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Old 07-30-2011, 06:07 AM
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GMC erratic shifts and speedometer

Everything seems normal until the transmission tries to shift. At that time, it seems to get hung up in neutral and the speedometer drops, sometimes to zero.

Not sure which is the cause, and which is the result. Could be the speedometer drops to zero and the transmission goes to neutral, or other way around.

I changed the VSS and checked wiring to the VSS buffer.

I still think it's an electrical problem, but I'm not sure. This all started after returning to 2WD after some 4WD action. Could the transfer case have something to do with it? Can the speedopmeter tone ring in the transfer case slip, or does it stop rotating if the TC slips into neutral?

I'm still tinkering, but I'm unfamiliar with this system from a 1995 GMC Sierra.

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Old 07-30-2011, 06:33 AM
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Quote:
Originally Posted by Argess
Everything seems normal until the transmission tries to shift. At that time, it seems to get hung up in neutral and the speedometer drops, sometimes to zero.

Not sure which is the cause, and which is the result. Could be the speedometer drops to zero and the transmission goes to neutral, or other way around.

I changed the VSS and checked wiring to the VSS buffer.

I still think it's an electrical problem, but I'm not sure. This all started after returning to 2WD after some 4WD action. Could the transfer case have something to do with it? Can the speedopmeter tone ring in the transfer case slip, or does it stop rotating if the TC slips into neutral?

I'm still tinkering, but I'm unfamiliar with this system from a 1995 GMC Sierra.
is the transfer case a mechanical shift or electronic shift?
have you tried putting the t-case in low range to see if it still does it? any grinding noises?
its pretty common for the fork inside the t-case to get worn out and that can cause neutralizing. i dont think that with what you explained that the problem is in the trans. i think you have an internal t-case problem..
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Old 07-30-2011, 06:44 AM
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The transmission is electronically controlled. I"m not sure about the transfer case. I"ll have to crawl under and look. I didn't see any wires going to it other than the VSS wires. Of course, I was so convinced it was the VSS, I really wasn't paying attention.

I did try shifting the 4WD lever a few times in case a switch was sticking, but no change.

The 1-2 shift sometimes is fine, sometimes does the "neutral and dropping speedometer thing". The 2-3 shift always has the problem.

I need to test drive some more, but the problem also seems to co-incide with letting off the gas a bit.

During the 4WD excursion, water got a bit high, so I pulled all the connectors I could find to check for water. All dry.

There's a bit of fluid on the front friveshaft yoke, apparently from the TC, but it's merely damp looking, not dripping, so that's likely normal for the age. But i did wonder if the TC is low on fluid. The description I read on it does mention an internal pump. So I'll check the fluiid level, although I'm not hopeful on that one.
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Old 07-30-2011, 06:57 AM
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Quote:
Originally Posted by Argess
The transmission is electronically controlled. I"m not sure about the transfer case. I"ll have to crawl under and look. I didn't see any wires going to it other than the VSS wires. Of course, I was so convinced it was the VSS, I really wasn't paying attention.

I did try shifting the 4WD lever a few times in case a switch was sticking, but no change.

The 1-2 shift sometimes is fine, sometimes does the "neutral and dropping speedometer thing". The 2-3 shift always has the problem.

I need to test drive some more, but the problem also seems to co-incide with letting off the gas a bit.

During the 4WD excursion, water got a bit high, so I pulled all the connectors I could find to check for water. All dry.

There's a bit of fluid on the front friveshaft yoke, apparently from the TC, but it's merely damp looking, not dripping, so that's likely normal for the age. But i did wonder if the TC is low on fluid. The description I read on it does mention an internal pump. So I'll check the fluiid level, although I'm not hopeful on that one.

if it has a handle to shift the t-case, then its a mechanical shift t-case.
you need to drive the truck in 4 low and see if the condition still exists. not just shift it between ranges. if you have no speedo dropout while its in low range and the trans shifts through all the gears, then its in the t-case.
as far as the trans goes, i know its an electronically controlled trans as the early 4l60e's came out in 1992.
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Old 07-30-2011, 07:13 AM
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It has a handle, and according to my book, it should be either a New Process or a New Venture 241 Transfer case. The pic of a 241 in the book looks like it.

I really appreciate the input, but I can't get all the answers to your questions today. It's pouring rain outside. So today is Google Day and "ask for help in the forum" Day.

