So i checked the ohms with a multi meter and everything was within specs. I'm thinking there must be a PCM issue as i have been unable to access it with a scanner. Although it does give codes as a flashing method.

 

Did you check for power at the valve body harness?

 

yes. replaced it

 

that doesn't mean theres a problem from the ignition switch to the transmission. what was the voltage you read?

 

If you think about these codes with some PCM logic.
Code 81 2-3 error, It commanded the shift and voltage remained high on the circuit. It goes to a high pressure mode and 2nd & 3rd are only commanded
Code 82 1-2 error. Same scenario
Both of these codes are electrical though. Two digit codes are OBD I and P- codes are OBD II codes. Codes 81 & 82 are equal to the late P0753 & P0758. I have uploaded a diagnostic chart for those codes/
Code 85, At anytime the PCM saw a gear ratio for xx seconds and did not recognize it based on the given parameters of 1st 2nd, 3rd or 4th.
86 low gear ratio error. The PCM is seeing a ratio that is outside of the parameter for 2.48 low gear
87 high gear ratio error. Same scenario as code 86 but at 1.1 high gear.
I have also uploaded a 4L80E chart for codes 85/86/87
I would start with the external wiring and looking at the ground sides. 12V is supplied at all times & the PCM supplies the ground.
Hope these help.

 

a solenoid 22.4
b solenoid pin 21.9
pressure control solenoid 3.9
torque convertor clutch solenoid 10.4

 

All you are measuring is the ohm's of each solenoid. That only proves that the solenoid is capable of switching states.
Now that you know this, You need the to verify the 2 main properties of a circuit. +12v & -12V.
1) This will require testing pin "E" of the wire harness for +12V
2) you need to know if pin "E" can carry a load. I have a 9005 headlamp bulb & pigtail rigged with a pass through connector of an old internal harness & alligator clip for a ground. If it light the bulb you have +12V and it will carry a load.


Next you need to know if the ground at the PCM actually can be made. If the wires running from the transmission to the PCM are shorted or open the shifts wont happen.
From the diagnostic pages I uploaded
[The Normally Open 4L60E/4L80E Shift Solenoids are ON/OFF solenoids which have a resistance value of 20 to 30 Ohms.
· The Shift Solenoids are supplied “Key On” power through case connector terminal “E”. The PCM/VCM controls the solenoids through the ground side on Circuits 1222 and 1223 at case
connector terminals “A” and “B”.
· There are times when the scan tool will indicate that the command to turn the solenoid on has been sent, when in reality it has not. Use a volt meter on Circuits 1222 and 1223 to insure that less than
one volt exists, indicating that the computer has in fact grounded Circuit 1222 and 1223. Voltage on the circuit, over one volt, means the computer cannot pull the circuit all the way to ground.
· In situations where a wrong gear start exists or certain gears are missing use the scan tool to check solenoid command in order to separate a mechanical from an electrical problem using the solenoid
firing order chart in Figure 119.
· Use the scan tools bi-directional control or a transmission tester to see if all the gears can be forced,
this helps to determine if the problem is inside or outside the transmission. Then compare it to the
solenoid commands that are given by the computer.
· Circuit numbers may vary, especially in “P” Vans, Chevy Forwards, GMC Tiltmasters, H1
Hummer and vehicles that were built as “Incomplete”.]
It's fairly simple but you could actually test each gear by bypassing the PCM and just grounding Pins "A" & "B" of the case connector yourself using the proper sequence.
1st gear wire "A" is grounded
2nd gear both "A" & "B" wires are UN-hooked from ground
3rd gear wire "B" is grounded
4th gear both "A" & "B" would be grounded.
or you could use an OBD 1 bi-directional scan tool if you can find one. I use an OTC Genisys Touch or a shift box I made years ago. If you look at the shift pattern on the box it is made for a 4L60E and a 4L80E is exactly the oppisite.

