GM trans tech
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Yes, Z28 cars had the corvette servo. The servo only applies the band. Burnt 3/4 clutches is most common failure point we see on 4L60E's. Lots of different reasons for them to fail.
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3,255 Posts
It's also acts as the accumulator for the 2-3 shift when pushing the band off.
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3,255 Posts
Your 3/4's are gone. Probably works okay when its cold, but get some heat in it and you lose the 3-4 apply.
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969 Posts
Bad/weak epc solenoid, input shaft loose in drum, torn/damaged/hard lip seal (if older model), damaged/broken sealing ring, etc, etc.
Is this a fresh transmission that keeps losing 3-4 over and over, or is it a 150k mile wore out transmission that just now lost 3-4?
Is this a fresh transmission that keeps losing 3-4 over and over, or is it a 150k mile wore out transmission that just now lost 3-4?
fairly fresh trans. gm trans tech building them for me had this in and out about five times for this concern. really getting old, hope I haven't spent the money for a rebuild for nothing. might start looking for different rebuilder after this is resolved.
I had a 4l60E do that to me this summer. trans cooler was restricted and clogged filter as well. that was after flushing and blowing it out. it looked good to me, it was not.
I pulled trans and after finding nothing concrete, reassembled
and bypassed cooler. all was well after i did this.
I pulled trans and after finding nothing concrete, reassembled
and bypassed cooler. all was well after i did this.
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844 Posts
Other things to check/fix:
- use the transgo hi rpm pump ring and spring kit, the stock pump slide spring gets weak with age and can/will cause a drop in line pressure somewhere above 5400 RPM. I just did one that started dropping at 5000 rpm and was down 40 psi by 5600 rpm.
- make sure the check ball capsule behind the servo seals with solvent. it's kinda tough to check, you gotta install the servo and fill the 3rd accumulator passage with solvent and look from the inside to make sure the check ball isn't leaking, there's a bleed hole right next to it though IIRC. this can cause a 3rd gear oil leak. some guys take the capsule out and plug the hole which is supposed to cause a momentary bind on the 4-3 downshift but I've never felt it
- check line pressure at WOT during the shift, you should see a slight bobble but no drop
- use the transgo hi rpm pump ring and spring kit, the stock pump slide spring gets weak with age and can/will cause a drop in line pressure somewhere above 5400 RPM. I just did one that started dropping at 5000 rpm and was down 40 psi by 5600 rpm.
- make sure the check ball capsule behind the servo seals with solvent. it's kinda tough to check, you gotta install the servo and fill the 3rd accumulator passage with solvent and look from the inside to make sure the check ball isn't leaking, there's a bleed hole right next to it though IIRC. this can cause a 3rd gear oil leak. some guys take the capsule out and plug the hole which is supposed to cause a momentary bind on the 4-3 downshift but I've never felt it
- check line pressure at WOT during the shift, you should see a slight bobble but no drop
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3,255 Posts
Two things I would look at for this repeat failure. First is to make sure the input drum isn't leaking where the input shaft is splined into the drum. You have to air check the drum with some fluid around the base of the input shaft to verify this. Second, make sure the correct position, and sealing of the encapsulated check ball that intersects into the servo bore. The valve body has to be removed to do this. Those are the two most likely causes for a repeat failure.
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