Need Help With Diag On A Transmission Issue

[obrienja]

My dad has a one ton with a lift bed on it that he uses around the farm. It is having transmission problems. I work on a lot of engines but not many transmissions.

 

His truck is a 4wd 1992 chevy cheyenne with a 350 engine. It is an automatic and I think the tranny is a 4L80e. It has 17 bolts. It has less then 30,000 miles on it. Last spring the tranny oil pan rusted through. He had it replace by a local shop. a couple of months ago he hit some wash board bumps on the way home from cutting firewood. The truck slipped into neutral. The shifter didn't move but all drive power went away. If you stop then shift into first or so then the truck will move and you can shift back into drive or overdrive and the truck will go. It may take 10 miles for it to happen again or it may happen in 3 miles.

 

So I though maybe the filter didn't get put in tight or replaced when the pan was replaced. I dropped the pan this weekend and put a new filter and gasket in. The fluid was red but seemed to smell bad. When I pulled the pan out to clean it and check for metal the bottom of the pan that is on the passanger side seemed to have a black coating on the bottom.

 

After I got the pan back on and filled with fluid I took it for a drive. When it shifted from first to second it jerked and slammed into gear. And never shifted out of second gear into third.

 

I don't know if it was shifting into third before I changed the fluid or not.

 

Any suggestions on what could be causing this problem. Can the tps cause the tranny to not shift into third?

 

The truck can sit for a month or two between uses then it will be used every day for a bunch of short trips. It will be ran 6 miles and sit while it is loaded then driven and dumped and sits again waiting for another load.

[obrienja]

Sorry forgot to put this question in my original post.

 

Does the black in the bottom of the pan mean the tor conv is bad? If so is there an upgrade or different one that should be put in? I know that some of the original 4L80e were not as durable as the latter ones. If I have it out are there other parts that should be swapped out to make it stronger/better?

[wavery]

You might want to check the transmission and engine mounts and shift cable bracket. If something is out of alignment, it could effect the shift cable.

 

As far as the black stuff in the oil pan, that could be ~normal if the fluid has never been changed before and the truck is in heavy service. The clutch plates and other moving parts will leave some normal wear residue in the pan. When they installed the new pan, they probably only replaced the fluid that was in the pan. There is about 6-8qts of (burnt) fluid left in the converter which would be carrying that residue. When you do a simple trans fluid change without getting the fluid out of the converter, you will have residue within 15 minutes of starting the vehicle again.

 

Having said all that, if the trans fluid smelled burnt, chances are you may have something else going on. Running a transmission on burnt fluid is like running it on a gallon of varnish or water. It will increase the wear on the trans tremendously.

[obrienja]

Thanks for the reply Wayne.

 

So the need to drain the tor con by pumping it out the front lines and refilling it at the dip tube. I will check the mounts and other stuff you mentioned.

 

I am wondering if maybe it has been stuck in second for awhile and maybe my dad hasn't noticed. He doesn't typically drive it fast it is not a freeway kind of truck. Most of the trips are on country roads. So he could have been driving it at high rpms for awhile. The truck is a beast and you can easily drive 45 mph without red lining it. so if he has been driving it a 3500 or 4000 rpm I assume that could toast some fluid.

[hrspwr]

Couple things to take a look at. First make sure that the fluid does not smell burnt as Wayne said. The hard shifting could be caused by high line pressures caused by a mis adjusted TV Cable(also sometimes called the kickdown cable). Here is an article that may help adjusting it. http://www.tvmadeez.com/tvcable_instructions/stockcable.php

[obrienja]

I don't know that I have ever smelled burnt fluid. I just know that when I opened the pan the smell was very unpleasant. It seemed to go away after a few minutes or I got use to it. I didn't smell it when I was cleaning the pan before I put it back in.

[hrspwr]

What the fluid milky looking?

[obrienja]

No the fluid was red like it just came out of the bottle. What does milky fluid mean?