I'll see if I can do the 4-Lo drive test shortly. Good idea!
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Old 07-30-2011, 06:14 PM
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Maybe I'm not used to shifting a GM in and out of 4WD, but it doesn't seem like it's working right to me.

First, I can't get it to shift into 4wd lo, unless I stop and put the Transmission in Neutral. In fact, I can't even get the TC to shift into Neutral unless the transmission is in Neutral.

Once in 4WD LO, I'm not sure if problem is there or not as the engine winds up with the vehicle speed staying low (more or less expected), but it does seem to stay in first gear a long time, and then shift to 2nd with a significant lurch.

Next, I can't shift the TC back, not even to Neutral unless I shut the engine off as well as leaving the transmission in Neutral. Any attempt to shift the TC case through neutral on the way back to the 2WD position creates a grinding of gears and the speedometer starts to rise even though the vehicle isn't moving.

A highway test in 4WD Hi shows the same erratic shifting, loss of power to the rear wheels, and a dropping speedometer as experienced in 2WD.

Tomorrow I'll check the TC oil level and look for linkage problems, but I'm starting to suspect an internal problem with the TC.
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Old 07-30-2011, 07:15 PM
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Quote:
Originally Posted by Argess
Maybe I'm not used to shifting a GM in and out of 4WD, but it doesn't seem like it's working right to me.

First, I can't get it to shift into 4wd lo, unless I stop and put the Transmission in Neutral. In fact, I can't even get the TC to shift into Neutral unless the transmission is in Neutral.

Once in 4WD LO, I'm not sure if problem is there or not as the engine winds up with the vehicle speed staying low (more or less expected), but it does seem to stay in first gear a long time, and then shift to 2nd with a significant lurch.

Next, I can't shift the TC back, not even to Neutral unless I shut the engine off as well as leaving the transmission in Neutral. Any attempt to shift the TC case through neutral on the way back to the 2WD position creates a grinding of gears and the speedometer starts to rise even though the vehicle isn't moving.

A highway test in 4WD Hi shows the same erratic shifting, loss of power to the rear wheels, and a dropping speedometer as experienced in 2WD.

Tomorrow I'll check the TC oil level and look for linkage problems, but I'm starting to suspect an internal problem with the TC.
you have to shut the engine off with the trans in neutral to shift through the ranges on the t-case. thats normal.
what doesnt make sense here is what i underlined from what i quoted from you.the speedo gets signal from the output shaft of the t-case. and unless the rear tires are already turning. you shouldnt be getting anything from the speedo.
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Old 07-30-2011, 07:24 PM
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Yes, that doesn't make sense, but that's what is happening when trying to go from 4WD Lo into Transfer Case Neutral... a grinding of gears (what's spinning as the vehicle isn't moving?).

Thank-you for the other info as to what's normal.

I'm going to do some more tinkering tomorrow. Check the fluid level of the TC, and monitor the 12VDC to the VSS Buffer while test driving. Hopefully more if I can think of anything.
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Old 07-30-2011, 07:42 PM
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Quote:
Originally Posted by Argess
Yes, that doesn't make sense, but that's what is happening when trying to go from 4WD Lo into Transfer Case Neutral... a grinding of gears (what's spinning as the vehicle isn't moving?).

Thank-you for the other info as to what's normal.

I'm going to do some more tinkering tomorrow. Check the fluid level of the TC, and monitor the 12VDC to the VSS Buffer while test driving. Hopefully more if I can think of anything.

if the engine is running the trans pump is working. and unless its in park, the output shaft will turn on the trans. the thing is, the v.s.s. is getting signal from the output shaft of the t-case. not the trans.
there also should not be any noise going from 4lo to neutral on the t-case.
also the linkage on those is very simplistic. and as long as its going between ranges, its not going to be a external issue, especially with the v.s.s. having drop out issues.
is there a speed signal trouble code in the pcm? you may be fighting a wiring issue or pcm problem as well.
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Old 07-30-2011, 08:00 PM
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Yes, I may be faced with tearing the TC apart. I wouldn't attempt an automatic transmission repair, but a TC doesn't look too complicated, or at least not any worse than a manual transmission.

Naturally, I want to rule out external issues first.

There is no error light lit on the dash, and although I do have an OBDII reader, the GM connector at the base of the dash is differerent, so I may need an adapter to check codes.
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Old 07-30-2011, 08:20 PM
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its pre obd II and will need the appropriate connector to pull codes. there is not an adaptor available to go from obd II to the proprietary gm diag connector.
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Old 07-31-2011, 07:05 AM
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Ah, a new day, sunny out, and a fresh start on troubleshooting.

