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91 4l80E Manual 2nd only

91r2500

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Thread starter #1
Hello, Welcome to my first thread

I am having an odd issue with my 4l80E.
When shifting automatically it skips second, it shifts through all other gears 1-3-4, does not slip, nor give any other problems.
When I shift it manually all gears work 1-2-3-4 no problem and if I put the shifter in 2nd and take off it will start off in 1st and shift to 2nd no problem. It will not automatically downshift to 2nd either, 4th to 3rd is fine and if I go slow enough it will go into 1st as well, it will downshift to second if I manually put the shifter into 2nd though.

I have replaced all of the solenoids and wiring harness in the trans., rewired the external harness (someone did the conversion to the newer harness but the connections looked iffy) While resplicing the connector I checked to make sure that the wires were in their proper places.

Anyone have any ideas?
Thank you in advance for any help.
 

THEFERMANATOR

FRANKENBURBAN
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Staff #2
Does 2nd work if you put it in 3? I checked my atsg and it doesn't show anything for no 2nd onlyin D4. I would want to put a scanner on it to check for codes, and to see if it's commanding the shift to change and if the PSM is showing the shift as changing when it's not. Normally a no 2nd is a broken intermediate sprag, but that gives no 2nd in any position. Only thing I can figure is a stuck 2-3 shift valve, but stuck valves don't normally occur on higher milage transmissions unless alot of clutch material gets introduced into the system in which case you get multiple sticking valves. I hate saying it, but none of my manuals or internet searches turned up anything for no 2nd in only D4.
 

Will L.

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#3
Any codes at all?
That is a head scratcher.
What does the fluid look and feel like?
Has it gone a long time without transmission service, then recently gotten one?
 

91r2500

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Thread starter #4
It has been this way since I bought it, a few years ago, I know I should have bothered with it sooner but it was a run it till it dies type of situation but it never died. The fluid has always been nice and pink, no signs of grit or shavings. I dropped the pan on it last week and replaced all of the electronics and replaced the internal harness (I thought it was a solenoid problem). Inside the pan was very clean, there was an extremely small amount of fine black silt in the bottom of the pan. It was one of the cleanest transmissions I have ever opened.

I tried running it with the shifter in D3 this morning, it acts like 2nd is not there and it doesn't expect it to be. No delays or other oddities just straight from 1st to 3rd. If I put the shifter in D2 it will shift from 1st to 2nd and then I can shift up to 3rd and 4th, the gears run out like they should even 2nd (more throttle holds the gears longer, less throttle holds gears shorter).

I'm beginning to think that I am the lucky winner of a unique issue. I have searched many forums and Google and have yet to see anyone with the same problem.
 

Will L.

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#5
Tps adjusted properly?

I wonder if something in the programming has it opening 2-3 under normal circumstances. I wonder if it could even do that, really.

Factory electronics or aftermarket?
 

91r2500

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Thread starter #6
TPS has been adjusted.
The ECM has a reman. sticker on it.
I was beginning to wonder the same thing, like there was a hole in the programming or something.
The only code it has is the O2 sensor, apparently someone put dual exhaust on it and decided to opt out on putting the O2 sensor bung back on it.
 

91r2500

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Thread starter #7
Is there a cost effective scan tool that will let me monitor the shift commands and other functions? I have been checking codes with the jumper method.
 

WarWagon

Well it hits on 7 of 8...
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AZ
#8
Check the wire condition next to the speed sensor plugs as the insulation likes to flake off. Also in the junction into the harness can short out. When I dropped in the stall converter I noticed it's kinda dark under the truck even with the best flashlights. :rolleyes: In the darkness it's possible to swap the input and output speed sensor wires for some real weirdness. Output speed matching idle speed in park on a scanner would tell you this...
 

91r2500

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Location
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Thread starter #9
When I rewired the external transmission plug I back pulled the entire harness so that it would come out under the hood to make it easier to work with and inspect everything. I noticed that the rear speed sensor harness on mine has a 4 wire connector that it goes to about 6" from where it plugs into the speed sensor, I am guessing if it was 4wd there would be one more speed sensor that goes in the transfer case? About 1 year ago I replaced the front speed sensor pigtail ,it had a broken wire (instant limp mode, no bueno). Am I correct in believing that my speedometer works off of the rear speed sensor? If the two speed sensors were plugged in backwards would my speedometer be crazy too? Just for s&g I could try swapping them and see what happens.
 

WarWagon

Well it hits on 7 of 8...
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#10
I don't recall the speedo being off when swapped. Just was thinking it couldn't handle the stall and needed aftermarket trans controller. Then I discovered my error. Years vary so YMMV.
 

91r2500

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Thread starter #11
Tried swapping the plugs and things got weird. Shifted weird, speedo was out of wack it read 15mph while parked and speed would increase as rpm did. OD did not seem to be working either.
 

THEFERMANATOR

FRANKENBURBAN
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Staff #12
Since you say O2 sensor, I'm guessing you have a gasser. If it's a 91 r2500 like your screen name suggests, then theres a few options out there for not too much money to scan it. You would use a program called tunerpro, the only hard part is the aldl cable to connect up to it. If you search gm obd1 scanner cable you will come up with alot of cables. You can also find directions on how to build your own online. I've never bought from them, but I know aldlcable has been around for a long time selling these cables. I hate to say it, but theres nothing I can think of that would cause this to happen except for mave a stuck 2-3 shift valve, but I wouldn't expect that to happen except in a plugged up gear box.
 

91r2500

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Location
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Thread starter #13
Yes it is. 5.7 tbi. I am going to drop the pan again and check out the 2-3 shift valve while I wait for the aldl cable to come in. Thank you all for the help. I will post an update after I get the pan back on.
 
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