I have a 4l80 e in a 91 GMC 3500 4x2 that sat most of its life. 21k original miles. It ran fine for about 500 miles over the summer hauling heavy loads. Reverse started acting up ( high rpms to get it engaged) It worked fine once engaged. This issue resolved itself with driving. Now 3rd and forth gear seem non existent. Reverse, 1st, and second are strong and proper. She will just tach out and not change at all into 3rd or 4th automatically or manually. I have changed both solenoids and did a fluid filter change. The only other work that has been done is a recall for the throttle position sensor. Any advice would be greatly appreciated.

 

has the internal harness and transmission connector been upgrade to the 94+ style?

 

No, I had it inspected and it seemed to be in good condition. Was there a known issue with the connector in the 1991 4l80e?
Should i update it?

 

Yes the 91-93 connectors are a known issue. I am not saying it is your problem but it is a good thing to do and "cross off the list". Rosta p/n 350-9000
i have done a few now and they seem OK quality. I assume you don't have access to a scan tool? is this GAS or DIESEL?

 

I actually work at a Ram dealer but dont have one that old. I can probably gain access to one.

 

Check out this, some troubleshooting techniques without a scan tool. https://www.thetruckstop.us/forum/th...2/#post-457552

 

I was able to gain access to a scan tool. Could not connect. The guy ask if my cigarette lighter was working? We check and of course it was not. He said it was on the same circuit. I have not torn into this yet, but have check fuses and they are good. Where would you suggest i go from here?

 

You can get a cigarette lighter and tie it into the battery. I have used a few scanners that came with this adaptor.

 

replaced the front speed sensor. no change.

 

have you updated the harness yet? have you got your scan tool working? have you put a pressure gauge on? what was the reason to change the ISS?

 

I have not updated the harness yet because i have not been able to read with a scan tool yet. I am attempting to do the least expensive and least invasive things first. I have had two different scan tools attempt it. One that provided its own power and one without. Both said since the cigarette lighter was not working that it could not read as they were on the same circuit. I have checked all the fuses and found no issues there. The transmission friend said to get the codes read first. So still working on that. I may have to give up and take it to the shop. I fear they will just try and sell me a new one at that point. I do have something new. The service engine light has come on. Steady not flashing. I did ask a friend of mine about this last night. He suggested the clutch fan could be stuck. That would keep it from shifting. I am checking this out today. he also made the following statement about the wiring. I don't know what he's saying and was too ashamed to ask him.. maybe someone here can decode this down for me. "The ignition switch put power to trans on the pink wire on trans if the ignition switch don’t send power default to 3 gear"

 

you can read the codes with A & B terminals jumpered on the ALDL plug. you can do this with a paper clip, small piece of wire, etc. then turn on the ignition on and the check engine light will flash the codes, starting with the first digit, pause, second digit long pause and then it will flash the next code. it takes a bit of getting use to but it works. if you unplug the main transmission plug a 4L80E will default to 2nd gear NOT 3rd. the same is if the power is not there. Checking the ignition switch and feed is a good start, but not really an issue until GM went to the newer style dash in '95. Like i said in a previous post upgrading the internal harness and main plug should be done regardless. It is a known failure point, GM only ran that style for 3 years. they are also leak prone.

 

OK, so i fixed the issue with the csty fuse but was still unable to connect obd1 to the system. I jumped the a&b as suggested and here are the codes.

13 O2 sensor or circuit
28 Transmission range pressure switch
81 transmission 2-3 error
82 transmission 1-2 error
85 undefined gear ratio
86 low gear ratio error
87 high gear ratio error

 

i think it's time to drop the pan and poke around with a multimeter and see if there's power.

chances are "code 13" is actually code 12, which will flash even if the check engine light is not lit.

 

I was always taught that any code with gear ratio errors means, it's time to pull it!!!

 

Well, time to pull the pan and replace this wiring. I know you suggested the Rostra. There are several online to choose from. Any brands or companies to avoid? I'm seeing the Dasbecan for half the cost of Rostra. Probably something missing or substandard part. I have already serviced the filter and replaced both shift solenoids. Is there anything else while i'm in there? I am not shy about asking for help and advice. I am not a transmission tech! Thanks in advance.

 

All three of those carry a Rostra part # 350-9000 only two show the 90 degree wire clamp usually required for the 4L80E.

 

So, I just replace the wiring with the Rostra Kit 350-9000. No change at all. Reran the codes and came up with the same ones as previous.
13 O2 sensor or circuit
28 Transmission range pressure switch
81 transmission 2-3 error
82 transmission 1-2 error
85 undefined gear ratio
86 low gear ratio error
87 high gear ratio error
It still will not shift beyond 2nd gear. Suggestions are very welcome.

 

Have you tested the transmission range pressure switch or replaced it? A defective range pressure switch can tell the PCM that the trans is in one gear when in all actuality it is in a different gear (exp. shifter is in D1 but PMC is being told it is in 4th gear by a defective sensor). This could be the cause of all of those error code. The switch can be tested with an ohm meter.

 

i guess that's the next step. Thank you.