit. During a 4 hour trip the problem may occur again without any change in speed 65-75, then go away....
     
  9. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    Nothing special as long as it's rated for vacuum.



    <shhhhhh> don't tell anybody but I had a bunch a vac line from a Supra and that's what I used.
     
  10. HarryH3

    HarryH3 1 ton status Author

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    The signal comes out through the 4-pin connector on the drivers side of the transmission. Before you drop the pan, check for any bad connections. The "signal" to the ECM is just a switch grounding the wire when the tranny goes into 4th gear. If the connection is loose, that could be the problem.
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    If it were a vacuum problem, it would exist in all gears that use lockup. (at least 3/4)

    My thinking anyways.
     
  12. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    Thanks Grim and HarryH3!

    I will take a look under the truck tomorrow night for that connector on the tranny.

    Short story (really)...
    I have been without my 89 Burb for over a week now. Last Monday a group of guys tried to steal it from my driveway at 8:30 PM. I guess my neighbor spooked them and they took off before they could get the Burb started.

    They broke the passenger window to unlock the doors. I guess they tried to push the broken window in but the tint kept the shattered window in place. They completely destroyed the steering column. Talk about just in the nick of time, the ignition was on and the radio was blaring... After the police took fingerprints I tried to start the truck but the ignition switch (column) was all the way forward and the truck wouldn't start. The next day I stuck a screwdriver into the slot where the ignition cam moves and the truck started right up.

    Anyway, it's been in the shop and I pick it up tomorrow... They caused $1000 worth of dammage but thankfully I have full coverage insurance on the truck. And BOY! am I glad I also have rental car coverage. Before I pick it up I am going to purchase one of those club type locks that locks the brake pedal. I have herd they are hard to remove (for thieves). I am also going to invest in that steering column collar they sell in JCWhitney....

    It's amazing how much I miss my Burb... Don't think I can part with it any time soon.

    /forums/images/graemlins/burb.gif
     
  13. azblazor

    azblazor 1/2 ton status Premium Member

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    According to my electrical schematic for 1987 the circuit 446 is "4th gear in" to the ECM. It originates from the 4 pin connector on the trany and is the Light Blue wire. It enters the ECM from the J1 C7 connection.

    Sorry about your breakin on the Sub. Ours was stolen a year ago last October. Where do you live in the valley?
     
  14. scrappyk5

    scrappyk5 1/2 ton status

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    my 88 is controlled by the vss. which
    tells the ecm how fast i am going to let the
    the tcc kick in. i dont know if that helps you any.
    i know this becouse when my speedo cable was going bad
    and bouncing around, it made the vehicle shift in and out
    of lock-up mode. once i replaced the cable and the gear ,
    the problem was solved .
     
  15. AZ_Overdrive

    AZ_Overdrive 1/2 ton status

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    Re: 89 Burb- What Tells Computer it\'s in 4th Gear?

    UPDATE:

    Just got my Burb back from the body shop. I called around to a few trany places and asked about the 4th gear switch working intermintently. They almost seemed offended I would ask such a question. All the responses were the same "the switch either works or it doesn't. If you have an intermintent problem, you have a short somewhere between the trans and the computer."

    So, I crawled under the rig this evening and found the connector on the drivers side of the trany. It looks almost completly melted. Also, the wires leading to the connector look pretty worn out. I will try to trace the wires over the trans to see if any are frayed. I wonder if the dealer still sells that connector with splice-in wires?

    Thanks for all the advice. I sure hope I can eventually figure this one out.
     

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