1994 2500 GMC Suburban Overdrive Failure

Hello to all. I have a problem. I just purchased a 2500 GMC 4x4 SLE Suburban. I knew that the overdrive was out, now it is my time to fix. Here is whats going on. As previous posts states, 1-2,2-3 shifts are fine. That infamous fouth is good while cold, gone when warm. I notice that my rpms are way up even in third in the freeway. 50 mph at about 4000 grand in third at one point. On the retrun trip the r's were about 2200 at 60 mph. I added some Lucas Slip fix stuff to the tranny. Seemed to make it better. i noticed posts about the vavle body. Could it be a sensor? The previoud owner replaced the flywheel and stater with out removing the tranny. He said he slid it back. I was Praying that he may have broken a wire connector somewhere. A friend said it may be the overdrive pressure switch or sensor. Any and all information is GREATLY appreciated! I am mechanically savy when it comes to turning wrenches. But braindead when it comes to trannys!
Thank you in advance for your help!
Also, mileage sucks.
Blessings
Chuck Butler aka ulremember

Hello to all. I have a problem. I just purchased a 2500 GMC 4x4 SLE Suburban. I knew that the overdrive was out, now it is my time to fix. Here is whats going on. As previous posts states, 1-2,2-3 shifts are fine. That infamous fouth is good while cold, gone when warm. I notice that my rpms are way up even in third in the freeway. 50 mph at about 4000 grand in third at one point. On the retrun trip the r's were about 2200 at 60 mph. I added some Lucas Slip fix stuff to the tranny. Seemed to make it better. i noticed posts about the vavle body. Could it be a sensor? The previoud owner replaced the flywheel and stater with out removing the tranny. He said he slid it back. I was Praying that he may have broken a wire connector somewhere. A friend said it may be the overdrive pressure switch or sensor. Any and all information is GREATLY appreciated! I am mechanically savy when it comes to turning wrenches. But braindead when it comes to trannys!
Thank you in advance for your help!
Also, mileage sucks.
Blessings
Chuck Butler aka ulremember

How many miles on it?
How is the fluid and fluid level?
Has the transmission ever been serviced?
Any extra metal or clutch stuff in the filter or bottom of pan?
It needs checked out with a transmission/engine capable scanner.
Check for codes and check shift times and adptive data and slippage in all gears.
MT

Thank you MT 2500.
I will check that as soon as I can get to my buddys house. He has the Modis reader by snap on. Also, I just found out I am getting 3-4 mpg in the city. Changing O2 today.
Again, thank you with the quick reply.
Blessings to all
Chuck aka ulremember

You are welcome and.
Good luck and let us know how it goes.
MT

MT-2500. I put in some of the Lucas Slip Stop. The O/D came back. But now, sometimes it leaves. I can be going along the freeway and everything is great 2500 rpms at 65 mph. (does this sound about right for thegearing and all).Sometimes I can get on the freeway and when it comes to the O/D shift, I feel it just "slip" out if that sounds right. When U get off of the gas, I feel it engage. As soon as I give it a lil gas , it just "slips" out. If I lock baby in third rides like a thirsty champ :). Put it in O/D, same thing. Get off of the freeway, O/D is back. Any ideas what it may be? Again thank you for your replies and insight!/
Blessings
Chuck aka ulremember

Yes rpm at 2500 at 65 mph is about right.
Can you feel Lockup clutch lock in.
You will have to get it on a transmission capable scanner to see what all is going on and what the problem is.
Post back any codes.
If the Lucas additive helped it it souds like you are losing pressure on 4 th gear circuit.
The lucas thickens up oil and helps build up low pressure.
Not good for a good transmission but will help one on last leg.
Good luck
MT

Well Happy Holidays to everyone first off.
MT-2500, It has been awhile since I have been on about this problem. I had a big scare this morning. No 4th no 3rd, just 2nd. On my way to work. So I went to my Union hall and decided to get under the truck and left. When I did, I had 3rd back. Slipping fourth (again). So I locked it in 3rd and ran about 50 mph at 2500 r's. Have been looking at all of the previous posts from different members. I decided to go with the filter change praying not to find an "aluminum" mine when I dropped the pan. Lo and behold the Goodness! Very minimal residue and no metal particulates. When I did open up the filter, I found the plastic pieces from the wiring covers. Why would they put that covering inside. Just breaks off from the heat. But anyhoots, I put the new filter in with some (I know I may should not have) Coastal Dex/Merc Blend they have. (money is laughing very loudly) It is Dextron IIE compatible. But baby shifts soooo much better so far. I drove it around for a bit to heat it up. No problems. I had been having the "shudders" when I would come to a stop. It is gone, so far. I want to thank you soooo very much for your insight and knowledge! Should I change the fluid again a lil later down the road (either because of the Coastal fluid, or just to help "flush" it out. I have read your post about flushing and informed my dad.)
If I do not hear from you and anyone else before this Christmas, please be safe and have a Blessed one!
Sincerely:
Chuck aka ulremember
Oh yeah, I forgot to clean the magnet. Is that a major concern?:uhoh:

replace the fluid again , clean the magnet , and use a clean cloth to wipe out the metal slime on the pan bottom...

don't use dexron III get the new dexron VI..this is superior to the dexron III...

since this did improve your transmission shift it is best to replace the fluid again to remove as much contaminated/damaged dexron III as possible...