 

unplugged the connector from the transmission and checked with a test light (it worked). While i was there i checked with a meter as well (11.9) this is acceptable i guess as the engine was not running. Just to be sure i used the same meter at the battery and it was 11.9-12

 

for starters 11.9V is a dead battery. just because you have battery voltage there doesn't mean it will pass sufficient amperage. this is why tranzman uses a 9005 bulb to test, this places a load on the circuit to check the integrity of the wire/ignition switch/etc

 

So at the risk of sounding stupid....the battery is fine...it raises the dump and starts the truck. is there another test i should be doing?

 

I understand that electrical can be confusing and misinterpreted. I am assuming you do not have access to a bi-directional scan tool.
Battery basics:
Automotive batteries are 12V by design. Each cell when fully charged is 2.2V x 6 = 12.6 or better. They have a voltage range but It is amps that actually does the work not voltage. CCA (Cold Cranking Amps) is the most desirable form of measuring the available working amps. Batteries with less than 100 CCA measure approx 11-11.5 volts so at 11.9 your battery has about 200-300 CCA. This is a far cry from a fully charged 800 CCA battery but it will start a motor in warm weather and still operate a dump motor how ever slowly it may be. Because a battery is just an amperage storage cell the voltage range is quite small @ 11V-13V that is only 2V difference from completely dead to fully charged. So put your battery on a charger and get it fully charged.

Yes there is something else you should do. I laid out and supplied a complete list of test's and how to do them.
Testing voltage to pin "E" at the transmission is just 1 test. Because circuits work on amperage not really voltage. The next step or test is testing that circuits ability to pass amperage through a wire.

Let's say you have a 18 gauge wire that contains 15 strands of wire. 14 strands can be broken and that 1 strand will test @ 12V or battery equivalent using a test light. That 1 strand will not carry the amperage required to light up a headlamp bulb, this is why I rigged up the headlamp. A headlamp is similar to a solenoid in it's load. It's fast and actually kills 2 tests in 1 operation.
Why won't the 1 strand light up the headlamp? Because most circuit testers are a low impedance design and have the ability to light up with less than .5 amp and that is not enough to operate a solenoid.
These tests will tell you that +12V supply is adequate. If it is not adequate you fix that.

If it is adequate move on to the next step.

The next step is to turn to the ground side. Grounds are as equally important as supply, sometimes more important. Have you ever seen a dim headlight. That's a ground issue not a supply issue.
if the PCM makes the ground for the solenoids you have to make sure both shift solenoid "A" & "B" ground wires are good from the case connector to the PCM, If they are not good you fix that.

 

I do not have access to a bi directional scan tool. I have been looking over the steps that you listed. I will not say i fully understand. All i had time for today was to do a simple continuity check between the PCM connector and the J1 connector at the transmission. All the wires gave me tone. Its my understanding that I will need to do an ohms test on this same location as there could be a load issue even though it was positive for continuity. Once again I am not the best on electrical. I have enough knowledge to get me into trouble. lol If you can give me a step by step for dummies version. There is too much information in the wiring schematics for me to get a grasp on what i should be testing. Pin location and ohms to check for on everything coming out of the J1 to the PCM connector. All i have is a multimeter to work with as well. Thank you for your patience.

 

I can't get much more step by step. I would print out both PDF files I uploaded for you. Both of these are about as simple as it gets. Read through them and highlight or circle the codes/information that only pertains to your situation. The only thing that they leave out is the actual pins & location. You are going to have to find a PCM pinout for your specific vehicle. They have supplied the circuit #'s. The circuit #'s will be on the PCM pinout. it will be up to you to connect the dot's and follow the print out.
A 1991 could have a broken apply plate in the direct/reverse drum. I stand by my original statement from post #15 "I was always taught that any code with gear ratio errors means, it's time to pull it!!!"
I could be wrong, but that follows my 40 years experience.

