I bought a '92 3/4 Ton 4x4 6.5L Diesel Chevy Truck w/automatic transmission 3 years ago. Which has worked great up until 2 months ago. Trans is acting up.

Problem: The auto trans get stuck in first gear unless i rev it really high and then it bangs in to 2 gear which if very bad. Also if I rev in first gear, going down the road and let up it seems to free wheel and not hold me back but if I rev it back up it catches and pulls great. (Also somedays it works great other days it sticks)

When this first happened I thought I would need a new trans, but the next day the trans acted brand new with no problems.

So I thought well lets try putting some transmission cleaner in. When I put the cleaner in and drove it around it immediately fix the problem so I thought dirty trans; lets change the oil and filter.

So I changed the oil and filter. The transmission ran like a top for about a week, then the syptoms came back.

Next I changed both shifting solenoids, thinking maybe they were the problem, but before i changed them I added another can of cleaner which fixed the problem again. I Still changed the solenoids thinking they might be weak.

Well again, I thought I fix it. Worked great for 2 weeks then started acting up again.

Now sometimes it sticks and sometimes it don't.

Check for loose wiring, nothing.

I am at a loss, any suggestions?

Thanks
Auctionguy33

I had a 97 1/2 ton that acted like that. Turned out to be a bad ground wire. I think there was a TSB on my year about it.

Any codes stored? Do you know how to retrieve codes?

To answer your question, Yes I know how to retrieve codes.
But didn't think to check because the check engine light didn't come on.

Anyway, went out to check codes with a Xp240 Auto Xray Scanner. Got a "Vehicle is not Responding".

Any thoughts?

UPDATE:

Been looking throught the forum and someone else kind of had the same problem. He fixed the problem by replacing the speed sensors on the trans.

On my truck my speedometer jumps to 10 to 20 miles an hour the first 5 or ten minutes in the morning when parked in idle. And the tranny problem seems to be worse when i pull something.

Could these sensors be the problem? or could it be the pcm is messed up because it will not communicate with my scanner?

Auctionguy33
:help2:

Yeppers,

Speedo gets signal from exit shaft speed sensor (rear). I had exactly the same problem and fixed it by replacing the sensor.

Local tranny shop wanted like $1800 to rebuild it. Cost me $45 to fix

Tim

Auctionguy, is your truck a diesel? If so, it sounds like you may have a computer problem or a power problem to the computer. '93 has a mechanical pump that doesn't need the computer, so it can be completely non-functional and still run, but the trans won't work. If that happens, the trans goes to 2nd and will coast if you accel and let off. The "vehical not responding" makes me wonder why you are not getting communication w/the PCM.

UPDATE:

Yes, it is a 6.5l turbo diesel.

I read through a Hanes repair manual and it said I could retrieve the codes manually. So I did.

RESULTS:

Two Error Codes 86, 85

According the the repair manual these codes only apply to '94 and later models.(I have a '92).

Code 85 detects the calculated transmission ratio different than the known ratio error ratio in any gear. The ECM uses this info to detect shift solenoid problems.

Code 86 detects the calculated transmission ratio higher the the known ratio in first or second gear. The ECM uses this information to detect shift solenoid problems.

I still don't know why my xp240 scanner wouldn't work.:confused:

UPDATE:

Yes, it is a 6.5l turbo diesel.

I read through a Hanes repair manual and it said I could retrieve the codes manually. So I did.

RESULTS:

Two Error Codes 86, 85

According the the repair manual these codes only apply to '94 and later models.(I have a '92).

Code 85 detects the calculated transmission ratio different than the known ratio error ratio in any gear. The ECM uses this info to detect shift solenoid problems.

Code 86 detects the calculated transmission ratio higher the the known ratio in first or second gear. The ECM uses this information to detect shift solenoid problems.

I still don't know why my xp240 scanner wouldn't work.:confused:
Clean your speed sensors!

