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Discussion Starter · #1 ·
First post. Recently bought a '93 K3500 mainly for the 6.5 engine with the idea to eventually put it in my '86 K2500 project. Has a hair over 200K on it. I would like to put some miles on it to determine if I need to do anything before I go to the work of putting it in the '86.
The trans has only the default 2nd gear at this time. Fuses are OK. Bought a code reader so I wouldn't get the speech (I totally understand). Codes are 22, 75, 81, 85 & 86.
Where is the switch that signals the proper gear-internal in trans, or dash indicator?
Could the speed sensor (where is it?) be causing this issue?
I haven't done the test on the harness plug yet. I'm an arthritic 66 years old and don't want to spend anymore of my remaining time rolling around underneath this than necessary. Was hoping some one could narrow it down to the most likely issue.
Thanks
 

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Welcome.
The 4l80e has a pressure switch inside it that tells the TCM what gear it should be in.
The gear indicator on the dash is mechanical.
You are probably losing 12v. batt to the TCM, going by the codes.
That 12v. batt ( ORANGE wire at pin A12 on the TCM) is from a fusible link on
the buss bar on the firewall under the hood.
 
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Welcome to DieselPlace
(y)
 

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Dunno if you got this figured out. But unplug the connector on the trans, big circle one, check for excess trans fluid. The oring that keeps the trans fluid at bay rots eventually allowing seepage.
If it’s juicy, pull it out real quick spray it down with come electrical contact cleaner on both sides male/female, dry it and plug it back in. Take it for a quick spin and she if she shifts.
 

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Discussion Starter · #5 ·
Finnaly got back to the project-too much stuff to do getting ready for winter.
Have voltage on the orange wire at TCM. Pulled plug on trans, no voltage on the truck side pin, so guessing either no voltage out of TCM or fault in harness? Which wire carries voltage from TCM to the trans, and if there is voltage out, can I just run a new wire from the TCM to the trans?
 

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The TCM controls the transmission solenoids by grounding them.
The TCM does not send them 12v.
The PINK/BLACK wire at the transmission connector should have 12v. ign. with KEY ON.
If not, check the TRANS fuse.
 

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At the TRANS fuse.
 

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Discussion Starter · #9 ·
Tore the harness by trans plug apart. Someone previous to me had butt connected the wires together (maybe a new harness end?). The 12v wire had come loose, so fixed that and feeling pretty confident about that being my problem I went to start and go for a spin. Up jumped a problem with lift pump. Found out here how to jump the relay (again thanks). Headed down the road, but still no shift, just 2nd gear. Any good ideas where to go from here?
 

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Tore the harness by trans plug apart. Someone previous to me had butt connected the wires together (maybe a new harness end?). The 12v wire had come loose, so fixed that and feeling pretty confident about that being my problem I went to start and go for a spin. Up jumped a problem with lift pump. Found out here how to jump the relay (again thanks). Headed down the road, but still no shift, just 2nd gear. Any good ideas where to go from here?
This may provide some info..
From the Trans FAQ link at the top of the section: 4L80E with 2nd Gear Only�Vehicle Speed Sensor
 

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Discussion Starter · #11 ·
So-due to the holiday, got a new vss installed today, refreshed the grounds, except the battery to frame ground, still no shift. Will look at the frame ground tomorrow, but think that's the unlikelyest ground issue. Really wanting to make this thing work but starting to think maybe the 7R400 could be made to work in my '86 project.
 

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So-due to the holiday, got a new vss installed today, refreshed the grounds, except the battery to frame ground, still no shift. Will look at the frame ground tomorrow, but think that's the unlikelyest ground issue. Really wanting to make this thing work but starting to think maybe the 7R400 could be made to work in my '86 project.
Ever check the big ole plug for fluid?
Fluid leaks into the plug and makes a bad electrical connection. Symptoms exactly like you’re describing.
Pop the plug off wipe it down spray it with electrical cleaner. Plug it in and run it around the block. If it starts shifting replace the internal wiring harness for the 4l80e. It’ll come with a new inner plug and o ring.
I have been through a similar issue to find out that plug was just too juicy.
 

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As long as it is clean that is all you need to do.
If ATF caused electrical shorts then the tranny would not work.
All the connections in the tranny are permanently submerged in ATF.
When dirt, salt, and debris get on the wet with ATF main connector,
then it may cause problems.
The biggest problem is ATF wicking up the harness and the wire insulation degrading
causing intermittent shorts and opens due to corrosion.
That wire does not handle ATF very well at all.
 
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