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Transmission, Reverse only works in Low 4wd slips in 2wd or 4wd high

rdunkin

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Have a strange problem that I wanted to see if anyone had a suggestion before I pull the trans and transfer case. Recently purchased a 94 GMCc2500 4wd with the 6.5 diesel engine. The problem occurs when I put it in reverse it moves but is slipping. If I shift to 4 wheel low it works fine. It also seems to have an issue when shifting from 2nd to 3rd and 3rd to 4th. It does shift but winds out a little before it does shift. If it wasnt for the fact that it works in low gear I would have already pulled the tran but just wondering if it could be anything fixable without having to pull. I believe it is the 4l80e.Any help would be great, Thnaks
 
Changed the filter yesterday to see if any help and there was no difference. My question would be should I pull the transfer case or go ahead and pull the whole thing. If reverse works fine in 4wd low wouldnt that indicate thet the tranmission is functioning. Since the issue occurs in 2wd or 4wd high im leaning to the transfer case as the cause. If it was the transmission I would think reverse would be affected regardless of what mode I was in on the transfer case. Thanks in advance for any suggestions.
 
I'd bet that the problem is the clutches slipping in the main transmission. The question is why. The low range gearing in the transfer case means the main transmission only has to transmit a fraction of the torque to move the vehicle that it does in high range. If the transmission filter is clean then I would suspect either a bad pump or worn out clutches in the transmission. I believe that reverse gear requires more pressure to actuate than the forward gears do so that would tend to point to a pump output problem. Perhaps there is a relief valve that is leaking by causing the pump outlet pressure to be too low.:???: Maybe some of the tranmission gurus will chime in with more helpful information.
 
I am curious about this too.

My son's '91 GMC 2500 gasser is doing the same exact thing. Slips like hell in reverse when in 2WD or 4WD Hi, but if he shifts the T-case into 4-Low it backs up fine. :WTF:

This is his work plow truck and without reverse he is out of business. :sick:
 
Follow up

Im leaning towards, worn clutches. If the pump was weak wouldnt it slip initially but as rpms increase the pressure should rise. On a flat service it will roll backwards in 2wd or 4wd high but it slips as soon as I hit incline. Does make sense now that I think about it working in 4wd low. There should be muck less strain on the clutches due to the low gearing. Looks like its time to see about a rebuild unless anyone else has any ideas. Thanks for the replies and I will follow up with the diagnosis.
 
You need to connect a tech 1 or tech 2 or gmtdscantech[scantools] to see what the ecm[computer] is seeing. A trans pressure gauge is very helpful also.With just the pressure gauge you should see a jump in pressure when put into reverse. If the ecm isn't seeing any input speed signal it will make no hydraulic pressure because it doesn't think that the torque converter is spinning. Check all connectors to the trans for pulled out or loose pins or cut or broke wires. Check all fuses.
 
follow up

You need to connect a tech 1 or tech 2 or gmtdscantech[scantools] to see what the ecm[computer] is seeing. A trans pressure gauge is very helpful also.With just the pressure gauge you should see a jump in pressure when put into reverse. If the ecm isn't seeing any input speed signal it will make no hydraulic pressure because it doesn't think that the torque converter is spinning. Check all connectors to the trans for pulled out or loose pins or cut or broke wires. Check all fuses.

Hooked up a scanner and discovered that the ecm is shifting correctly into all geras however it still slips in reverse and 3rd. The odd thing is that the longer I drive the truck the less it slips in reverse. Upon first start it slips alot but after about 15 minutes of driving it slips much less. I thought the truck was not shifting into drive but now can see that it is shifting into 3rd but it seems like it wont engage until abou 2000 rpm, at this rpm I can only get to about 30mph before the rpms keep increasing. anyway I m pretty sure now it is the clutches, I have removed the valve body and changed all of the electronics and still the same thing. Time to go to the trans shop and see about a rebuild, unless someone else has any suggestions.
 
Just checking to see if you had any luck with your sons truck, I posted what I discovered below but it looks like I need a rebuild, Thanks , Just curious what you found out.
 
