1994 2500 GMC Suburban Overdrive Failure

[MT-2500]

Originally Posted by 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 ) 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.

[ulremember]

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

[MT-2500]

Originally Posted by ulremember

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.

[j cAT]

Originally Posted by ulremember

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....

[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

[MT-2500]

Originally Posted by 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.

[ulremember]

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

[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