Transmission problem

[Eduardo]

Hello,

I have just acquired in an auction a 1993 Chevrolet Cheyenne 3500 4x4 6.5 turbo-diesel. It is a high-kilometers vehicle (430000 and counting) but the engine seems to work fairly well. However, I am having some problems with the automatic transmission, namely:

1) Shifting from 1st to 2nd gear is very rough.
2) All shifting (from 1st to 2nd, then to 3rd and 4th) happens at 3600RPM, which I find quite high (in the yellow area of the tachometer).

I took the truck this morning to an automatic transmission specialist, and he told me that the good news is that the transmission is mechanically OK (all the gears are there and working) but the bad news is that the computer would not talk to him (I think that meant he could not read the computer codes).

Today I did a search in the forum and tried to find a similar case to mine, but no luck. Nevertheless, I crawled under the truck and removed and cleaned two sensors from the tranny (two magnetic pickups) and also removed and tested a switch from the front part of the transfer case (the switch was OK). That did not make any difference.

I also saw a maze of wires going inside the tranny, but decided not to mess with them.

I also opened the computer, but could not see anything obviously wrong.

This is probably unrelated, but I observed that the reverse signal light stays on continuously. In my previous truck (a Silverado 1987 6.2 diesel) I believe the reverse switch was installed in the steering column, but have no clue where it is in this one.

Any idea of what could be causing the above transmission problem? Any advice?


Thank you for your help. [img]smile.gif[/img]

[farmerherb]

Ed, Iam also having trouble with my trans.(95 K2500) I have replaced the main computer and ignition switch, I was told both would fix my problem Not. I have ordered the inner wiring harness(25.00 on Ebay) also today I plan to replace all the solenoids and pressure switch in the pan. I have a funny feeling its a wiring problem I'm trying to fix, but where? How did you check your solenoids?

[Hye]

I remember reading here that there is a wiring harnes that goes across the top side of the tranny that rubs on the cab and eventually lets go. Maybe this is your problem?

[JohnC]

Sounds like the Throttle Position Switch is out of whack. Go here How to adjust the TPS

[Eduardo]

Thank you all for your responses. I will check out the harness, and I also will chek the Throttle Position Switch. However, I first need to know where the TPS is located, so I would appreciate if anyone could give me some indications regarding where to look for it and how it looks like.

Farmerherb, I did not check the solenoids, which I presume are inside the transmission.

[arrowheadracing]

Originally posted by JohnC:
Sounds like the Throttle Position Switch is out of whack. Go here How to adjust the TPS

So if the TPS is out of whack will it shift at a higher rpm ? Wouldnt there be anymore symptoms ? As far as engine running wise.

[DA BIG ONE]

You need to pull the codes somehow.

Very common, but seldom mentioned is manifold pressure switch (also called pressure switch assembly)failure in an otherwise good 4L80, shifting will be all over the place, it will sometimes bang gears, then followed by what I call a self destruct mode "grinding of gears when coming to a stop".

I have seen so-called tranny rebuild shops not know what the manifold pressure switch code (P1810)means thinking it is engine related instead. Then the poor owner having to pay for a rebuild when only the switch needed replacement.

[Eduardo]

I found in a Haynes manual for gasoline models where the Assembly Line Data Link connector is placed, then I jumpered the ground and diagnostic test terminals. The computer is indicating codes 85 (transmission gear) and 86 (transmision gear ratio error).

According to the Haynes manual, " code 85 will set if the ECM detects the calculated transmission ratio different than the known ratio error in any gear. The ECM uses this information to detect shift solenoid problems "; and " code 86 will set if the ECM detects the calculated transmission ratio lower than the known ratio in first or second gear. The ECM uses this information to detect shift solenoid problems ".

Any ideas of where to go after this? Looking for a wiring problem in the transmission harness, perhaps? A problem with the input or output shaft sensors? Or something more serious?

Thanks in advance for any input.

[Eduardo]

Originally posted by Eduardo:
I found in a Haynes manual for gasoline models where the Assembly Line Data Link connector is placed, then I jumpered the ground and diagnostic test terminals. The computer is indicating codes 85 (transmission gear) and 86 (transmision gear ratio error).

According to the Haynes manual, " code 85 will set if the ECM detects the calculated transmission ratio different than the known ratio error in any gear. The ECM uses this information to detect shift solenoid problems "; and " code 86 will set if the ECM detects the calculated transmission ratio lower than the known ratio in first or second gear. The ECM uses this information to detect shift solenoid problems ".

Any ideas of where to go after this? Looking for a wiring problem in the transmission harness, perhaps? A problem with the input or output shaft sensors? Or something more serious?

Thanks in advance for any input.

I also found the following on the ATSG manual for the 4L80-E:

Code 85: Undefined ratio

Probable cause: Code 85 will set if the PCM/TCM's calculations indicate an unexpected gear ratio (does not include overdrive)

Default action: Set pressure to maximum


Code 86: Shift Solenoid "B" Stuck "ON"

Probable cause: Code 86 will set if the PCM/TCM has commanded 1st or 2nd gear, but a ratio calculation indicates 3rd gear.

Default action: Set pressure to maximum.


Now what?

[JohnC]

Originally posted by arrowheadracing:
So if the TPS is out of whack will it shift at a higher rpm ? Wouldnt there be anymore symptoms ? As far as engine running wise.

It won't affect the way the engine runs. Remember, we're talking about a '93 (or was it '92?) anyhow, mechanical injection. The TPS takes the place of the vacuum modulator on a gas setup. In '94 the PCM took over all control based on the APP module.

[patrick m.]

i would say get yourself a set of solenoids and an internal harness, BUT there is the possibility that your tranny is spraying debris like a tree chipper, which would quickly ruin the new electrics

[DA BIG ONE]

Differential gear changes, tires too tall, or too short could cause these symptoms too. Make sure of this first.

However, when in default increased pressures can and will cause more problems.

I somehow remember that the "speed sensor reluctor ring" is brittle and can be easily damaged during a rebuild. But, I would have to see some evidence this would cause those symptoms yours has! Does it look like it has had a rebuild,or?

[eracers999]

The 86 code is the one that needs to go. All solonoids and the internal harness replaced if the the pan only has a normal amount of trash in it.
The code 85 i wouldent be too worried about, i've had a code 85 ever since the performance chip install. Code 85 since day 1 37k ago on a new transmission. I believe that the default action of code 85 is relied on to keep it from slipping under high tork load. Pulling hard in converter lock up at or around 2k rpm you need all the psi you can get. Does the unit in question have a performance chip installed??

Kent

[farmerherb]

Ed, I have had many shift problems with my 4l80E (you can check my other posts) I replaced all the solenoids and the manifold pressure switch($100 for all) takes about an hour or two. My trans. seems to be all better. It has 309,980 miles on it, but each time I've changed the fluid(Amsoil) there has been little debris in the pan. You can get the inner wiring harness on Ebay for $25.

[Eduardo]

Well, it looks like the problem has been solved.

I took the truck back to the shop and first they replaced both solenoids. This cleared the code 85 from the computer, but code 86 and the hard shifting remained there. The speed sensors were tested and found OK. Then, the TPS was tested and found reading 4.8V at idle, and that figure would not change regardless of the position of the accelerator pedal. Obviously, this made the computer