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Honestly with the troubles with wiring and more you are having with this thing , You might consider just buying a stand alone controller like USSHIFT.COM, They are relatively inexpensive compared to even the time effort and money you have in this thus far and would solve all but the reverse with absolution. Comes a time to cut your losses and bite the bullet . Just my thoughts looking at all the issues you have and are having .
__________________ Frank formerly of Performabuilt, Now just me, What can I build for you today? Call or message me. Click sig pic for my facebook
Blows my mind remember when 92 93s were brand new and these 4L80s and this stand alone control was some space age cool stuff for the day. I did my first 4L80 swap in 95in high school using this system you are using behind a 87 carbd 454 and man that was some cool **** for the day. rolled over 93 diesel donated it's parts for my truck
I think the wiring is almost perfectly fine since it started shifting 1-4 perfectly. I gave it a break for a day and today dropped the pan and found a total fail! The reverse plug was in backwards so I plan to look over all the valves and see if I messed up anything else! Surely this was the reason that reverse was acting funny?
I refuse to give up and say i didnt know what the problem was. I also have other work and have been giving this less time everyday.
Last edited by Guy with a Chevy; Mar 23, 2025 at 06:09 PM.
Blows my mind remember when 92 93s were brand new and these 4L80s and this stand alone control was some space age cool stuff for the day. I did my first 4L80 swap in 95in high school using this system you are using behind a 87 carbd 454 and man that was some cool **** for the day. rolled over 93 diesel donated it's parts for my truck
very cool, definitely limited info on that stuff on "the net" back then. had to get ahold of shop manuals and figure it out. i remember going to the local tech college and bugging the instructors to let us have access to their factory manuals. would have to take notes, sometimes they would photo copy stuff for us. definitely alot better than the haynes/chiltons the library had.
Originally Posted by Guy with a Chevy
I think the wiring is almost perfectly fine since it started shifting 1-4 perfectly. I gave it a break for a day and today dropped the pan and found a total fail! The reverse plug was in backwards so I plan to look over all the valves and see if I messed up anything else! Surely this was the reason that reverse was acting funny?
I refuse to give up and say i didnt know what the problem was. I also have other work and have been giving this less time everyday.
i think you need this determination to work on transmissions. you can at least take away the experience from a job that will probably not pay what you have invested into it.
On new style connector pin "E" you should have 12 Vdc. check with a test light or headlight bulb and make sure it is BRIGHT. The force motor should have 12Vdc at pin "C". Also check resistance to ground (on the battery to the 4L80E case) and make sure you have low resistance (under 1 ohm, ideally 0.5 ohm or less). I would verify wire integrity back to the controller, possibly have a bad one. it should be behind the glove box on top of the heater box, at least that's where the gasser stuff is on the early GMT400s. definitely get a factory manual and/or diagram.
What if pin C is not getting 12 volts? Should it have 12v with key on or does it have to be running?
Back in those day's my dad was a tech at the small town local chevy dealer I had full access to gm manuals and parts dept and wrecked trucks for parts and brand new trucks for reference. Did a lotta late model stuff in older platforms back then when I was working my *** off living at home in high school. miss those days in a way.
Posted pic of trans connector and wire layouts.
Force motor high and low run directly to pcm.
Are you still getting force motor codes if so start as mentioned earlier replace that main connector if nothing else so you can first hand verify pin by pin it's all correct.
I wish I had grown up in those days! The mechanic that had the this truck before me told me this evening he thought it was electrical issues causing the problems and that he didn't have the time to get to the truck that's why it was brought to me. He is definitely old school because he started in 65 and mentioned where the hell you going to even find a diagram for this thing lol. I mentioned I had it found on alldata which I guess his shop isn't too familiar with. With his experience maybe he doesn't need it?
I'm probably going to throw a TPS at this 6.5 since the TPS went completely dead today after calibrating it from .4 volts to .6 at idle. The drive after the check engine light came on and it started hardshifting and would go into 4th and drop back to third. Went to check the tps again and the reading showed 1-60 milli volts no matter what but confirmed why it was showing a code 22 before I even messed with it. Very weird since it was shifting perfectly yesterday
Posted pic of trans connector and wire layouts.
Force motor high and low run directly to pcm.
Are you still getting force motor codes if so start as mentioned earlier replace that main connector if nothing else so you can first hand verify pin by pin it's all correct.
There was never a force motor code.I was going by these somewhat matching diagrams. They seem to have everything the same but the force motor so I went as far as swapping the high and low today with no results maybe because the tps is no good and it is a full line pressure fail safe?
I believe replacing the TPS fixed the problem!!! May have also had something to do with the ISS grounding out on itself but from what I have found that creates no upshifting at all compared to a bad TPS with high line pressure and hardshifts. Either way I buttoned up the ISS/VSS wiring downstairs by cutting out bad sections, soldering the connections and finally shrunk the heatshrink thats been in place on the pigtail waiting. Bouncing line pressure guage readings were 80-100PSI@ idle 150-175 forward gears and 250-275 Reverse.
I brought this 93' C3500 6.5 TDI with a 4l80e problem here because the guys on diesel place don't talk transmissions like you guys do here plus they don't have Frank. I found a diagram with the VSS signal going straight to the TCM before Frank confirmed a somewhat obvious speculation of the TPS causing the pressure and shifting issues. Without live data it was a fun process of tracing and probing connections to verify what was and not. Really surprised the damn TPS was working perfectly before it took a dive, even made me grab another multimeter to check my mind right after calibrating it.
I am happy I didn't give up on this truck as I was starting to get the feeling I should. You must have heard my crying in the posts I made lol
Remember orientation of 4l80e valve body check ball end plugs or you will lose reverse!
I believe replacing the TPS fixed the problem!!! May have also had something to do with the ISS grounding out on itself but from what I have found that creates no upshifting at all compared to a bad TPS with high line pressure and hardshifts. Either way I buttoned up the ISS/VSS wiring downstairs by cutting out bad sections, soldering the connections and finally shrunk the heatshrink thats been in place on the pigtail waiting. Bouncing line pressure guage readings were 80-100PSI@ idle 150-175 forward gears and 250-275 Reverse.
I brought this 93' C3500 6.5 TDI with a 4l80e problem here because the guys on diesel place don't talk transmissions like you guys do here plus they don't have Frank. I found a diagram with the VSS signal going straight to the TCM before Frank confirmed a somewhat obvious speculation of the TPS causing the pressure and shifting issues. Without live data it was a fun process of tracing and probing connections to verify what was and not. Really surprised the damn TPS was working perfectly before it took a dive, even made me grab another multimeter to check my mind right after calibrating it.
I am happy I didn't give up on this truck as I was starting to get the feeling I should. You must have heard my crying in the posts I made lol
Remember orientation of 4l80e valve body check ball end plugs or you will lose reverse!
I once had a guy loose all line pressure , He had changed the boost valve and in process the end cap at other end of the valve fell out and into case. Everyone argued that could not have happened but when he finally did pull unit . Guess what he found. Weird stuff happens. Glad you got it solved. BTW you can get data from the ALDI plug in the 93 with DATACAT. Just FYI for future reference.
__________________ Frank formerly of Performabuilt, Now just me, What can I build for you today? Call or message me. Click sig pic for my facebook
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