We ended up getting the transmission rebuilt. Found that the reverse clutches were fried and one of the ring gears was damaged too. Had them replace everything that didn't look good and $2100 later the tranny is working again. The guy that rebuilt it gave us a 2yr/unlimited mileage warranty. I hope we won't need to use the warranty.
 
to all the ones that have a problem with a 4l80e sliping in reverse the problem is that the band struts have back out of the case causeing the band pin to become to short there it two ways to fix this problem you can pond the band struts back in abought 1/4 inc or you can weld on to the pin i just fix one to day by just adding a little weld to the pin and makeing it longer by a 1/4 inc and it works good
 
Im wondering why my manual says to put trans/column into neutral first before shifting transfer case/4WD into 4H or 4L. I got 99-03 ford f250 4x4 SD and wondering, IF I PUT TRUCK TRANS/COLUMN INTO NEUTRAL, AND THEN SHIFT TRANSFER CASE/4WD IN TO 4H or 4L (4WD), HOW CAN I SHIFT TRANS/COLUMN TO REVERSE ? Get what im saying ? MANUAL SAYS NOTHING ABOUT REVERSE, ONLY PUT TRANS/COLUMN INTO NEUTRAL, PRETTY SURE THEY MEAN TO KEEP IT IN NEUTRAL UNTIL SHIFT BACK TO 2H, but that would mean to not ever put it in reverse (no 4wd reverse), I CANT FIND ANY INFO. wondering if there is some kind of issue with it ? also, if transfer case/4wd is in 4h or 4l (4WD) and you accidentally shift trans/column to D (drive), what would happen ?
 
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Im wondering why my manual says to put trans/column into neutral first before shifting transfer case/4WD into 4H or 4L. I got 99-03 ford f250 4x4 SD and wondering, IF I PUT TRUCK TRANS/COLUMN INTO NEUTRAL, AND THEN SHIFT TRANSFER CASE/4WD IN TO 4H or 4L (4WD), HOW CAN I SHIFT TRANS/COLUMN TO REVERSE ? Get what im saying ? MANUAL SAYS NOTHING ABOUT REVERSE, ONLY PUT TRANS/COLUMN INTO NEUTRAL, PRETTY SURE THEY MEAN TO KEEP IT IN NEUTRAL UNTIL SHIFT BACK TO 2H, but that would mean to not ever put it in reverse (no 4wd reverse), I CANT FIND ANY INFO. wondering if there is some kind of issue with it ? also, if transfer case/4wd is in 4h or 4l (4WD) and you accidentally shift trans/column to D (drive), what would happen ?

When you are siting still there is a load on the back of the transfer case from the tires in friction with ground, so no free play there.
When you are in park, drive, or reverse: there is a load from the transmission applied to the transfer case, no free play there.
You need one side of the transfer case to have free play when switching hi/ low or 2wd/4wd. Since you can't get the load off the tires through the driveshafts, you need neutral to take the load off the transfer case.

There are some transfer cases that are designed to shift while under load but they are more expensive to build and wear out sooner, so many knowledgeable owners of them still shift to neutral in the tranny before moving transfer case.
 
When you are siting still there is a load on the back of the transfer case from the tires in friction with ground, so no free play there.
When you are in park, drive, or reverse: there is a load from the transmission applied to the transfer case, no free play there.
You need one side of the transfer case to have free play when switching hi/ low or 2wd/4wd. Since you can't get the load off the tires through the driveshafts, you need neutral to take the load off the transfer case.

There are some transfer cases that are designed to shift while under load but they are more expensive to build and wear out sooner, so many knowledgeable owners of them still shift to neutral in the tranny before moving transfer case.
Thanks Will, i appreciate good people, So you are saying i can shift Transfer case/floor shift into 4WD/4H/4L IN ( TRANS/COLUMN ) NEUTRAL, AND THEN SHIFT TRANS/COLUMN INTO REVERSE, (again, i am asking about how to shift to reverse, use reverse in 4wd ), AS LONG AS IT SHIFTED FIRST IN NEUTRAL ?
 
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Start engine. Hold foot on brake. Shift transmission to neutral. Shift transfer case to 4wd. Shift transmission to any gear you want to use, including reverse or park.

When you want to put back in 2wd, follow Same process.
 
Start engine. Hold foot on brake. Shift transmission to neutral. Shift transfer case to 4wd. Shift transmission to any gear you want to use, including reverse or park.

When you want to put back in 2wd, follow Same process.
Thanks will are you an ex soldier ? i did 8 years army, , how did you learn that, no manual states that and i have asked mechanics.
 
I was taught to drive in an old jeep on a desert mountain off road. I knew how to rebuild a transfer case at 6, almost half year before I shifted one driving it.
 
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