The truck is in my brother's barn and I asked him to check the fuse to see if that is blown and if that is why the tranny is staying in second. It is 45 miles from where I am at so I am not going to get a chance to look at it for a few days.

[hrspwr]

If I am notmistaken these trannys did not use the computer to shift(cant remember the cutoff point when they went to 4L60E, think it was 95-96). Milky fluid would mean that you have water in the fluid. I would check the TV cable to make sure that is set properly once it left the shop last time. Next I would take a look at the shifter linkage. Because if the fluid is good and there is no slipping detected when shifting gears, that would be my next two things. Other than that take it to a shop that you trust to see if there is something internal that might be causing the issue.

[obrienja]

I was able to look at the truck again this weekend. The transmission is a 4L80E. It has two speed sensors. The one in the back appears to have a problem. When I pulled the electrical connection to test the resistance in it there was tranny fluid in the connector. It appeared to have complete resistance no current was going through it. My service manual for my 91 astro has information in it for this tranny apparently it was an option on the astro. My book called for something in the range of 300 to 2000 ohms. The front sensor reading was 1430.

 

So I am assuming the back sensor is bad. Can the sensor rust through or develop a leak. I can't image that the tranny housing has cracked but I didn't have my mirror to look up there.

 

Could a bad sensor prevent the engine from shifting into third and allow it to slip into neutral? My book tells how to take apart and assemble the transmission but not much info on diag of the tranny.

[obrienja]

I found this on http://www.454ss.com/Articles_new/red/4L80...on_Problems.asp

 

It sounds a lot like my problem but not exactly. Maybe I have a problem with the 3 4 clutch in addition to the speed sensor problem.

 

P-R-N-D work, upshifts are erratic and occur at random engine speeds, will not upshift from first gear (no upshifts at all): This problem seems very serious in nature but in fact this is something very simple to fix. In addition to erratic shifting, the trans control program will default to backup shifting mode if you drive it long enough with this bad sensor, and as a result line pressure through the valve body will be boosted to maximum value, creating harsh shifting. Your transmission uses two main sensors to gather information on when to shift, which is the TPS (throttle position sensor), located on the throttle body, and the input speed sensor, located on the driver side gear case about 8 inches up from the pan, to the rear of the bellhousing. The input speed sensor (and output speed sensor, mainly used for vehicle speed calculations, both of which carry the same part number) is suspected of failure, according to a GM bulletin, due to its poor open-magnet design. This design allows tranny fluid to seep into the sensor and damage it. The sensor with the updated, completely sealed design may be purchased from your local GM dealer for a cost in the mid-$30 range. When changing it, put a pan under the tranny to catch the fluid that will leak out when you remove the sensor. The bolt that retains the sensor is 10mm.

[obrienja]

Well I went out to replace the sensor yesterday thinking it would be a quick deal. However it seems nothing ever is. A mechanic friend told me the sensors break of sometimes and this one did. I assume I have to drop the pan and pull the guts out to push the sensor out from the inside.

 

Is that correct?

 

Oh yeah the bolt holding it in was an 8 mm not a ten like the above note says.

[obrienja]

Well I fixed it. When I tried to take the sensor out it broke off. I sprayed it down with pb blast and let it sit for a couple of weeks. I used a dental pick to work around the sensor. I ended up braking the plastic of from around the magnet inside the sensor. There was enough of it to get a pair of pliers on and was able to twist it and work it out. I popped in the new one and fixed a bad ground to the fuel pump that was unconnected to the tranny problem. And everything worked great.

 

So the back speed sensor was bad and the computer wasn't getting any input from it and was going into limp mode. When I had tested it it was open and that is how I knew it was bad. The back speed sensor was apparently a know problem but because my dad had put so few miles on it it took almost 20 years to go bad.

 

I am hoping this will increase my gas milage also. It is running at lower rpms then it was before it started having obvious problems

 

Thanks for the help.