4l80e Shifting Issue
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4l80e Shifting Issue 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.
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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 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?
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Check out this, some troubleshooting techniques without a scan tool. https://www.thetruckstop.us/forum/th...2/#post-457552
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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?
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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.
Thread Starter
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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.
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
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From: Roxana, IL
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
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
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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.
TECH Enthusiast
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From: Roxana, IL
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.
QHQSQC GM
Dasbecan
Rostra
QHQSQC GM
Dasbecan
Rostra
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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.
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.
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From: Poplarville, MS
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.
Last edited by bbond105; Mar 15, 2023 at 11:05 PM.