Hi all,

I have a '95 GMC K2500 Suburban with the 6.5TD.

I'm having a weird problem, propably related to bad connections but I'd like to know your opinions.
When driving off, and engaging cruise control, it sometimes disengages after about ten minutes of driving.
Sometimes when trying to engage the cruise control after ten minutes of driving, it won't.
I notice a hard shift when locking up also.
Almost always it can be cured by shutting off power and restarting.
Then the cruise control works fine and lockup also works fine and smooth.
Another weird problem is that sometimes, but not as often as the cruise control, the tranny won't shift to from second to third.
Again this can be cured by cutting power and restarting.
Does anybody have any idea where I should start looking.

Can anybody tell me where to find a good manual, since their almost impossible to find over here in Belgium.


Thanks

Replace the vehicle speed sensor on the rear driverside of 2wd 4L80E.

Try disconnecting and reconnecting the transmission harness connectors, burnish the contacts.
Symptom is the temperature sensor circuit is opening, signalling HOT to the PCM, which then goes into "LIMP - Cool Down" mode - no cruise, hard shift, hard TCC lock-up.

Check Helm, Usa - they are discontinuing '95-back manuals, which are on now on sale, or try E-Bay.

The fact that it is being cured by restarting indicates that a code is setting. Jumper the 2 top right terminals of the data link connector together, turn the ign on and count the blink codes.

Thanks for the info guys,

GMCSID, I'll have to wait until I get the manual I ordered to take a look at the data link connector.
Helm's manuals aren't available right now so I ordered a CD from cdmanualwarehouse.com
I read somewhere that there was an online service available also but I can't remember the name.
In the meantime, I've taken a look at the contacts.
Jeez, I realy miss my old '84 Burb. No electronic thingies to mess with.. :-(
I've cleanded the contacts and tried to eliminate moisture to enter by putting vaseline on the edges of the connectors. I hope that helps.
There was some corrosion on the contacts. Or at least I think so. They're so tiny. I scraped it of until they were shiny again.
There's a sensor on the output shaft of the transfer case and there are two on the tranny.
The one in front has a connector, but the one at the back doesn't or I must be missing something.
I can't seem to remember whether it had one or not (Alzheimer???)
Anyway, I looked evereywhere, even opened the protection of the wiring harness to see if something was broken off or missing...nothing,

Is this normal?

Thanks again for the help.

Helm's manuals aren't available right now so I ordered a CD from cdmanualwarehouse.com.Do you know if these are the Helm manuals on CD, or some other publication? For the price, I wouldn't mind having a CD of my FSM from Helm to make copying easier. If you don't mind, could you let me know how you like it when you get it?

look under the dash, driverside.
Pin A to B, Key on, codes will flash on check engine light.
Write them down, then step on fuel pedal 'till 12 flashes.


Online service is alldata and they're worth the $25, just print all the information and a second year subscription is unnecessary.

I have a 95 C-2500 Pickup and had the shift problem but when it was cold. I changed out the sensors that are on the driver side of the transmission going into the transmission one is in the tail shaft the other further too the front. One bolt holds in place. The are about $30-35 each on is the 1-2 shift sensor the the other is the 3-4 shift sensor. You can also take them out and clean them up and see if it helps. I had problems with the cruise and the wires that attach to the harness on the stearing colum which the wires were pulling out. It take a special tool (I think it's a deep socket reverse torx ) to change it.

Hi,

I've cleaned all the contacts but as suspected, it didn't change a thing.
Can anyone confirm that the second sensor on the tranny (on the tail end, just befor the transfer cas) isn't connected?
The codes I'm getting are :
68 Transmission Component Slipping
71 Set/Coast Switch Fault (Cruise Control)
87 I can't find any info on this
It doesn't start with code 12 and I can't seem to clear the codes.

Thanks for all the help

Hi,

I've cleaned all the contacts but as suspected, it didn't change a thing.
Can anyone confirm that the second sensor on the tranny (on the tail end, just befor the transfer cas) isn't connected?
The codes I'm getting are :
68 Transmission Component Slipping
71 Set/Coast Switch Fault (Cruise Control)
87 I can't find any info on this
It doesn't start with code 12 and I can't seem to clear the codes.

Thanks for all the help
I have 2x4 but I believe it's 1-2 shift sensor and there is a small plug that plugs into it.

The two sensors on the tranny are input and output speed sensors they are acctually the same part #'s. Shifting is controlled by the pcm using info brought to it by these sensors. The input sensor is on the drivers side just behind the bell housing, one bolt with 10mm head holds it in, the plug comes out at a 90* down. The output sensor is on the transfer for 4x4's and it looks identical and is held in place by the same type bolt.

Hi,

I've cleaned all the contacts but as suspected, it didn't change a thing.
Can anyone confirm that the second sensor on the tranny (on the tail end, just befor the transfer cas) isn't connected?
The codes I'm getting are :
68 Transmission Component Slipping
71 Set/Coast Switch Fault (Cruise Control)
87 I can't find any info on this
It doesn't start with code 12 and I can't seem to clear the codes.

Thanks for all the helpDTC 68 - Transmission Component Slipping
DTC 71 - Set/Coast Switch Fault (Cruise Control)
DTC 87 - High Ratio Error


All the gears in the transmission have a set ratio. When the input shaft speed measured by the input speed sensor and the output shaft speed measured by the output speed sensor do not match you get a undefined ratio, in your case it is a consistantly high ratio depending on the slipping component so you are setting a 87 with a 68. The trans can't hold its one on the freeway and is slipping so it kills the cruise as well setting the 71 code. My first move would be to put a Tech1 or 2 on the thing before more damage occurs and see where the slipping is coming from and see if it is repairable before a complete rebuild is needed. Although it may be to late. I got a code 85 and 68 when my TCC clutch stopped working. That was my second 4L80E with many work overs in between, so I switched to a NV4500 and it has made my life easier and better, not that I recommend switching to you though.