Go to the FAQs at top sticky threads this forum will tell you how to read codes and what they are, paper clip is all that is required for OBD-I code reading provided SES/check engine bulbs in dash are good

UPDATE:

I am going to clear my ECM's error codes. Then clean my speed sensor and make sure there is a good connection. Then run it a few days to see if it acts up. I will then check for error codes if there are any, I will replace the speed sensor, reset the ECM again, and then report back here.:D

Thanks for all the help and input so far.:thankyou2

Auctionguy33

UPDATE:

Been looking throught the forum and someone else kind of had the same problem. He fixed the problem by replacing the speed sensors on the trans.

On my truck my speedometer jumps to 10 to 20 miles an hour the first 5 or ten minutes in the morning when parked in idle. And the tranny problem seems to be worse when i pull something.

Could these sensors be the problem? or could it be the pcm is messed up because it will not communicate with my scanner?

Auctionguy33
:help2:I just went through a rebuild on my 4L80E 4 wd tranny

UPDATE:

I unplug and remove the input sensors, inspected, cleaned them.(They looked clean before I started.) The connectors were in good shape(No corrosion.)

I put the sensors back, reset ECM and ran it this week. (Expecting to have to replace the sensors.)

It hasn't acted up yet. I checked the ECM; no error codes.

Must of healed itself.

I will post again next week if it still hasn't acted up. Sooner if it does.

Thanks Again for all the input!!!!!!!!!!:) :)

To answer your question, Yes I know how to retrieve codes.
But didn't think to check because the check engine light didn't come on.

Anyway, went out to check codes with a Xp240 Auto Xray Scanner. Got a "Vehicle is not Responding".

Any thoughts?
There are some important codes that get stored which don't light the check engine light or show in 1st code list on a reader, I call them "soft codes" usually an intemittent or lower tier code, that if left unattended can turn into a "hard code" one which turns on the light, on a MT2500 tool & (T2 I think) those are stored in a file called history.

Also an anomally I have with my MT2500 I can't view transmission codes if I set scan tool up with me having a K1500, if I tell it I'm a K2500 I get full transmission and bi-directional testing capability for transmission. My thought is Snap-On programmers did not realize my truck came as a K1500 with a Diesel & 4L80 trans as that is a HD 2500/3500 normally configuration, so maybe you are having some scan tool program issues as well.

Also an anomally I have with my MT2500 I can't view transmission codes if I set scan tool up with me having a K1500, if I tell it I'm a K2500 I get full transmission and bi-directional testing capability for transmission. My thought is Snap-On programmers did not realize my truck came as a K1500 with a Diesel & 4L80 trans as that is a HD 2500/3500 normally configuration, so maybe you are having some scan tool program issues as well.Thanks for the info. I will put the hounds to work on it.(Surfing the Web for answers.)

SHORT UPDATE:
The Trans still has not acted up so I am still waiting.:)

UPDATE:

Took the truck for a Sunday drive and the trans acted up big time.:mad: Checked the errors codes, 85 and 86 showed up again.

So I will now replace one of the speed sensors on the trans.

The question I have is which one should I replace first, the input or the output sensor?:think:

I am thinking that I should change the output sensor first or should I just change both of them?

Is there a way to check to see if the sensor is bad other than the trans isn't shifting right?

Any thoughts?

i would recommend replacing the input sensor, its been my experience that the input sensor ususally is the on that has the shortest life expectancy.

hope it works.

I had replaced both of mine when I did it. From the GM dealer, the cost was right around $30 for both sensors and they had them in stock.

If the speedo is acting up as well, its the back one. If the speedo is fine, its the front one.

Best bet is to change them both like ShawnR said.

Tim

DTC 85 undefined ratio: code85 will set if the PCM/TCM calculations indicate an unexpected gear ratio doesn't include overdrive, default:line pressure set to max.

DTC 86 shift solenoid stuck on: code 86 wiill set if the PCM/TCM has commanded 1st or 2nd gear but the ratio calculation indicates 3rd gear.

UPDATE::)

I had only one sensor on order, so I replaced the input sensor since it was the easiest one of the two to replace. I then cleared the codes and took it for a drive.

WOW! What a difference the shifting is. I can barely feel or hear the trans shifting. :cool2:
I think I have found and fixed the problem this time.(Crossed Fingers)

I will give an update at the end of the week, or sooner if something goes wrong.

Thanks:thumb:

Did you reset your codes with the app pedal?
tranny will be in limp/default mode till they clear.