Well the great pleasure is over! Overdrive is doing the same thing. I notice though that the shift points seem a little higher. While I am driving normally, the 1-2 shift and the 2-3 shift are fine. As I let it go into overdrive (without accelerating) it goes into 4th. At times the shift will hold. But as soon as I give it a lil bit of gas, I can feel it slip out. The R's raise up with very little acceleration, and then it "slides' right out of gear and the r's raise. Now, as soon as I let off of the gas. I feel 4th engage again. As long as my foot is not on the gas. If I "lock" it into 3rd, I can ride all day.Could this be the pressure solenoid for the overdrive. It really feels like the "locking" portion or the whatever does its thing is not holding the pressure. Also, if I let my foot off of the gas abruptly, it kinda "slams" back into gear. Not violent though. I can just feel it like a kick/nudge in the back end. I have a friend that has a Modis Reader, but he has not been close enough by to bring it by. He is a mobile mechanic. Thanks jCat for the reply. Any insights as to which solenoid it may be. I checked all of the aires for breaks and burns, but found nothing. The truck sat up three years prior to me buying it due to the gas mileage, which is why the neighbor sold it. Could it be varnish on the plunger of the overdrive solenoid?

Thanks again.

Get it on the scanner and find out what is going on.
Post back any codes.

Will do that for sure MT! Gonna get my buddy over here with his when he gets a chance. Should'nt I be looking for readings and values while I test drive. I have no Check Engine light on. Never has.
Blessings and thank you again MT and jCat

Will do that for sure MT! Gonna get my buddy over here with his when he gets a chance. Should'nt I be looking for readings and values while I test drive. I have no Check Engine light on. Never has.
Blessings and thank you again MT and jCat

you will need a scanner that can read drive train dtc...some scanners do not..

I'm back!
Hello to all! I finally got a hold of my friends Modis Scanner. I hooked it up and went on a FEW test drives to record the operation of the Tranny. Heck, it would not shift. According to the data stream, I was in 2nd after the first shift and would not downshift back to 1st after a stop at the red light. What I did find was tha if I turn the ignition off for a few, it would indicate that I was in 1st. After the shift, it would go into second and stay. No matter what speed or how I accelerated or deccelerated. The dat stated that all of my solenoids were off. Pressures looked good for the most part (scanner has desired values). What concerns me is that the solenoids switched off when I hooked the scanner up. When I disconnected it, the tranny works like it used to. No overdrive. Now, as I looked through the troubleshotting section, it says that the symtoms point to the accumulator. Not holding pressure. I am looking at getting the solenoid kit and accumulator.
Any suggestions as to why the tranny locked in second while the scanner was hooked up?
Blessings and Happy Easter!
Chuck aka ulremember

I'm back!
Hello to all! I finally got a hold of my friends Modis Scanner. I hooked it up and went on a FEW test drives to record the operation of the Tranny. Heck, it would not shift. According to the data stream, I was in 2nd after the first shift and would not downshift back to 1st after a stop at the red light. What I did find was tha if I turn the ignition off for a few, it would indicate that I was in 1st. After the shift, it would go into second and stay. No matter what speed or how I accelerated or deccelerated. The dat stated that all of my solenoids were off. Pressures looked good for the most part (scanner has desired values). What concerns me is that the solenoids switched off when I hooked the scanner up. When I disconnected it, the tranny works like it used to. No overdrive. Now, as I looked through the troubleshotting section, it says that the symtoms point to the accumulator. Not holding pressure. I am looking at getting the solenoid kit and accumulator.
Any suggestions as to why the tranny locked in second while the scanner was hooked up?
Blessings and Happy Easter!
Chuck aka ulremember

No really.
Locked in second gear is fail safe mode.
Like a hard code sset.
Did you pick up any codes past present or history?

Hello MT. No codes at all. First thing I checked. Here is the latest. I was going to work and I decided to check the "lockup" I think it is called. I was traveling at about 45 mph and in 3rd (no computer hooked up), I tapped the brake. I could feel the "lockup" engage and the r's rose about 200 rpms. Now, on my way home, I put the tranny in OD, 2400 rpms, 3rd gear and at about 45 mph. The tranny downshfted into 2nd and stayed there the duration. About 6 miles untill I got home and shut her down. I am going to guess that everything is going to be back to "normal" when I do leave.
Question, I read a topic with many replies to it. I forgot where. It may have been GMCforums. A person suggested changing the brake switch due to that has three sets of contacts and just because the brake lights work does not mean that the switch is good. He mentioned that it controls the cruise control and mine does not work. Also, the previous owner installed one of those "high dollar" ( in my opinion), alarms systems. The one that does it all, start, windows, ect. Could it be? You should see the underside of my dash spaghetti.
Again, thanks to all who have replied and I really appreciate this and I hope this may help someone in the future (not wishing it on anyone :nono:) Oh yeah, I just realized I never had 1st. When I put it in 1st (locked) it takes off like before but with a tell tell of some slipping. Solenoids? Lemon or a grapefruit?
Blessings to all and your families
Chuck aka ulremember

