• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

1993 gmc 6.5 td with 4l80e

P

POLO612

New Member
Messages
10
Reaction score
1
Location
Houston
i have had this truck for about 8 months and with that time i have put 22,000 miles on it and the trans. started acting up. it will throw OD out once it gets hot and i would just turn it off and start it back up and i would be right back on my way. Well one day it happend and i turned it off and started back up and took off but it never got OD back then lost 4th and well took it to trans. shop and they rebuilt it and put new sensors and wire harness upgrade and check the computer and it still dose the same it runs fine till it warms up and it wont get 4th or OD and now we have installed a bigger trans cooler and still the same im guessing on changeing the oil pump drive gear thats on the motor it has a speed sensor on it for some reson i unplug it and the trans hit all gears but the torque converter wont engage with the speed sensor unpluged so any one have an idea its poping up a code of 68:confused:
 
If you can get it on a scanner and see what your throttle position sensor and coolant temp sensor are reading. These are 2 common problem areas that cause the trans to go into limp mode like you describe and are often overlooked.
 
tps and temp

sorry forgot to say that i replaced the tps and calibrated it and that help a little now on the temp not shure what it is reading but i think it should be normal since its at a transmission shop and they should have notice if it was getting to hot. well ive spent all this money and still acting up so dont know if i should spend another 150.00 on this oil pump drive gear that has a speed sensor atached to it? What sucks is that this truck is what pays my bills and puts food on the table so its starting to worrie me now that ive had truble with it for three weeks it now has 175,000 miles!:nonod:
 
The coolant temp sensor is often overlooked, and if it reads hotter than the engine actually is it WILL lock out overdrive.
 
Well i replaced the oil pump drive gear that carries the speed sensor on the motor and well that set me back another 158.00 and now its reading good rpm but still no 4th or OD the trans shop is now telling me theres one thing left and they want to pull my trans back out cuz code 68 is still popping up and the book says that the 4th gear housing bolt could be broke and causes it to not cacth 4th or OD. NOTE this is the second trans they rebuild and put in my truck i know there loseing money big time but with 20 yrs. of experience what the hell?
 
The sensor with a 2 wire plug at the T-stat housing is the CTS. If the engine is below a certain temp or above a certain temp OD will be electronically disabled via the PCM.
 
well the trans guy is telling me that the only temp. sensor is the one on the wire harness and the harnesses are new iner and outer and all senssors as well and code 68 is still in affect. when i do a test run the scanner shows when gears are shifting and once its in 4th it will last for about 2min. then it shows ??????? just like that. and 4th is gone! what esle can it be im fixing to burn it!!! any body want this damn truck it will sell cheap without the wheel lift light bar or scanners if prob. is not fixed by tuesday!!! I think its time for a cummings or a duramax
 
Last edited:
The sensor FERM is talking about is not in the transmission. It is in the engine compartment, located in the water crossover next to the thermostat.
 
Trans shop says that its an electrical prob. but i dont think they know whats going on at all any one have any other ideas?
 
code 68 is most likely an internal problem inside of the trans. When you had the scanner on it what was the measured gear ratio? Was the pressure switch manifold replaced at any point during this(the one inside of the trans pan)? I had this code before in mine and it was in the valve body causing it as mine was also a fresh rebuild. There is an accumulator circuit in the valve body that gets worn out and needs to be bored and a new valve installed on high milage units. It causes a pressure drop when in OD that causes slippage to occur and this code trips, also a bad torque convertor will throw this code.
 
On a 93[mechanical ip] the temp sensor on the coolant crossover isn't a sensor for the pcm. It is a temp switch for the high idle-cold advance. It only uses temp input from internal trans temp sensor.
 
Oki Dokie here, not to bring up a dead horse here but my 93 just went through the same thing to no avail. I found a bad wire splice on the TPS harness on the blue wire and tried to clear codes to no avail. Now my remote start is starting to create it's own mind and start the truck every so often on it's own. Sounds like a ghost in the machine. I have not messed with grounds yet but am confused as to why the codes wil not clear after the steps to clear and not even run the truck yet. Code 68 is really pissing me off. I have had the tranny rebuilt by what is supposed to be a reputible shop, new TPS installed and tuned by GMC as well as a new and tuned TCM by GMC.
Any other thoughts guys?

Thanks,
Mark
 
BTW, my low coolant light is kicking on and off but my coolant level is just fine. I realize this is different that the coolant temp sensor and both have been replaced over the last few years however, could they be connected?

Thanks again,
Mark
 
AFAIK there is no direct connection between the two. did you check the wire for the light. It might be grounding out somewhere and tripping the light.
as for the funny transmission problems. did you check the ground for the computer? mine(6.2) was shared with the TPS, the fuel heater and a couple of other things. needless to say when it got corroded that's went funny.
 
just a number said:
AFAIK there is no direct connection between the two. did you check the wire for the light. It might be grounding out somewhere and tripping the light.
as for the funny transmission problems. did you check the ground for the computer? mine(6.2) was shared with the TPS, the fuel heater and a couple of other things. needless to say when it got corroded that's went funny.
Click to expand...
Thanks,
I will check those items as well. Unfortunately there is not alot of light around here anymore and I don't have a garage to work out of. I am really starting to think grounds are going to a big part of my issues.

Mark
 
just a number said:
check in 6.5/6.2 reference section. there's a post where all the ground are. save ya some time hunting.
Click to expand...
Thank you. I've already been there as TD had advised me some time ago. Very helpful info there. My biggest problem right now is daylight and time.

I would still like to know why I can not clear the code when following the "brake pedal and gas pedal mash for 15 seconds +" and I have not even started the truck again. I don't believe this to be a ground issue.
Any thoughts?

Thanks,
Mark
 
You must log in or register to reply here.
Back
Top