Doing a little more research last night, one very common problem with these TCs is a broken snap ring on the outboard end of the output shaft.

Apparently this can cause the TC to slip in and out of gear. There's even a snap-ring repair part to prevent this from re-occuring.

http://www.drivetrain.com/parts_cata...ase_saver.html

Although I never found any mention of the speedometer being erratic, I suppose it makes sense that with everything loose, the tone ring could be floating on the shaft, or even moved away from the VSS sensor at times.

I think before I do anything else, is to remove the VSS sensor, and see if I can feel the tone ring through the hole and see if it spins or moves (when it shouldn't). Doesn't look like a terribly long job to remove the dirveshaft, and then the TC tail section to inspect and possibly change the snap-ring if necessary. Of course nothing ever goes the way one expects it to... LOL.

Of course, there's the possibility that the TC internals are now shot, but that will show itself eventually. It certainly is difficult to shift the TC.

ps: that's odd about the connector not being a standard OBDII on a 1995. But as their is no "check light" ON, I would assume there are no codes, although I think OBDII may have codes, but not activate the Check LIght until the code has remained active for a certain time.
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Old 07-31-2011, 08:26 AM
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Quote:
Originally Posted by Argess
Ah, a new day, sunny out, and a fresh start on troubleshooting.

Doing a little more research last night, one very common problem with these TCs is a broken snap ring on the outboard end of the output shaft.

Apparently this can cause the TC to slip in and out of gear. There's even a snap-ring repair part to prevent this from re-occuring.

http://www.drivetrain.com/parts_cata...ase_saver.html

Although I never found any mention of the speedometer being erratic, I suppose it makes sense that with everything loose, the tone ring could be floating on the shaft, or even moved away from the VSS sensor at times.

I think before I do anything else, is to remove the VSS sensor, and see if I can feel the tone ring through the hole and see if it spins or moves (when it shouldn't). Doesn't look like a terribly long job to remove the dirveshaft, and then the TC tail section to inspect and possibly change the snap-ring if necessary. Of course nothing ever goes the way one expects it to... LOL.

Of course, there's the possibility that the TC internals are now shot, but that will show itself eventually. It certainly is difficult to shift the TC.

ps: that's odd about the connector not being a standard OBDII on a 1995. But as their is no "check light" ON, I would assume there are no codes, although I think OBDII may have codes, but not activate the Check LIght until the code has remained active for a certain time.

some codes dont throw a CEL. its just part of the programming.
as far as OBD II goes, there are some cars/trucks that had OBD II in 95, just not that many.
good idea on checking the reluctor, with what your explaining to me, it also wouldnt hurt to see if the output shaft on the t-case moves back and forth any.
did you ever check the fluid level and condition of the t-case?
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Old 07-31-2011, 10:42 AM
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Havn't got too far yet today, but I did remove the VSS sensor and felt inside. The reluctor ring feels tight, but seems like it's pushed forward, although perhaps it's normal. The rear edge of the ring is about center with the VSS sensor hole, and the ring is quite wide with the front edge lining up somewhere forward of the VSS sensor hole.

It does seem like the whole output shaft is about 1/4" forward. But then making the facts meet a guess, isn't always productive.

So I guess I'll take the driveshat out and the TC tailshaft off and have a look.

Little steps and the joy of not knowing what I'm doing...LOL.

Didn't check the fluid yet, but the end of the sensor was wet with fluid, so some must be in there. I will check it.

Can't see what method the TC uses to shift. Best I can tell is something on the top of the TC goes straight up through the tunnel and feels solid, like a rod, not like wiring. So no doubt that's the linkage and goes to the Shift Lever, or is the shift Lever.
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Old 07-31-2011, 12:09 PM
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Took driveshaft out. Didn't have to remove TC tailshaft as I could see inside. Snap-ring is still in place and output shaft has very little play in and out. I'd say less than 1/16".

Checked TC fluid level and it was fine. Nice clean red ATF.

Not sure what that thing is on top of TC, but not shift linkage. I found shift linkage on side of TC.

I guess something still could be wrong with TC, but it's back to looking at electrical problems for a while. VSS signal shouldn't drop out like it's doing.

But, first, to put the driveshaft back in...
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