1990 chevy k1500 tranny stopd going into overdrive? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-05-2011, 05:48 PM
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1990 chevy k1500 tranny stopd going into overdrive?

i have a 1990 chevy k1500 4x4 std cab 5.7L elc shift 700 auto trans, it has ben doing good sence iv had it, just one day i noticed it didnt go in to OD?? but every other gear is doing good.. no shuttering no noises just not going into OD??its full of fluid and looks and smells clean, need help guys, i drive this truck everyday to work getting 19mpg without OD whew around 12-15mpg...thanx guys..

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Old 09-05-2011, 07:42 PM
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Loss of 4th gear? Has a strong 3rd gear? Does not slip, or do anything else funky in 3rd? 3rd & 4th use the same set of clutches.. If 3rd is holding really well, we can rule out the clutches.. Next, the band applies for 2nd & 4th. If the band is burnt, or worn, it may not be able to hold for 4th gear to apply. How is 2nd gear? With this in mind, the 4th piston teflon seal may be worn. You can easily pull the 2-4 servo out of the side of the case and inspect the 4th piston & seals. Next, the input sprag. It holds in all forward gears except 4th. If the input sprag is damaged internally, it may be holding, or binding up, when it is supposed to be releasing.. Could also be a stuck 3-4 shift valve in the valve body. Lastly, it could be a damaged sealing ring on the turbine shaft. Or the 3-4 accumulator piston pin bore is worn and allowing a leak so that 4th does not have enough pressure to activate the band.. This list could go on, and on, and on. Unfortunately, you are going to have to do some investigative work. Start with the simple stuff-pull the 2-4 servo out and inspect the piston & seal. Then replace the 3-4 accumulator piston with a new piston and seal. While you have the valve body down to replace that 3-4 accumulator piston (rides in the case under the valve body & separator plate), go ahead and pull the 3-4 shift valve out of the valve body and check for a sticking/binding valve, broken shift valve spring, or anything else in the bore, or on the valve, that may be causing it to not shuttle. If these things check out, you could try another valve body just for grins. If another valve body does the same thing, you will at least have ruled that out, the accumulator piston out, and the 2-4 servo out. Anything else will require that the trans come out and be disassembled to figure it out.

If it has reverse, and 2nd, we can rule out the sunshell as the cause of failure. My friend, depending on mileage on this transmission, I would start thinking about an over-haul. You could spend several hundred trying to diagnose the problem on this trans, and still not get it fixed.

Have you taken it out on the freeway and really pushed the speed to see if it will shift into 4th gear at higher freeway speeds. When I say higher, I'm talking 70+MPH. Do this on a SAFE roadway and see if it goes into 4th then.
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Old 09-06-2011, 07:36 AM
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Quote:
Originally Posted by gofastz
i have a 1990 chevy k1500 4x4 std cab 5.7L elc shift 700 auto trans, it has ben doing good sence iv had it, just one day i noticed it didnt go in to OD?? but every other gear is doing good.. no shuttering no noises just not going into OD??its full of fluid and looks and smells clean, need help guys, i drive this truck everyday to work getting 19mpg without OD whew around 12-15mpg...thanx guys..
from what your describing, id say your not getting lockup (torque converter clutch engagement) the loss of gas mileage is the clue to me.
4th gear on these transmissions dont typically give out without 3rd gear going away as well.
does the truck have a SES light on? have you had it scanned for codes?
hows the fluid level and condition? have you had any recent service work done to it?
this is all important info to give for the best help to be given back to you on figuring out whats wrong with your transmission.
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