Hello MT. No codes at all. First thing I checked. Here is the latest. I was going to work and I decided to check the "lockup" I think it is called. I was traveling at about 45 mph and in 3rd (no computer hooked up), I tapped the brake. I could feel the "lockup" engage and the r's rose about 200 rpms. Now, on my way home, I put the tranny in OD, 2400 rpms, 3rd gear and at about 45 mph. The tranny downshfted into 2nd and stayed there the duration. About 6 miles untill I got home and shut her down. I am going to guess that everything is going to be back to "normal" when I do leave.
Question, I read a topic with many replies to it. I forgot where. It may have been GMCforums. A person suggested changing the brake switch due to that has three sets of contacts and just because the brake lights work does not mean that the switch is good. He mentioned that it controls the cruise control and mine does not work. Also, the previous owner installed one of those "high dollar" ( in my opinion), alarms systems. The one that does it all, start, windows, ect. Could it be? You should see the underside of my dash spaghetti.
Again, thanks to all who have replied and I really appreciate this and I hope this may help someone in the future (not wishing it on anyone :nono:) Oh yeah, I just realized I never had 1st. When I put it in 1st (locked) it takes off like before but with a tell tell of some slipping. Solenoids? Lemon or a grapefruit?
Blessings to all and your families
Chuck aka ulremember

A bad brake switch can affect lockup.

But if it is locking or staying in second gear it is going into fail safe mode.

A good transmission capable scanner should have codes past present or history on why it is going into fail safe mode.

Got it. Now here is one more thing that I found out. I was travelling yesterday and decided to put the selector in 1st. I was driving at about 20-30 mph when the transmission went into 1st. Kinda scared me. I then proceeded to shift to second. Nice crisp shift. I was able to repeat it twice. After I was finished with the work I was doing, about three hours, I could not get it to do it again. Could it be electrical? Speed sensor maybe? Stuck 1-2 solenoid. I posted earlier that when I had the scanner hooked up, all of the solenoids were in the off mode.
Thank you for your help!!
Blessings
Chuck

Got it. Now here is one more thing that I found out. I was travelling yesterday and decided to put the selector in 1st. I was driving at about 20-30 mph when the transmission went into 1st. Kinda scared me. I then proceeded to shift to second. Nice crisp shift. I was able to repeat it twice. After I was finished with the work I was doing, about three hours, I could not get it to do it again. Could it be electrical? Speed sensor maybe? Stuck 1-2 solenoid. I posted earlier that when I had the scanner hooked up, all of the solenoids were in the off mode.
Thank you for your help!!
Blessings
Chuck

Get it on a good transmission capable scanner and test it out.
Post back any codes.

Got it. Now here is one more thing that I found out. I was travelling yesterday and decided to put the selector in 1st. I was driving at about 20-30 mph when the transmission went into 1st. Kinda scared me. I then proceeded to shift to second. Nice crisp shift. I was able to repeat it twice. After I was finished with the work I was doing, about three hours, I could not get it to do it again. Could it be electrical? Speed sensor maybe? Stuck 1-2 solenoid. I posted earlier that when I had the scanner hooked up, all of the solenoids were in the off mode.
Thank you for your help!!
Blessings
Chuck

since it appears to be intermittent it is very possible the electrical connector to the transmission is damaged or not properly seated ...remove inspect/clean and reinstall....

some have found the connector to be the problem in cases like this....

Hello to all. I am in the process of checking electrical to the PCM. Where is it? I have removed the alarm system that was in there. I have read that it may be the ignition switch. Maybe?
TIA
Chuck aka ulremember

Hello to all. I am in the process of checking electrical to the PCM. Where is it? I have removed the alarm system that was in there. I have read that it may be the ignition switch. Maybe?
TIA
Chuck aka ulremember

Lot of maybe out ther But
You need to test it out on a good scanner.

Hello MT. I did hook one up. Locked it in second. All solenoids went to "off" I just found a "bunch" tranny fluid in the connector. Drove with out it hooked up. 2nd only so far. Now I am going to hook up the connector again and test drive. I have 13 volts with engine running on the pink wire at the connector, 11.5v engine off. when I hooked the connector up with the engine running, engine r's picked up. So, as soon as my brother leaves, I will drive. Always happens, when I get on a roll: Phone or company comes over argh!
Thank you a bunch MT and jCat!
Blessings
Chuck aka ulremember

Just checked the speed sensors. Both checked good. Had it on the jack, backprobed. Well calling it a night. Aslo. no cruise. I have voltage at the controller.
Thank you for all of your help. I really appreciate it!
Blessings
Chuck aka ulremember