 

ok using the following instructions i came up with voltage on both circuits.
Referring to Figure 124, unplug the PCM connector containing circuit 1222 or 1223, and with the ignition on, probe the terminal ends for the 1-2 or 2-3 shift solenoid with a voltmeter with the negative meter lead to ground. If the voltage supply at terminal “E” is good and the solenoids and circuit wires 1222 and 1223 are also good, battery voltage will be present at the VCM/PCM connector terminal ends.


I found a ground ribbon cable broken from the frame to the firewall but the one that branch off that to the engine was still intact. Im heading off to the parts store to get that before i do anything else.
What would you suggest is next step as i have battery voltage on both circuits?

 

you need to load test pin E with a light bulb like tranzman showed earlier. not a test light, it needs to be a bulb that will draw significant amperage to load the circuit. fixing grounds is a good idea..you may want to run a 14 to 10 ga wire from the negative on the battery to a bell​​​​​​housing bolt to ensure you are getting good ground. this ground can totally be temporary for now. have you charged your battery and load tested it to see if it is still good?

 

Battery tested and good. Circuit carries the load. I also got my hands on a computer and switched it out temporarily, no change. The prom has not been changes as im having trouble location one. The parts guy said hes not seen much failure in prom in my model. unless someone has another idea, i may be about ready to take it to the garage.

 

Ok, so here is an update on my 1991 GMC 3500 4x2 with 4L80E. I finally just gave up and sent it to the shop. The manifold pressure switch (only one i didn't replace) was bad along with a wiring issue internally causing the 3rd and 4th gear issue. got it home and the reverse gave out on the first haul. Reverse had been working flawlessly along with 1st and 2nd before it went into the shop. (The only reverse issues i had ever experienced was before the 3-4 issue. It was taking high rpm to get it into reverse). I of course called the transmission shop back and got the standard needs an overhaul, $3500 ( nope)

Description: While hauling a dump load, the truck had operated perfect. I hauled the load about 35 miles up hill and down to the site. Speeds were normal at up to 60 mph. no lights or overheating. While backing up to dump the load the transmission suddenly stopped pulling. it felt like when a manual transmission jumps out of gear.

Current Issue: when shifting into reverse, the transmission shift smooth and quietly. However it will not move. Higher rpms will result in the attempt of the truck to move but will not. All forward gear operate as normal. Even hauled a few heavy loads of course not getting in a reverse situation and it performed well. This of course let me to believe there must be an issue with the reverse servo pin length due to wear even though the truck only has 23000 actual miles.

First pull: Pulled the pan, inspected the pin, to find no wear. I did put a new O-rings and reverse servo gaskets in while i had it apart thinking there may be bleed by causing a pressure issue. Same issue as before.



Second pull: The transmission is out and torn down completely on the bench. I expected to find a broken reverse band. Band was fine and not worn at all. The whole thing actually looked clean and very good. Everything except the valve body gasket. It was very brittle and some pieces had flaked off. This leads me to think pressure issues again.



Question: I'm about to clean everything out to be sure of no blockages, order a gasket set and put this thing back together. Brand? supplier? any suggestions that may lead me to do something different?

 

Check that the pins have not backed out through the case, saw a 400 with this issue recently rev band

 

So upon inspection I found blue hot marks on the reverse drum. Also the roller bearing seem very easy to pop out of part? Do I need a new part or is this normal?

 

the roller do come out pretty easily and have no effect on reverse, With the bluing that means the servo is almost applying, So hydraulic leak or band pin too short . Their is a tool to check the band clearance , However when unit down and install band and rear drum with roller I do it the old fashioned way visually and by feel and use a pin that gets me as close as possible without dragging on the band by putting the servo assembly on and checking along with air to apply the servo. But the bluing tells you that is the issue.
Also make sure the direct clutch has been what they call dual fed (really means made so it uses both chambers in the drum or 3rd and reverse.) Make sure the port right side of the center support bolt is blocked.