Rob P.

I have a 91 GMC 2500 2wd with a 4L80e transmission. I had it rebuilt by a shop and it has "problems."

1) When it's cold it will not reverse. It goes into gear but "locks up" and won't move. After the trans warms up, it will reverse. The trans shop is BS'ing me and says that this is because my brakes are dragging or stuck. I don't believe them. I believe that the trans is engaging both drive and reverse at the same time.

2) The trans also drifts in and out of a forward gear while in park or neutral. It engages the forward gear so strongly that it can be driven in neutral (with high revs). This becomes less of an engagement when the trans warms up. The trans shop is giving me more BS & tells me that the drop in engine speed isn't because it's going into gear but because my computer is causing my idle speed to fluctuate. I don't believe this either because you can shift to neutral, & after the engine speed drops, you can drive away without doing anything else.

3) Sometimes (rarely) I have no gear engagement when shifting to drive right after starting the engine in the morning. If I shift back to park and then drive again, I (usually) get engagement. Sometimes this takes 2 tries but it has always engaged after the 2nd try.

4) The trans has now developed some noises. Some sound like small stones/gravel hitting the driveshaft - ting ting ting. Other noises are like a loose heat shield buzz while stopped in gear and a rattle at about 35 mph. These noises come and go randomly. Driveshaft U-joints were checked & are good.

So, I took my problem child to another shop. They had it for 2 days and looked and tested and consulted various other trans guru's and couldn't find anything which would cause this problem. Then they charged me $500 for the "I dunno" diagnosis.

So I'm now asking complete strangers if anyone has any ideas on what could be going on in there. This truck hauls my 2-horse trailer and does other light duty tasks. As it is right now I do not trust it for long distance trips whether or not I'm towing. It's not modified in any way other than tire tread pattern (less of a highway pattern and wider).

Ideas?

Gilbert@Ace Racing

If you were to bring the vehicle to me to diagnose the symptoms you described the first thing I would do is to drive it of course and verify your complaints. From there I would get it in the air to remove any load from the unit and see how it responded. At the same time I would install a pressure gauge on it and also watch the Pressure Control Solenoids commanded voltage to see if indeed it corresponded with the pressure I was actually seeing. That information is paramount to understanding the problem. However, all that only speaks to how I would go about diagnosing it and not necessarily what your issues are...

There are more than a handful of possibilities based on what you said is going on... but it all speaks to some sort of an internal pressure leak or minor cross leak in the hydraulic control system of this unit. The 80e is know to have issues with the center support not sealing well, there could easily be excessive wear at several points in the valve body that was over looked and I could go on...

But in the end the reality is you need to get it to someone that understands what you are talking about, someone that will take the time to ride with you and try to get his mind around the symptoms. As you have found out, throwing money and time at it is not the answer. So many times i see clients who are in the same or similar boat you are in. Meaning, they have had work done only to have issues that are unresolved and are paying someone who is not getting the job done.

Feel free to call me, I'll discuss it in detail with you if you like. What part of the country are you in bud?

g

Rob P.

I'm in California (just north of LA). The shop I took it to for diagnosis is a small one-guy shop who is highly recommended as someone who can diagnose weird problems. He said he put a gauge set on it and the pressures were fine and stable. He dropped the VB and checked the case and VB for cracks (none). He did find a "ding" in the case which almost crossed from the reverse pressure gallery to a "no pressure" return gallery. He told me that if this was a problem there would be no reverse gear because it would not build pressure. As it stands this would mean no reverse instead of both reverse and fwd gear engagement. He double gasketed the VB to be sure that there was no pressure bleed-off from the dinged area.

VB was fine. VB separator plate was fine. All electronics "function check." (<- my words) All VB checkballs are in place and follow the TransGo kit instructions. VB was fully disassembled and checked for broken parts/wear. All gears engage/shift fine with the exception of "lock-up" in reverse when cold and the intermittent fwd gear engagement while in park/neutral.

I've been reading about issues in the 4L80e all weekend. I know (now) that the rear bushing can walk and contact a sensor/gear in the tailshaft housing. I'm wondering if this could be the noise I'm hearing? It sounds EXACTLY like bad U-joints squeaking (chirp chirp chirp) and speeds up when accelerating BUT drops in speed after the shift and then stops. This tells me the trans is the noisemaker and not the driveshaft. (Plus the physical inspection of the U-joints showing them to be good.) I did find that the bolt which holds the yoke to the tailshaft was only finger tight when I dropped the driveshaft. I torqued it to 50 ft/lbs when I reinstalled the DS. There was no change from this.

The trans is going to be serviced in the next week or so. (I'm out of town for 2 weeks at least. Thankfully I have another car.) The shop already knows that they can have the truck for several days if necessary. Is there anything I can ask them to check while they are in there which would help with diagnosis? I don't know if I'm hoping that they don't find metal in the pan or not at this point.

What REALLY torques me off are the idiots who rebuilt it LYING to me like I don't know anything about cars or mechanical systems. Just so that they don't have to fix it.

So, if the center support were leaking from a torn/damaged lip seal would this cause the double gear engagement?

Vince B

iTrader: (1)

Rob when the trans-go kit was installed was it a stacked plate separator design? I used some of these in the past and got away from this design because I was worried about cross leaks. The issue I see with this style plate is when you drill the feed holes to firm up the shifts burs for the bit going through the plate make it uneven. What I have done in the past is pitch this plate and just use the springs that come in the kit when building a trans. Then ditch the second ring on the center support, leave the lip seal off of the direct drum and then block the reverse side feed hole in the case. By doing this you now have a dual fed direct drum. Works great and this is a trick that was used in TH400 units years ago and works just fine in an 80E.

Not saying this is your issue but might be something to think about if another shop goes into this unit. I hope you get this fixed and please let us know what becomes of this. Vince

Gilbert@Ace Racing

That's good information... glad to see you have had it to someone who is interested in solving the problems.

Now, I know you said that the pressures were deemed to be good. But, did that technician actually log them for future reference? That would be very useful right about now.

There are some good clues in your commentary, what I am concerned about is what you are calling "lock-up" in reverse when it is cold, or below operating temperature. Let me give you some food for thought.... In a racing application there is a feature we call a Transmission Brake. It's function is to hold the car perfectly still at the starting line while the footbrake is released and the throttle is wide open. The reason it works so beautifully is that reverse and low gear are the same ratio, therefore when reverse is applied all of the forward energy going into the transmission is canceled. I bring that up because I am curious to know if the vehicle will not move or requires a significant amount of throttle input to move when this happens. Meaning, low may not be applied, but one of of the other forward elements may be and that itself could cause the intermittant symptoms in neutral.

Vince has a good point about the laminated valve body plate. And it sounds like you are in good hands with the guy you took it to. I don't mind visiting with him if you would like to pass on my contact info... I might recognize something he overlooked, who knows. :-)

g

Rob P.

Gilbert:

When cold it takes very significant throttle to move the vehicle in reverse. And when it does move, it "lurches" rather than move smoothly. You can also feel it straining to move. No abnormal noises though. I will talk to the shop about this and the separator plate when the truck is there for service.

These issues will take awhile to resolve. And, as I said, I'm out of town so I'm doing this by "remote control."