NEW PROBLEM:

To answer the question on clearing the codes; I just disconnected the battery for a minute which resets ECM. I then manually check the codes and they were all clear.

Replacing the input sensor has fixed the hard shift between 1st and 2nd:).

But now the transmission shifts between Overdrive and 3rd gear while going 55 mph:(.(Back and forth, back and forth.) This doesn't happen everytime at 55 just some of the time.

I checked the error codes again and came up with the same 85 and 86.

Should I just change the output sensor, or is it something else?:help:

Should I just change the output sensor, or is it something else?:help:
In order, yes and probably not.

Tim

UPDATE::)

Had to order the sensor. It will be here tomorrow. I'll install it and report back this weekend.

Thanks!

UPDATE:http://www.dieselplace.com/forum/images/smilies/help.gif


I put in the sensor and reset the ECM. It worked for a while but then it started acting up againhttp://www.dieselplace.com/forum/images/smilies/banghead.gif . I checked the codes again and came up with the same 85 and 86.

I talked to a chevy machanic. He said the next thing to change would be the pressure regulator solenoid.

Any thoughts?
http://www.dieselplace.com/forum/images/smilies/help.gif

QUESTION?

I have been doing some thinking and I need some input.http://www.dieselplace.com/forum/images/smilies/help.gif

Should I pull the pan again and replace the Pressure Regulator(Cost $75) or, should I just drive the transmission out till it gets worse and actually breaks?

Because half the people say the bands are slipping($2000 Trans replacement), the other half try to sell me parts($25 here, $40 there, and $75 here).

By replacing the Pressure Regulator, Am I trying to heal a dead horse?http://www.dieselplace.com/forum/images/smilies/think.gif

Any input would be helpful.

ok when it works does it seem to slip at all or does it seem to work perfect ?? if when it works it works well trans is probably ok it will be an electrical problem!! now that said please update your signature with your truck info year ect.it will make helping you much easier .now depending on the year we are talking about there is possably 2 output speed sensors on your truck one on the trans and one on the transfer case if you have access to a scanner it will make sorting this out much easier

ok had to go back to beginning to see what truck was. now what 2 sensors did you put in the one at the front above shifter linkage "input" and where was the other one on back of trans above wiring harness conecter or on transfercase??

OK Updated My Signature.

Now to answer the questions. I replaced the input and output sensors that are on the trans. I did not replace the speed sensor on the transfer case.

When it works there is absolutely no slipping which makes me think, like you; electrical.

I have an xray xp240 Scanner. It reports the "vehicle is not responding". I am still trying to make it work.

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ok if you can get your scanner working i would be watching input and output on scanner as well as the speedo for a signal dropout or one that is slow to react. i wouyld not think it is the force motor if it waqs that you would have a code 78 the other thing that i do to try to find your problem is to actually measure the input and output signals with an ac voltmeter by back probing the connectors on the pcm problem could also be a faulty "drac" module

UPDATE:

Well I tried to get my XP240 Scanner to work with not luck. I even email Autoxray with my problem and they replied in a nutshell "It sux to be you.":mad:

So I took it to a GM mechanic he scan it, looked up the error codes and asked what I had replaced and then suggested that I might want to either replace or clean the Pressure Manifold Switch(the part in the trans that tells the ECM what gear it is in). He said sometimes in older trans, they get dirty from disk wear and it won't work properly unless you clean it. Cost to replace $70 in parts.

Well I went down to the local auto parts store and there just happen to be a transmission mechanic there. He said that GM had an update for the L80E Trans which could solve my problem. He just had it done to his '97 Chevy Truck. He asked if I had the update done. I said I didn't know for sure because I bought it used. I would have to ask the original owner.

The Update: (Something to this effect) To pull the transmission valve body and rebore a known sticky valve cylinder and replace the valve. Cost: Around $200.(This is to fix hard shifting between 1st and 2nd gear and other shifting problems.)

Has anybody heard of this update or had it done?
And does anybody know a good scanner that will scan the 6.5L Diesel?

I woulndn't want to be the guy that has to pull the valvebodie with the tranny in the truck. It's hard enough on the bench upsidedown.