This is my first time posting on this site but I am so lost on what to do about this issue. So let me start by talking about the truck. I inherited my grandpas 1994 c1500 that had been sitting for about 2 years before he passed and now another 1 year so 3 total years not running. It was parked due to a bad transmission. I have recently started getting it back together starting with the transmission problem. I purchased a rebuilt 4l60e and installed it myself. On initial notice of the new transmission it would shift into drive no problem. 1st to 2nd gear is beyond smooth and normal. Shifting into 3rd gear is also normal. When trying to shift into 4th gear I get to about 3500-3600 rpms and it won’t shift until I let off the gas and re apply and then it will shift into 4th gear. Torque converter was new also when I put the trans. Trans fluid is normal also. I do have a check engine light on with codes 35 & 43. I am lost on where to go from here. Any help is great

 

Regarding the DTCs (Diagnostic Trouble Codes)...
If your scan tool/ device reads GM OBD I Codes and Not generic OBD I Codes, Then:

-35 pertains to the Engine Idle Air Control Circuit/ System (Engine idle speed).

Do you have any issues with the way that the engine idles?
Does it idle rough at any time?
Does it idle too high; or too low?
Does the idle speed never change?

First you should inspect the Engine Throttle-Body and the Idle Air Control Solenoid for being in need of cleaning.

-43 pertains to the Engine Electronic Spark Control Circuit/ System (Knock detection and Ignition retard).

Do you hear the engine making any banging or knocking sounds while at idle or driving?
Do you hear any knocking or pinging sounds when under heavy acceleration?

If No noises are heard then the Knock Sensor (and circuit) and the Electronic Spark Control Module (and circuits) need to be tested.
You will need a scan tool and a 12v test light to do this... we can guide you through the steps when you are prepared with these tools.

Regarding the 4L60E Transmission...
It is common to hear a person say that the transmission will not up-shift until lifting completely off of the throttle; during a 2nd to 3rd gear shift.
Not so much on a 3rd to 4th gear shift.
Please double check/ make sure that this is an issue from 3rd to 4th gear (and Not 2nd to 3rd gear) before we continue.

 

Let me go further into detail about the truck because it will help answer a lot of the questions.

This truck was my grandfathers that he left to me after he passed away. It had been sitting for about 2 years before he died & another year after he passed. I recently finally had the heart to actually look at it and now just trying to get it running & driving. All I was told about this vehicle when I received it was “it has a bad transmission”. I am also going to note that I live in central Oklahoma & the vehicle is in South Louisiana.

I did buy a rebuilt transmission from a local shop near me, drove down to Louisiana with it in my trunk and installed it myself. I am very confident it is 3rd gear going into 4th gear that is struggling to shift. Also noting this transmission is under warranty, it just will take me uninstalling it and bringing it 600 miles back for repair & 600 miles back too try again. I am hoping it could be something simpler then an entire new rebuild or me having to go that route. The builder is thinking it is the servo that is bad and causing this issue, could this be so? Is this could be, I can change the servo myself and not have to pull it again.

Regarding the rest of the issues & engine codes this is what I have found so far.

engine did idle high at first. This corrected itself when I cleaned the throttle body, changed spark plugs, air filters ,flushed coolant and changed oil & filter.

truck didn’t have a thermostat at all installed. Installed one and got the temperature gauge working .

Gas gauge is broken for some reason

exhaust is rusted through & through causing an exhaust leak

interior lights turn off when head lights turn on

 

Adding below,

no knocks from motor at all, it’s running very strong now. Truck also has about 240k miles on it. I am thinking the knock sensor has just gone bad.

 

Yes the 4th gear piston of the 2-4 servo could be the issue...

You would have to remove it carefully an inspect the seal that is on it.
Be prepared for a shower... there is a lot of ATF in the servo Bore!